Earthquake

An earthquake is a sudden shifting of masses of rock beneath Earth's surface. This motion releases enormous amounts of energy and sends out shock waves that cause the ground to shake. Geologists believe that no spot in the world is completely safe from earthquakes. The great internal forces of the planet are constantly changing the shape of Earth's surface. In fact, Earth has been resounding with earthquakes for more than four billion years.

Thousands of earthquakes occur each day. While not all of these earthquakes are significant, some are powerful enough to wreck cities and kill thousands of people. In the twentieth century alone, deadly earthquakes have claimed more than a million lives.

The 1906 San Francisco earthquake

On April 18, 1906, as the first rays of dawn began to spread across the sky, residents of San Francisco, California, were violently awakened as an earthquake shook the ground beneath the city. Most present-day geologists believe the earthquake, which came in two stages, measured 8.3 on the Richter scale. Other geologists believe the magnitude of the earthquake was slightly less, measuring about 7.8. (The Richter scale measures the magnitude of an earthquake or size of ground waves generated at the earthquake's source; a magnitude of 7.0 produces major damage, and a magnitude of 8.0 produces almost total damage. Every increase of one number on the Richter scale means a ten-fold increase in magnitude.)

Minutes after the earthquake struck, fires from broken gas and electrical lines began raging through the city. They burned uncontrollably for three days. Initial reports listed the number of dead as 700 to 800, but present-day research has led historians to conclude that more than 3,000 deaths were caused directly or indirectly by the earthquake and fires. Out of a population of 400,000, more than 200,000 people were injured and more than 250,000 were left homeless. Some 28,000 buildings were destroyed. The estimated total property damage from the earthquake and three-day fire was placed at more than 400 million dollars (in 1906 dollars).

WORDS TO KNOW

aftershock:

ground shaking that occurs after the main shock of an earthquake.

asthenosphere:

region of the mantle below the lithosphere, composed of partially melted rock.

continental drift:

geologic theory that all continents were part of a single, original landmass before they slowly separated and gradually drifted apart.

convection current:

circular movement of a gas or liquid between hot and cold areas.

crust:

outermost layer of Earth, varying in thickness from 3.5 to 50 miles (5 to 80 kilometers).

epicenter:

the point on Earth's surface directly above the focus of an earthquake, where seismic waves first appear.

fault:

crack in Earth's surface where two plates or sections of the crust push and slide in opposite directions against one another.

fault creep:

slow, continuous movement of plates along a fault, allowing pressure to be released.

focus:

the underground starting place of an earthquake, also called the hypocenter.

foreshock:

ground shaking that occurs before the main shock of an earthquake.

intensity:

description of the physical damage caused by an earthquake.

liquefaction:

the transformation of water-saturated soil into a liquidlike mass, usually by the action of seismic waves.

lithosphere:

the rigid outermost region of Earth, composed of the crust and the upper part of the mantle.

magnitude:

the power of an earthquake, as recorded by a seismograph, or seismometer.

mantle:

thick, dense layer of rock that lies beneath Earth's crust. The mantle is about 1,800 miles (2,900 kilometers) thick and accounts for about 84 percent of Earth's volume.

modified Mercalli scale:

scale developed by Italian seismologist Giuseppe Mercalli to measure the intensity of an earthquake based on the amount of vibration felt by people and the extent of damage to buildings.

plate tectonics:

geologic theory that Earth's crust is composed of rigid plates that "float" toward or away from each other, either directly or indirectly, creating the major geologic features on the planet's surface.

Richter scale:

scale developed by American seismologist Charles Richter that describes the amount of energy released by an earthquake.

seismic waves:

vibrations that move outward from the focus of an earthquake, causing the ground to shake.

seismograph:

instrument used to detect and measure seismic waves. Also known as a seismometer.

A city's history is changed in one day

The area from present-day San Francisco south to Monterey Bay had been home to Native Americans for more than ten thousand years before the first Europeans arrived on the land. About forty culturally diverse native tribes (now known collectively as the Ohlone, a Miwok word meaning "western people") occupied this area when Spanish explorers and missionaries established a presidio (fortress) and a mission on the site of present-day San Francisco in 1776.

The Spanish settlement, named Yerba Buena, remained in Spanish hands until 1821, when it came under Mexican control. U.S. naval forces occupied the village in 1846 when the Mexican War (1846–48) broke out between the United States and Mexico. When the war ended in 1848, the United States gained control of the village and renamed it San Francisco.

At about the time the war ended, gold was discovered in California. The population of San Francisco was then about eight hundred. Two years later, because of the mad rush to find gold, the city's population had grown to some twenty-five thousand. California became linked to the east by the Pony Express (mail delivery system comprised of relay teams of men riding ponies between Missouri and California) in 1860, and by the first transcontinental railroad in 1869.

By the beginning of the twentieth century, San Francisco was a booming metropolis with an art museum, numerous universities, a large commercial and fishing port, and a population of about 400,000. In just fifty years, it had become one of the largest cities in the United States. On April 18, 1906, however, San Francisco's rapid growth was brought to a halt, and the city's landscape and history were forever changed.

Lies on a fault

A fault is a crack in Earth's surface where two plates (sections of the crust) meet. At the San Andreas Fault, on which the city of San Francisco is situated, the Pacific plate and the North American plate push past each other. The Pacific plate moves slowly to the northwest, while the North American plate moves equally slowly to the southeast.

The San Andreas Fault came into being about fifteen to twenty million years ago. The entire fault system forms a continuous narrow break in Earth's crust that runs more than 800 miles (1,287 kilometers) from northwest California south to the Gulf of California. The fault extends downward at least 10 miles (16 kilometers) into the planet's crust. On the surface, the fault appears as an area of crushed and broken rock measuring a few hundred feet to 1 mile (1.6 kilometer) wide.

The plates move at an average rate of about 2 inches (5 centimeters) per year. Rather than sliding smoothly past each other like oiled blocks, the plates make sudden, jerky movements. First one plate moves, then the other. Sometimes the plates lock against each other and are unable to move. Pressure builds up between them. After many years, perhaps centuries or more, the plates overcome the built-up pressure and suddenly move with great force. When that happens, a massive earthquake occurs. Such was the case in 1906 along the fault near San Francisco.

Awakening to a nightmare

Before that fateful Wednesday morning in April, the Pacific and North American plates were locked, and pressure had been building along the northernmost section of the San Andreas Fault. At 5:12 am local time, the internal forces on the fault finally broke free. Like a compressed spring that is suddenly released, the two sides of the locked fault abruptly tore the ground apart along a 290-mile (467-kilometer) stretch from San Juan Bautista to Cape Mendocino. The ground west of the fault shifted northward as much as 21 feet (6.4 meters) in places. Tremendous amounts of energy were released, and the earth shook violently. The epicenter of the earthquake—the point on Earth's surface directly above the focus, or the place where energy is first released—was near San Francisco.

The foreshock, an earthquake tremor that occurs before the larger main shock, lasted about twenty seconds. It was strong enough to be felt throughout the San Francisco Bay area. The main shock hit the area about twenty-five seconds later and lasted for almost one minute. The strong shaking, punctuated by fierce jolts, was felt from southern Oregon to southern California and as far east as Nevada.

Numerous buildings in San Francisco collapsed during the main shock. Many poorly constructed buildings situated on land that had been filled with loose stones and dirt sustained the worst damage. The newly constructed six million dollar city hall was ruined, and the Sonoma Wine Company building collapsed, spilling 15 million gallons (57 million liters) of wine.

Shaken from their slumber, many people ran into the streets dazed and horrified. Others were killed instantly or mortally wounded when bricks from nearby buildings or chimneys crashed through the roofs onto them. San Francisco Fire Department Chief Dennis T. Sullivan was one of the victims. While he was sleeping upstairs in a fire station, bricks from the California Hotel next door crashed through the roof of the station and fell on him.

What people saw as they rushed onto the streets was a cityscape turned into a wasteland. American writer Jack London (1876–1916), who had been born in San Francisco and lived nearby, came into the city soon after the earthquake to witness the damage. His account appeared two weeks later in the national magazine Collier's.

London described the initial scene: "The streets were humped into ridges and depressions, and piled with the debris of fallen walls. The steel rails [of the streetcar and cable car tracks] were twisted into perpendicular and horizontal angles. The telephone and telegraph systems were disrupted. The great water mains had burst. All the shrewd contrivances and safeguards of man had been thrown out of gear by thirty seconds' twitching of the earth-crust."

Consumed by fire

Even those buildings left standing after the earthquake were not out of danger. Just minutes after the initial tremors waned, fires began to break out across the city. Downed electrical lines, toppled stoves, ruptured gas lines: all combined to set San Francisco ablaze.

Firefighters desperately sought ways to extinguish the raging fires. Hampered by a lack of water (most of the city's water mains had been ruptured), they decided to use dynamite to blow up damaged buildings in the paths of the fires. The idea was to create a firebreak by flattening the buildings and thus depriving the fire of fuel. The plan failed. Not only did the dynamited buildings fail to stop the fires, but the explosions added to the inferno.

By mid-afternoon, one huge blaze had taken over the heart of the city. It had become so large and so hot that it began to create its own wind. Jack London, looking at the burning city from a boat anchored in San Francisco Bay, described the strange, dreamlike scene: "It was dead calm. Not a flicker of wind stirred. Yet from every side wind was pouring in upon the city. East, west, north, and south, strong winds were blowing upon the doomed city. The heated air rising made an enormous suck. Thus did the fire of itself build its own colossal chimney through the atmosphere."

Most intense earthquakes in U.S. history

While the San Andreas Fault in California is well known as the source of many great earthquakes in the United States, the most intense earthquakes in the country over the past two hundred years occurred elsewhere. In the winter of 1811–12, three earthquakes centered on the New Madrid Fault in Missouri affected an area sixteen times larger than the 1906 San Francisco earthquake.

The three principal earthquakes that struck New Madrid, a city in southeastern Missouri on the banks of the Missouri River, took place on December 16, January 23, and February 7. Present-day scientists estimate that the quakes ranged in magnitude from 8.4 to 8.7 on the Richter scale. The intense ground vibrations caused by the main earthquakes and their many aftershocks bent trees until their trunks snapped, opened deep cracks in the soil, caused landslides on bluffs and low hills, created waves on the Mississippi River that overturned many boats, and changed the elevation of land in the area by as much as 20 feet (6 meters). The last, and most intense, of the three earthquakes altered the course of the Mississippi River and created a depression in the northwest Tennessee landscape that filled with river water to become Reelfoot Lake.

Fortunately, few lives were lost because the area was sparsely populated. Seismic waves and their effects, however, were felt for thousands of miles. Stone and masonry buildings as far away as 155 miles (250 kilometers) suffered severe damage. Structural damage was recorded in Pittsburgh, Washington, D.C., and coastal South Carolina. The vibrations surprised people in Chicago and Detroit and even caused church bells to ring in Boston, some 1,100 miles (1,770 kilometers) away.

The aftermath

The fires burned for three days. Sixty percent of the residential buildings in the city and the entire business district had burned. Almost 500 square blocks had been destroyed, and another thirty-two had partially burned. Overall, more than 2,590 acres (1,036 hectares), or about 4 square miles (10.4 square kilometers), had been ravaged by fire.

In the days following the earthquake and fires, hundreds of thousands of people fled San Francisco and the surrounding area. Most of those who remained were homeless but found shelter at makeshift camps set up around the city. Donated supplies poured in from all over the United States, staving off possible famine and more deaths.

Scientists predict that San Francisco may experience another massive earthquake midway through the twenty-first century. They base this estimate on data from the last 1,500 years indicating that major earthquakes occur along the San Andreas Fault about once every 150 years. Nonetheless, the area is prone to frequent, moderate-sized (yet damaging) earthquakes. The 1989 earthquake centered at Loma Prieta, 50 miles (80 kilometers) south of San Francisco, is one such example.

Recent events: Catastrophic earthquakes since 2000

Though many earthquakes occurred between the 2000 and 2005, the 2004 Indian Ocean earthquake and the 2005 Kashmir earthquake were especially destructive and deadly.

The Sumatra-Andaman earthquake of 2004

The Sumatra-Andaman earthquake, also known as the Indian Ocean earthquake, occurred on December 26, 2004. It was also one of the most powerful earthquakes on record. The event registered 9.0 on the Richter scale and was later upgraded to a magnitude between 9.1 and 9.3.

The Sumatra-Andaman earthquake struck off the coast of Indonesia, but its effects were far-reaching. The event is perhaps best remembered because it triggered a series of deadly tsunamis, or giant waves, which devastated countries that border the Indian Ocean. At the time of the earthquake, there was no reliable early warning system for Indian Ocean tsunamis, so the giant waves took people by surprise, battering coastal areas.

Casualties were especially high in Indonesia, Sri Lanka, India, and Thailand. When the waves hit, people were going about their business as usual. Tourists played on beaches, fisherman worked, and families went about their ordinary routines. People did not expect the giant waves that came ashore, destroying buildings and homes, and carrying debris like cars and construction materials farther inland. Well over 100,000 people died in Indonesia alone. The damage caused by the tsunamis displaced over a million people and caused billions of dollars worth of damage.

The destructive earthquake set records. The event created the longest fault rupture on record, creating an opening in Earth's seabed almost 800 miles long. The earthquake was also the longest-lasting on record with a duration of 500 to 600 seconds (about 10 minutes). By contrast, many small earthquakes last only one second. According to scientists, the average ground movement along the fault was at least 16.5 feet (5 meters). In some places, the ground moved as much as 20 feet.

Scientists say that the Sumatra-Andaman earthquake literally shook the entire planet. Scientists were able to detect movement from the event in every place in the world that seismic activity is measured. Thanks to new technology in earthquake measuring instruments, like global broadband seismometers, scientists were able to measure the Sumatra-Andaman event with more precision than ever.

The destructiveness of the Sumatra-Andaman earthquake created widespread human suffering. This tragedy has motivated the scientific community to develop a reliable warning system for tsunamis in the Indian Ocean.

The Kashmir earthquake of 2005

Another historic and disastrous earthquake happened on October 8, 2005, in the Kashmir region of Pakistan. This region is one of the most seismically active in the world. The 2005 earthquake had a magnitude of 7.6 on the Richter scale. It was the strongest earthquake on record in Pakistan. It was followed by a series of powerful aftershocks, measuring as much as 6.3 on the Richter scale. The epicenter of this powerful earthquake was about 60 miles north-northeast of Islamabad, Pakistan's capital.

The Kashmir earthquake was one of the most destructive of all time. In addition to causing widespread destruction, the earthquake killed almost 80,000 people. At least as many people were injured. The earthquake left three million people homeless just before the onset of the winter, which is bitterly cold and severe in the region, which includes parts of the Himalayan mountain range.

The gravest damage caused by the Kashmir earthquake was said to be in the village of Balakot, which is near the epicenter of the earthquake. Portions of the village, which is located on a hillside, literally slid down the hill. Most of the village was completely flattened by the earthquake. The residents of Balakot were among the millions displaced by the earthquake.

In addition to nearly decimating many mountain villages like Balakot, the Kashmir earthquake caused a number of landslides. These landslides cut off crucial thoroughfares across the mountains to the north of Islamabad. Because winter was coming, aid workers feared a second wave of deaths from the earthquake. They worried that people left homeless would die from exposure to the cold or from starvation. Fortunately, predictions that millions of displaced people would die turned out to be wrong. Many people were spared by a winter more mild than most. Aid agencies also found ways to transport food and supplies to isolated communities.

Still, the Kashmir quake devastated the region. Eyewitnesses reported that landslides continued well into the month of October, making mountain areas especially dangerous. Extensive farming infrastructure was destroyed, causing not only food shortages but also losses of livelihood.

Dangerous science: What causes earthquakes?

The answer to what causes earthquakes is found in both the structure of Earth's surface and the forces that rage inside the planet. The interior of Earth—from the planet's center to its surface (a distance of about 3,975 miles [6,395 kilometers])—is divided into layers, defined by their material composition. The core, at the center of the planet, is composed of a solid inner portion about 780 miles (1,300 kilometers) thick and a liquid outer portion about 1,380 miles (2,220 kilometers) thick. Surrounding the core with a depth of about 1,800 miles (2,900 kilometers) is the mantle. The crust, or surface layer surrounding the mantle, varies in thickness from 5 to 25 miles (8 to 40 kilometers).

The mantle is divided into two sections. The upper section, directly below the crust and about 40 miles (65 kilometers) thick, is solid. The section beneath it is soft, or partially melted. The crust and the rigid section of the mantle together compose what geologists call the lithosphere (pronounced LI-thuh-sphere). The soft section of the mantle is called the asthenosphere (pronounced ass-THEE-nuh-sphere).

Most geologists believe that convection currents (circular movements of fluid between hot and cold areas) in the asthenosphere are the driving force behind earthquakes and other movements on the surface of the planet. The heat energy at the center of Earth—where temperatures are estimated to exceed 9,900°F (5,480°C)—is carried to the surface by convection currents. As they near the crust, the currents cool and sink back toward the center to be heated once again. The pressure created by the action of these currents is released on Earth's surface through volcanoes and earthquakes.

Plate tectonics

Plate tectonics is the geologic theory that Earth's crust is made up of rigid plates that float on the surface of the planet. (Tectonics comes from the Greek word meaning "builder.") The plates make up the lithosphere and float on the underlying asthenosphere. There are seven major plates and several smaller ones that are in constant contact with each other. When one plate moves, it causes other plates to move. The movement of the plates toward or away from each other either creates the major geologic features, such as mountain ranges or faults, at Earth's surface.

Plate tectonics is a relatively new scientific theory. This theory is built on the idea of continental drift, introduced in the 1920s by German geophysicist Alfred Wegener (1880–1930). Wegener believed that all continents were part of a single, original landmass—a supercontinent he called Pangaea (pronounced pan-JEE-ah)—before they gradually separated and drifted apart. His concept was based on the fact that several of the planet's continents seem to fit together like pieces in a jigsaw puzzle. This is particularly apparent when examining the eastern coast of South America and the western coast of Africa. Wegener, however, could not provide a convincing argument as to what made the continents shift around Earth's surface.

Reports from the past: Prehistoric earthquakes point to the future

In July 1998, two scientists published a study in the journal Science in which they asserted that there had been at least two giant prehistoric earthquakes near present-day Los Angeles. The scientists estimated that the earthquakes occurred within the last 15,000 years on the Sierra Madre Fault, which runs 12 miles (19 kilometers) north of the city. It was previously believed that earthquakes had not occurred on that fault.

The scientists, by measuring different layers of soil in the faulted region, determined that the ground had moved more than 16.5 feet (5 meters) on average during the ancient tremors. Based on that figure, they estimated that the earthquakes had ranged in strength from 7.2 to 7.6 on the Richter scale.

The scientists made no predictions of future earthquakes, merely pointing out that it was possible for an earthquake to eventually strike the city. If an earthquake were to occur today in the same place as the prehistoric earthquakes, the scientists reasoned, strong tremors would be sent directly into the heart of Los Angeles. Such an earthquake would cause immense destruction in the heavily populated city.

That question was answered by the theory of plate tectonics. The theory states that tectonic plates, moving about Earth's surface in response to pressure beneath them, interact with each other in one of three ways: they converge (move toward one another), diverge (move away from one another), or transform (slide past one another). The boundaries where plates meet are known as plate margins.

Faults

As two rock plates slide past one another, a crack or fault develops at the plate margin. Most earthquakes occur along faults (also called fault lines). The principle types of faults are normal, reverse, and strike-slip. A normal fault forms when two plates are being pulled or stretched apart. A reverse fault, in contrast, forms when two plates are being pushed together—the compression forces one plate up and over the other. A strike-slip fault, the most common type of fault, forms when the edge of one plate grinds against the edge of another as it slips sideways. The San Andreas Fault, the best known fault in the continental United States, is a strike-slip fault.

When plates along a fault move slowly and continuously, allowing pressure to be released, the movement is called fault creep. More often, however, the lower parts of the plates move slowly and continuously along the soft portion of the mantle while the upper parts of the plates, where the surface is solid rock, remain locked in position. Such a configuration causes stress to build up in the crust. When that stress becomes greater than the forces holding the surface plates in position, the crust can rupture. The stored energy is then released through ground vibrations—better known as earthquakes. There are more than one million earthquakes a year around the planet, although most are too faint to be noticed.

Earthquake zones

On Earth, about 90 percent of earthquakes occur around the outer rim of the Pacific Ocean. This area is where the edges of the Pacific plate, the portion of the Earth's crust that lies beneath the Pacific Ocean, come in contact with several other plates. On its western side, the Pacific plate slides beneath the Eurasian and Indo-Australian plates. Along its northeastern side, the Pacific plate comes up against the North American plate. The north-central edge of the Pacific plate rides above the Nazca plate (located between the Pacific plate and the South American plate).

Not only does the outer rim of the Pacific Ocean have frequent earthquakes, but it also contains three-quarters of the world's volcanoes. For this reason, this circular area is also known as the Ring of Fire (or Circle of Fire).

Seismic waves: The motion of earthquakes

Vibrations transmitting the shock of an earthquake are called seismic (pronounced SIZE-mik) waves. These waves travel both underground and along Earth's surface in all directions, like ripples from a stone dropped in a pond. The underground area where energy is first released, triggering an earthquake, is called the focus (or hypocenter). The focus may be as far as 430 miles (700 kilometers) beneath Earth's surface. The point on the surface directly above the focus is called the epicenter.

When an earthquake occurs, two main classes of seismic waves are generated. The first class waves, called body waves, are generated below ground, from the source of the earthquake, and travel to the surface. Body waves consist of P (primary) waves and S (secondary) waves. The P waves travel fastest, up to 4 miles (6.4 kilometers) per second, and are the first waves to reach the surface. They stretch and compress the rock in their path and cause the ground to move vertically (upward). P waves release their energy to the atmosphere, resulting in the common thundering or rumbling sound associated with earthquakes.

S waves travel at about half the speed of P waves, or 2 miles (3.2 kilometers) per second. They move from side to side as well as upward, causing the ground to move horizontally as well as vertically. For this reason, S waves are far more destructive to buildings than are P waves.

S and P body waves combine near the epicenter to form surface waves or L (long) waves, which travel along Earth's surface. Although surface waves move slightly more slowly than body waves—less than 2 miles (3.2 kilometers) per second—they cause greater damage. Surface waves can set off avalanches, landslides, and tsunamis.

Seismic waves can travel great distances. For instance, in late 1811 and early 1812 a series of intense earthquakes hit the United States near New Madrid, Missouri. Vibrations could be felt more than 1,000 miles (1,600 kilometers) away. Although waves lose energy as they travel, they can still cause major destruction as they ripple outward.

Aftermath: What are the effects of earthquakes on Earth and its inhabitants?

Great earthquakes leave telltale signs in the surface of Earth, especially in the area around the epicenter. A horizontal or vertical change in the ground level can often be seen near a fault. The most dramatic change is the creation of a scarp, or cliff. Depending on the type of rock and the amount of uplift caused by the earthquake, a scarp may be up to 1 mile (1.6 kilometer) tall.

Experiment: Understanding plate tectonics

Earth's crust is made up of large tectonic plates. These plates are always moving. They move slowly, but they move with incredible force. To understand what happens when plates move into each other, try this experiment. Get a sheet of newspaper, and find a large, hard, flat surface, either a table or the floor, where you can spread the newspaper flat. Put your palms down on the newspaper about a foot apart and slowly push them together. The newspaper will crumple and fold upward in the middle. When tectonic plates fold upward, mountains are created. When they crumple together, earthquakes are the result.

Earthquakes can also produce fissures, or long cracks, in the ground. Often the ground on one side of a fissure is uplifted several inches or even several feet. In addition to being moved vertically, the ground on one side of a fissure can also be moved horizontally. One example of vast changes in ground level occurred in southern Alaska on March 27, 1964. On that date, an earthquake measuring 8.6 on the Richter scale struck the area, permanently resculpting 77,220 square miles (200,000 square kilometers) of land surface. Almost 620 miles (1,000 kilometers) of coastline from Kodiak Island to Prince William Sound was either uplifted or lowered. In some places, the vertical uplift was as much as 36 feet (11 meters); in others, the subsidence, or lowering of ground level, was as much as 7 feet (2 meters).

If a fissure occurs on a hill, it may set off a landslide. Landslides or mud slides are rapid downhill movements of soil and rock. These slides may encompass whole sections of a hill or mountain. During the landslide, the loosened soil and rock rushes down the slope, often destroying everything in its path before finally coming to rest at the bottom of the slope.

Underground structures such as springs and wells may be disturbed by earthquakes. Springs may stop flowing, either temporarily or permanently, and new springs may form. The water in wells may be muddied or its level changed. All of these effects can occur even at a great distance from the epicenter of an earthquake. An extremely intense earthquake may even force water, mud, and sand out of the ground, forming what are called earthquake fountains and sand blows.

What makes earthquakes deadly?

The damage created by an earthquake is most evident in human-made structures. An earthquake is more likely to cause injuries, fatalities, and damage the closer the epicenter is to a populated area. Earthquakes occurring in crowded cities leave behind buckled and broken sidewalks and streets, downed trees and power lines, and collapsed homes and buildings.

Pre-Renaissance artwork destroyed by earthquakes

Priceless frescoes attributed to Florentine painters Giotto (c.1266–1337) and Cimabue (c.1240–c.1302) were severely damaged when two earthquakes struck central Italy within hours of each other on September 26, 1997.

The earthquakes, which measured 5.5 and 5.6 on the Richter scale, killed eleven people and caused damage to nearly 1,500 buildings. The first earthquake struck at 2:33 am local time. Its epicenter was close to the town of Foligno in Umbria, a region dotted with historic towns. The second earthquake struck some nine hours later. Tremors from both earthquakes were felt in Rome, more than 62 miles (100 kilometers) away.

Frescoes are paintings made on damp plaster walls. When the plaster dries, the painting becomes fixed to the wall. The frescoes by Giotto and Cimabue were part of the walls and ceiling of the thirteenth-century Basilica of San Francesco in Assisi, one of early Christianity's most important shrines. A renowned tourist attraction, the basilica is famous for its twenty-eight frescoes depicting the life of Francis of Assisi (c.1182–1226), founder of the Franciscan religious order.

The first earthquake caused deep cracks in many of the precious frescoes. Just as friars and structural experts were inside inspecting this damage, the second earthquake struck, causing an even greater tragedy. Huge sections of the vaulted ceiling of the basilica collapsed, killing four people and destroying many of the frescoes.

On November 28, 1999, just over two years after the earthquakes struck, a commemorative mass was held to reopen the church. The bell tower, the vault sections, and some of the frescoes had been repaired or reattached. Restorers estimated it would take at least another two years before other damaged frescoes, including Cimabue's St. Matthew and Giotto's St. Jerome, would be reassembled.

What usually kills people in an earthquake is not the earthquake itself, but the collapse of buildings, roads, and other structures. The greatest destruction is brought about, often miles away from the epicenter of an earthquake, through the process of liquefaction (pronounced liquh-FAC-shun). When earthquake vibrations encounter layers of loosely packed soil containing water, the vibrations cause the particles of soil to lose contact with one another. This allows the water to bubble through, transforming the ground into a watery mass resembling quicksand.

Structures and roads built on this type of soil cannot be supported by this liquid mixture and thus sink or slide from their foundations.

Kobe earthquake of 1995

The largest earthquake to hit Japan in the last half of the twentieth century occurred on January 17, 1995. At 5:46 am local time, an earthquake measuring 7.2 on the Richter scale struck the region of Kobe and Osaka in south-central Japan. The region includes Kobe, Japan's sixth largest city and the world's sixth largest port, with a population of some 1.5 million. The Kobe earthquake, dubbed the Great Hanshin Earthquake by the Japanese media, killed more than 5,500 people and injured 25,000 others. More than 300,000 people were left homeless. The Kobe earthquake was the deadliest natural disaster in Japan since 1923—the year that the Great Kanto Earthquake killed about 140,000 people.

The epicenter of the Kobe earthquake was located about 20 miles (32 kilometers) south of the city of Kobe near Awaji Island. The minor strike-slip fault on which the earthquake occurred runs from Awaji Island through Kobe. The earthquake caused the ground to move 7 inches (18 centimeters) horizontally and 4 inches (10 centimeters) vertically—the greatest ground displacement ever recorded in Japan. In the twenty-four hours after the main shock, 716 aftershocks were recorded.

The Kobe earthquake damaged or destroyed one out of every five buildings in the area, totaling more than 180,000 buildings. The violent ground shaking, which lasted for about twenty seconds, caused an estimated damage of about 147 billion dollars (U.S.).

Damage occurred throughout a 62-mile (100-kilometer) radius from the epicenter. This area encompassed the cities of Kobe, Osaka, and Kyoto, with Kobe being the most severely affected. Kobe, like much of modern urban Japan trying to keep up with a growing population, was built on very soft soil and landfills. The worst damage occurred near the waterfront where severe liquefaction took place, causing industrial buildings to sink and crumble. Ninety percent of the port's 187 berths were destroyed, and most large cranes along the waterfront were either damaged or destroyed.

Major utilities were severely disrupted in Kobe and surrounding cities. At one point, almost one million households were without electrical power, another 800,000 households lacked gas, and some 250,000 telephone lines were disconnected. Severed gas lines started fires that burned for days. Approximately 150 fires broke out in Kobe and surrounding areas in the hours immediately following the earthquake. Firefighters, hampered by a lack of water because of thousands of breaks in the water line system, were ill-equipped to fight the blazes. Streets clogged with collapsed buildings and traffic congestion also blocked firefighters' efforts.

Houses built in the style of traditional Japanese architecture readily collapsed, killing their inhabitants. Heavy roofs composed of thick mud or tile fell as the thin walls supporting them buckled. Other structures throughout the city were also destroyed by the earthquake. Railway lines were twisted and bent, and train cars were thrown like toys. Elevated portions of the area's highway system collapsed or caved in. Impassable roads, bridges, and rails slowed the arrival of aid from nearby cities. In the period immediately after the earthquake, the only way of transporting emergency supplies was by bicycle or on foot.

The Kobe earthquake hit when most people were asleep. Had it come later in the day—when people filled trains, freeways, and sidewalks—the death toll would have been far higher.

Another serious consequence of earthquakes is fire. Broken gas lines, downed electrical wires, and overturned stoves and heaters start many fires (as was the case in the 1906 San Francisco earthquake). Because

water mains and streets are damaged at the same time, the water needed to put out the fires is unavailable. In a populated region, the damage caused by fires frequently exceeds that caused by the ground shaking.

A secondary effect of an earthquake that occurs under the ocean is a tsunami (pronounced tsoo-NAH-mee), also called a tidal wave. A tsunami

Turkey's earthquakes of 1999

One of the deadliest earthquakes of the twentieth century hit northwest Turkey on August 17, 1999. At 3:02 am local time, a violent tremor measuring 7.4 on the Richter scale shook the ground for forty-five seconds. The epicenter of the earthquake was located near the city of Izmit, 55 miles (88 kilometers) east of Istanbul. Almost half Turkish population lived in the area affected by the earthquake. When it was over, nearly 76,000 buildings had collapsed, an estimated 17,500 people were killed and more than 33,000 others injured. Almost 500,000 were left homeless. The Turkish government, which called the natural disaster one of the greatest in Turkish history.

Turkey lies atop some of the world's most unstable geology. The North Anatolian fault runs through the densely populated, industrial area of northern Turkey. This fault is a small slab of crust that marks the boundary between the Arabian plate to the south and the larger Eurasian plate to the north. Between the two plates lies the relatively small Turkish microplate, on which most of the country of Turkey is located. As the Arabian plate moves slowly northward, it pushes the Turkish microplate to the west relative to the Eurasian plate at a rate of about 1 inch (2.5 centimeters) per year. At times, the microplate sticks, then jerks free with great energy. This occurred on August 17.

The North Anatolian fault is more than 800 miles (1,287 kilometers) long and 10 miles (16 kilometers) deep. It is a perfect example of a strike-slip fault, where the ground moves side to side. After the August 17 earthquake, geologists measured the North Anatolian fault and found that in some places one side of the fault had shifted more than 12 feet (3.7 meters) relative to the other side.

Earthquakes are common in geologically unstable Turkey. According to historians, over the last 5,000 years major earthquakes have shaken northern Turkey about once every 175 years. Seven destructive earthquakes have occurred along the North Anatolian fault since 1939, the year in which an earthquake struck the eastern province of Erzincan and killed 30,000 people. Since then, the locations of the earthquakes along the fault have moved progressively westward.

The earthquake on August 17 was the most powerful to strike the area since 1939. Lives and buildings were destroyed up to 200 miles (322 kilometers) from the epicenter. Tremors from the earthquake were felt as far east as Ankara and across parts of the Balkans.

In central Istanbul, the majestic museums and historic mosques suffered no visible damage. In areas on the outskirts of the city, home to some twelve million people, newly built apartment complexes crumbled. In some places to the southeast of Istanbul, the destruction was nearly total.

Rescue teams representing thirty-eight international organizations and forty-two countries arrived in the country shortly after the disaster. But many areas in desperate need of help received nothing for days, forcing the living to dig through the rubble with their bare hands in search of their family members, friends, and neighbors. Smashed sewer lines and a lack of fresh water and portable toilets raised the risk of the spread of infectious diseases among the hundreds of thousands of people left homeless by the earthquake.

On November 12, an earthquake measuring 7.2 struck the northwestern town of Duzce just after nightfall. Located in a hilly region about 115 miles (185 kilometers) east of Istanbul, Duzce was on the eastern fringe of the region hit by the August 17 earthquake. Hundreds of thousands of survivors from the first disaster were still living in tents in the rainy winter weather when the new earthquake struck. The Duzce earthquake lasted for thirty seconds, cutting communication lines and crumbling the main road leading into the town. More than 5,100 people were injured and more than 750 buildings were destroyed. The death toll exceeded 700.

is set in motion by a vertical shift in the ocean floor, which pushes the water ahead of it. The vertical shift generates waves that can travel across the ocean with speeds of 250 to 500 miles (400 to 800 kilometers) per

hour. Tsunamis start out small and grow larger as they near land. It is typical for a large tsunami to measure 60 to 100 feet (18 to 30 meters) in height by the time it crashes onto land.

Earthquakes usher in year 2001

Within the first twenty-six days of 2001, major earthquakes struck two locations at opposite ends of Earth. The first quake shook the tiny Central American nation of El Salvador on January 13, and the second quake hit western India thirteen days later.

Measuring 7.6 on the Richter scale, El Salvador's earthquake killed some 700 people, injured more than 2,000 people, and left 65,000 families homeless. Damages totaled 1.5 billion dollars—about half of the gross earnings of all Salvadorans.

El Salvador's earthquake affected the capital city, San Salvador, and villages in the countryside alike. A large portion of the casualties occurred in Las Colinas, a suburb of San Salvador. A cluster of homes at the base of a hill there were buried under a wall of soil and mud that collapsed in an earthquake-triggered landslide. (The removal of trees from the hillside to make room for a coffee plantation and luxury homes was considered partially to blame for the landslide.) San Francisco Javier and San Augustin, both about 100 miles southeast of the capital, were representative of outlying towns in which virtually every building, most made of adobe walls and tin roofs, was reduced to rubble.

India's earthquake, a 7.9 on the Richter scale, had far greater consequences than El Salvador's. The death toll in India exceeded seventeen thousand, there were more than sixty thousand injuries, and one million people were left homeless. The earthquake, India's worst in half a century, occurred at about 9 o'clock in the morning on Republic Day (a national holiday honoring the Indian constitution).

The epicenter of the earthquake was below the heavily populated state of Gujarat, near the border with Pakistan. Buildings in cities throughout Gujarat, constructed without following earthquake-proof building codes, teetered and fell or collapsed in heaps. The hardest-hit city, Bhuj, lost 6,000 of its 150,000 residents and its hospital. Not a single building in Bhuj escaped without serious damage. In the town of Anjar, 400 children marching in a Republic Day parade were buried alive by debris from tumbling houses and high-rise buildings.

Throughout the region, the earthquake caused water supplies to become contaminated; destroyed roads, bridges, and railroad tracks; and downed telephone and power lines. The destruction not only made it hard for survivors to find food and water, but it made it difficult for relief workers with needed supplies to reach affected areas.

The human factor: How do people contribute to earthquake damage?

The most obvious way that people contribute to earthquake destruction is by building cities and towns within earthquake zones (areas situated on or near faults and other plate boundaries). Because scientists have identified these zones only within the last one hundred years, most cities were built with no knowledge of the danger. Complicating matters, wetlands and other moist lowland areas around many cities have been filled in with soil

to create more living space. These filled-in areas are highly prone to liquefaction and destruction during earthquakes.

Along with ground instability and the possibility of liquefaction, the design of many buildings makes them susceptible to earthquake damage. Structures that are insufficiently braced or not tightly secured to their foundations can be damaged easily by earthquakes. The materials used to construct the buildings is another factor. For instance, wood is more flexible than brick and cement; therefore, wood-frame buildings are better equipped to withstand ground motion. Brick and cement structures often fracture and collapse during large earthquakes.

Further, some scientists said that human development and coral mining created conditions that worsened the impact of the Sumatra-Andaman earthquake. That 2004 earthquake, whose epicenter was in the Indian Ocean, triggered massive tsunamis that pulverized many countries that border that ocean. Normally, a coral reef acts as a natural barrier between the land and powerful ocean waves. When a reef is compromised or destroyed, there is no barrier to protect the land. Thus, some scientists think that the tsunami that resulted from the Sumatra-Andaman earthquake was taller and more powerful in areas where reefs had been compromised.

Artificial earthquakes?

Scientists have recently begun exploring the occurrence of artificial or human-made earthquakes, such as those caused by quarry blasts or other large explosions. Another possible trigger of earthquakes is the underground detonation (for the purpose of testing) of large nuclear bombs. The tremors generated by this type of explosion are strong enough to be felt more than 100 miles away from the testing site. It is therefore plausible that such tremors could trigger the release of built-up pressure between two nearby plates, resulting in an earthquake.

The underground storage of hazardous liquid wastes is yet another problem. (Wastes deemed hazardous are those detrimental to human health or the environment because, for example, they catch fire at low temperatures, are extremely acidic, or undergo violent physical and/or chemical changes when mixed with water. Such wastes must be disposed of in accordance with strict standards developed by the Environmental Protection Agency.) Scientists believe that improper underground disposal of hazardous wastes may have caused an earthquake beneath northeastern Ohio on January 31, 1986. For the twelve years before that date, hazardous liquid wastes had been disposed of in two underground wells located almost 1.3 miles (2 kilometers) below ground. A total of about 350 million gallons (1.3 billion liters) of hazardous waste had been pumped into the wells. The pressure created by that volume may have ruptured the surrounding rock, causing cracks that reached an area where tectonic pressure had been building up. Whatever the cause, an earthquake measuring 4.9 on the Richter scale rocked this area in Ohio.

Technology connection: Measuring and predicting earthquakes

From the beginning of recorded history, people have tried to explain, predict, and measure earthquakes. Ancient people believed that the movement of various animals such as giant catfish, snakes, spiders, or turtles that lived beneath Earth's surface created earthquakes. One of the first people to attempt to explain the action of earthquakes based on natural phenomena was Greek philosopher Aristotle (384–322 bce). He believed that winds within the planet caused shaking at the surface.

The first known earthquake-measuring device was invented by Chinese scholar and poet Zhang Heng (78-139) in the second century ce. The device, now called a seismoscope, is a huge bronze vase measuring 6 feet (1.8 meters) in diameter. Eight dragon heads are sculpted around the top. In the mouth of each dragon is a small bronze ball. Directly below each dragon head sits a bronze frog. Inside the vase, each dragon head is attached to a bar, which connects to a single pendulum (a hanging object that freely swings back and forth) in the middle of the vase. The pendulum swings from the movement of even the slightest tremor. When the pendulum swings, it pulls back one of the bars. The dragon's mouth opens, and the ball drops into the open mouth of the bronze frog directly below. The frog holding the ball indicates from which direction the tremor came.

Present-day scientists measure an earthquake's power by two standards: intensity and magnitude. While the intensity of an earthquake is determined by the amount of damage caused, magnitude is measured by using seismographs, also known as seismometers, or other devices that detect ground movement. Intensity is a measure of an effect; magnitude is a measure of released energy.

Seismology

Scientists use an instrument called a seismograph, or seismometer, to measure the waves caused by an earthquake. A seismograph consists of a heavy weight or pendulum hanging over a constantly revolving drum wrapped with recording paper. Attached to the end of the pendulum, with its tip touching the paper, is a recording pen or needle. When an earthquake occurs, the seismic waves cause the pendulum to swing, and the pen records that motion on the paper. The bigger the seismic wave, the larger the swing of the pendulum, and the wider the zigzag line made by the pen on the paper. Today, the motion of the pendulum can also be recorded as digital data on a computer.

Seismographs, or seismometers, are usually placed underground in deep holes away from the artificial vibrations caused by cars, airplanes, and other modern machinery. In 1931, only 350 seismographic stations existed around the world to measure earthquakes. By 2000 there were more than 4,000 stations recording data. That data is transmitted via satellite and computer to scientific institutions around the world.

The most recent seismographic technology includes the broadband seismometer, which is able to record a broad range of frequencies. These devices have allowed scientists to collect even more specific data on recent earthquakes like the 2004 Sumatra-Andaman earthquake and the 2005 Kashmir earthquake. This electronic device confines a small mass between electrical forces. When the Earth moves, the device measures how much force is required to hold the mass steady.

Mercalli scale

In 1902 Italian seismologist Giuseppe Mercalli (1850–1914) developed a scale to compare the surface effects of earthquakes. Mercalli's measurement of earthquake intensity was based not on scientific measurement but on the damage done to buildings. Updated in 1931 by American seismologists Harry Wood and Frank Neumann, the scale (now called the modified Mercalli scale) rates the intensity of earthquakes from levels I (detected only by seismographic instruments) through XII (destruction of all buildings).

On the modified Mercalli scale, a rating of II or III describes a weak earthquake that is felt by few people. At this strength, ground vibrations are similar to the passing of a large truck. People on upper floors of buildings would most likely feel such small tremors as a slight swaying motion. Tremors of this strength may also cause hanging objects like chandeliers to swing.

Medium-strength earthquakes with a modified Mercalli rating from IV to VI are felt by most people and usually wake those who are asleep. Earthquakes of this intensity can cause walls to crack and dishes and windows to move or break. During this type of earthquake, unstable objects may fall over or drop to the floor, heavy furniture may move across the floor, and people may feel off-balance as they walk.

Earthquakes with a VII or VIII rating on the modified Mercalli scale can cause considerable damage to poorly designed and built structures. Even well-built structures may suffer moderate damage. An earthquake of this strength can cause chimneys, factory stacks, columns, and walls to

tumble, and unsecured houses to move off their foundations. The ground motion may cause wet ground to develop cracks and sand and mud to spurt up through those cracks.

Serious damage and partial-to-total collapse of buildings—even specially designed structures—is common in earthquakes rating IX or higher on the modified Mercalli scale. Underground water pipes, reservoirs, dams, and embankments break or become damaged. Railroad rails may bend, and noticeable cracks may appear in the ground. Liquefaction occurs, causing buildings and roads to sink into the ground. Landslides occur on steep slopes. At level XII of the scale, the ground ripples in waves, objects are thrown into the air, and the courses of rivers may shift.

Richter scale

In 1935 American seismologist Charles Richter (1900–1985) developed a scale to measure the magnitude of earthquakes. Richter and colleague Beno Gutenberg (1889–1960) sought a standard method of measuring and comparing earthquakes. Up to that point, the Mercalli scale was the only earthquake-measurement tool. The problem

with the Mercalli scale, however, was that it depended on nonscientific factors. It was greatly influenced by the soundness of building construction and the behaviors of the people living in the earthquake area. The Mercalli scale also made it difficult to rate earthquakes that occurred in rural areas where there were few buildings or people.

Richter and Gutenberg developed a way of measuring an earthquake's power, rather than its effects on humans or buildings. The Richter scale, ranging from 1 to 10, uses seismographic readings to compare the energy released by a specific earthquake to the energy released by other earthquakes. Each whole number increase in value on the scale indicates a ten-fold increase in the energy released and a thirty-fold increase in ground motion. Therefore, an earthquake with a value of 6.0 on the Richter scale is ten times more powerful than an earthquake with a value of 5.0, and so on up and down the scale.

Most earthquakes that are reported measure between 3.0 and 8.0 on the Richter scale. Those that have a rating less than 3.0 are not usually felt. Earthquakes measuring between 3.0 and 6.0 cause minor to moderate damage. Strong earthquakes, causing destruction in areas with large populations, measure between 6.0 and 6.9 on the scale. Earthquakes measuring 7.0 to 7.9 are major. (California is struck by earthquakes in this range about once every eighteen years.) Any earthquake that measures 8.0 or above on the Richter scale is considered a massive earthquake, one that causes widespread destruction. An earthquake of this magnitude generally occurs somewhere on the planet once every five to ten years.

Earthquake prediction

Of the more than one million earthquakes that shake the surface of Earth each year, the vast majority measure 3.4 or below on the Richter scale and cannot be felt by people. The few earthquakes that register high on the Richter scale, however, can be disastrous. Great earthquakes have accounted for some of the most dreadful natural disasters in recorded human history. In the past eight hundred years, seventeen earthquakes have caused fifty thousand or more deaths apiece.

Seismologists in many countries constantly monitor the stresses within Earth's crust. Ultrasensitive instruments placed across faults at the surface measure the slow, almost imperceptible movement of plates. Other instruments measure phenomena that sometimes precede earthquakes, including changes in tide and groundwater levels, fluctuations in the magnetic properties of rocks, and the swelling or tilting of the ground.

Earthquake rattles Washington State

At 10:54 am on February 28, 2001, northwest Washington state was rocked by its most powerful earthquake in fifty-two years. The forty-second tremor measured 6.8 on the Richter scale. It was centered between Olympia and Tacoma, some 30 miles from Seattle, on the southern border of Puget Sound. The quake produced just one fatality (one woman died of a heart attack) and approximately 235 injuries; only a handful of the injuries were serious, and none were life-threatening.

Damage caused by the earthquake was estimated at more than two billion dollars. Examples of damage included crumbled bridges, buckled roads and sidewalks, and broken glass and fallen plaster in buildings. Some of the worst destruction occurred at the seventy-four-year-old capitol building in Olympia, about 12 miles from the earthquake's epicenter. The building's dome was cracked in several places, and engineers were unsure as to whether or when the statehouse could be used again.

Geologists noted that the damage in the region was relatively minor for a quake of that magnitude. They attributed the gentleness of the earthquake to the location of its epicenter, which was more than 30 miles beneath the surface. "We were very, very lucky," stated Washington governor Gary Lock in the New York Times of March 2, 2001. "There could have been utter catastrophe had it been higher, closer to the surface."

Also deserving credit for the prevention of greater damage was the strict earthquake code for building construction in the region. Those regulations are intended to protect buildings from all but catastrophic earthquakes. Almost all buildings built or upgraded to be in compliance with the code escaped without serious damage.

For more than thirty-five years, the U.S. Geological Survey (USGS) has operated seismographic stations throughout the world. In the mid-1990s, the USGS and the Incorporated Research Institutions for Seismology (an association of more than ninety universities) developed the Global Seismographic Network (GSN). This worldwide network is composed of 128 stations in more than eighty countries on all continents. Its purpose is to make readily available high-quality data regarding earthquakes. Within minutes after data is received and recorded, scientists can analyze the information to determine the location and magnitude of any large seismic event that has occurred anywhere on the planet.

Even with this state-of-the-art equipment, scientists cannot predict the exact time and place an earthquake will occur. Earthquake researchers continue to sharpen their ability to correctly interpret the significance of seismic activity. At present, scientists rely primarily on information about past earthquakes in order to determine the likelihood of future earthquakes.

Some people look to peculiar animal behaviors as a sign of impending earthquakes. Certain animals have heightened sensitivities to electricity, magnetic fields, odors, and vibrations. In China it is said that prior to earthquakes snails come out of the ground, rats leave houses, birds fly from their nests, and dogs bark constantly. In Japan, there are reports of goldfish swimming frantically just before earthquakes. From a scientific standpoint, however, the connection between animal behavior and earthquakes has not been proven.

A matter of survival: How can we live through earthquakes?

Earthquakes can neither be stopped nor controlled, but humans can minimize the destruction earthquakes cause. A seemingly simple solution is for people to avoid living in earthquake-prone areas. This solution is not very practical, however, since many cities—some with large and growing populations—already stand in these areas.

A more realistic solution is to design structures resistant to earthquake damage. In many earthquake zones, strict building codes for new structures have been adopted. For existing buildings, internal and external braces have been added to strengthen them. Buildings have also been anchored to their foundations to keep them from slipping off during an earthquake.

Builders in earthquake zones now use metal straps or braces to help strengthen a building's (especially a tall building's) resistance to earthquakes. They use materials and designs that can absorb or withstand ground vibrations. They also put layers of rubber and steel underneath new buildings to lessen the effect of an earthquake on the building itself.

Bridges can also be constructed to withstand earthquakes. San Francisco's Golden Gate Bridge, for instance, has a flexible structure that allows it to sway but not break during most earthquakes. Cantilever bridges (type of bridge made of two sections that extend outward from banks or piers and join together in the middle), like the San Francisco-Oakland Bay Bridge, in contrast, are less flexible and more prone to collapse during earthquakes.

Personal safety

An earthquake is unpredictable and terrifying, but people caught in one can follow a few steps to help reduce personal injury. Safety experts recommend the following:

• First and foremost, do not panic. The shaking of the ground is not harmful; falling objects are.
• If you are indoors when an earthquake hits, stay there. Crawl beneath and hang onto a heavy desk or table. If that is not possible, move into a hallway or crouch against an inside wall. Stay away from windows, fireplaces, and heavy standing objects. Get out of the kitchen—stoves and heavy appliances make it a dangerous place. Do not run downstairs or rush outside while the house or building is shaking.
• If you are outside when an earthquake hits, move quickly into an open area away from buildings, power lines, walls, or other structures that might fall.
• If you are driving when an earthquake hits, move the car out of traffic as quickly and carefully as possible and stop. Do not park on or under a bridge or overpass, or under trees, light posts, power lines, signs, or anything else that might fall. Stay inside the car until the tremors stop.

• If in a mountainous area, move into a clearing or open area. Be mindful of landslides, falling rocks, or other loose debris that could come rolling down a slope.

After the tremors of an earthquake subside, remember to do the following:

• Check the utilities. If the smell of gas is present in the air, turn off the main gas valve and open the windows, if possible. Do not light matches or lighters. Do not turn on electrical equipment or appliances. A spark from any of these may cause an explosion and fire. Leave the house or building and report the gas leak.
• Use the telephone only to report an emergency. If there is an emergency and the telephone lines are down, send someone for help.
• Stay out of damaged buildings. Aftershocks (tremors that occur after the main shock of an earthquake) can topple already weakened structures.
• Advance preparation for an earthquake, like for any disaster, can greatly reduce the chances of injury or death. Develop an earthquake plan at home or at school that explains the safest course of action should an earthquake strike. Keep on hand an emergency kit that includes a flashlight, a battery-powered radio, extra batteries, first-aid materials, a fire extinguisher, canned or packaged food (with a manual can opener), plastic containers filled with drinking water (allow at least one gallon or three liters per person per day), warm clothes, matches, candles, and a camp stove or barbecue with extra fuel (stored carefully).

[See AlsoTsunami ]

For More Information

BOOKS

Bolt, Bruce A. Earthquakes. 3rd ed. Salt Lake City, UT: W. H. Freeman, 1993.

Bronson, William. The Earth Shook, the Sky Burned. San Francisco, CA: Chronicle Books, 1959; reprinted, 1997.

Keller, Edward A., and Nicholas Pinter. Active Tectonics: Earthquakes, Uplift, and Landscape. Upper Saddle River, NJ: Prentice Hall, 1996.

Newton, David E. Earthquakes. New York: Franklin Watts, 1993.

Ritchie, David. The Encyclopedia of Earthquakes and Volcanoes. New York: Facts on File, 1994.

PERIODICALS

"Assessing Quake's Damage." New York Times (March 2, 2001): p. A21.

Earthquakes

WHY EARTHQUAKES OCCUR

Earthquake conditions

The planet Earth is believed to consist of a thin crust 2–3 mi thick under the oceans and as much as 25 mi thick beneath the continents that covers the large, solid sphere of the rock mantle, which descends to about 1,800 mi. Below the mantle is the fluid outer core, and, at about 3,200 mi depth, the apparently solid inner core. The province of earthquakes recorded thus far is from the crust to a maximum depth of about 450 mi.

Conditions thought to prevail in this hot, dark, high-pressure land cannot be simulated in existing laboratories—at the base of the mantle, pressure is about 11,000 tons per square inch and temperature is 10,000°F. These diamond-smashing pressures produce a rigidity in mantle rock about four times that of ordinary steel, with an average density about that of titanium.

This very solid mantle rock seems to behave, over periods of millions of years, like a very sluggish fluid. Something, perhaps the temperature difference between the white-hot region near the core and the cooler region near the crust, drives slow-moving cycles of rising and descending currents in the mantle rock itself.

Evidently, these currents rise beneath the thin-crusted ocean floor, thrust up the mid-ocean ridges, and generate the stresses that produce their spine-like transverse cracks and shallow earthquakes. This is believed to be the force that causes material to well up through the crust, replacing and spreading the old sea floor, and pushing drifting continents apart.

Where the currents begin their descent at the edges of continents, they produce compressive pressures, and massive folding in the form of trenches and mountain ranges. These regions are the sites of the deeper earthquakes, and of most volcanism.

Earth stresses and strains and then releases

Stresses generated in the crust and upper mantle by convective currents are stored in the form of strain—physical deformation of the rock structure. Under normal circumstances, the "solid" rocks deform plastically, releasing pent-up energy before it builds to catastrophic levels. But, when stresses accumulate too rapidly to be removed by plastic flow, some structural compensation is necessary. Large blocks of material are slowly forced into highly strained positions along faults, and held in place by a supporting structure of stronger materials. These energy-absorbing zones of weakness continue to shift, like longbows being pulled to the breaking point. Finally, more stress causes the supporting rocks to rupture, triggering the "cocked" fracture back toward equilibrium. The sides of the rebounding fault move horizontally with respect to one another (strike-slip), vertically (dip-slip), or in combinations of such motion, as in the large-scale tilting that accompanied the Alaska earthquake in March 1964.

Foreshocks and aftershocks

Sometimes all the energy to be released goes out in one large wrench, followed by trains of smaller tremors, or aftershocks, produced by continuing collapse and slippage along the fracture. Sometimes the fault shift is preceded by the small structural failures we detect as foreshocks. The magnitude 5.9 earthquake that shook Fairbanks, Alaska, on June 21, 1967, was preceded by a magnitude 5.6 foreshock, followed by a magnitude 5.5 aftershock, and then, over the next 24 hours, by more than 2,000 smaller aftershocks. Small tremors were detected for days after the initial event. However, all small tremors or earthquake "swarms" do not necessarily indicate that a big one is on the way. The Matsushiro, Japan, swarm maintained an intermittent tremble for more than a year, probably doing more psychic than physical damage. Of more than 600,000 tremors recorded between August 3, 1965, and the end of 1966, 60,000 were strong enough to be felt, and 400 were damaging. During the most active period, in April and May 1966, Matsushiro felt hundreds of tremors daily, all under magnitude 5.

Whatever the time period involved, the energy of strain flows out through the shifted fault in the form of heat, sound, and earthquake waves.

How earthquake waves travel

There are four basic seismic waves: two preliminary "body" waves that travel through the earth, and two that travel only at the surface. Combinations, reflections, and diffractions produce a virtual infinity of other types. The behavior of these are well-enough understood that wave speed and amplitude have been the major means of describing Earth 's interior. In addition, a large earthquake generates elastic waves that echo through the planet like vibrations in a ringing bell, which actually cause the planet to expand and contract infinitesimally.

The primary (P) wave is longitudinal, like a sound wave, propagates through both liquids and solids, and is usually the first signal that an earthquake has occurred. Where the disturbance is near enough or large enough to be felt, the P wave arrives at the surface like a hammer blow from the inside. This is the swiftest seismic wave, its speed varying with the material through which it passes. In the heterogeneous crustal structure, P-wave velocity is usually less than 4 mi per second—nearly 15,000 mph. Just below the crust, at a layer called the Mohorovicic discontinuity (the Moho), these speeds jump to 5 mi per second and subsequently increase to about 8.5 mi per second (more than 30,000 mph) through the core.

As the compressional phase of the P wave passes through the earth, particles are pushed together and displaced away from the disturbance. The rare factional phase dilates the particles and displaces them toward the earthquake source. For an object embedded in the ground, the result is a series of sharp pushes and pulls parallel to the wave path—motions similar to those that passengers feel when a long train gets under way.

The secondary (S) wave is transverse, like a light or radio wave, and travels about half as fast as the primary wave. Because S waves require a rigid medium to travel in coherent rays, their apparent absence below the mantle gives credence to the theory of a fluid core. About twice the period and amplitude of the associated P waves, these shear waves displace particles at right angles to the direction of wave travel. The vertical component of this movement is somewhat dampened by the opposing force of gravity; but side-to-side shaking in the horizontal can be quite destructive. Where the motion is perceptible, the arrival of the S waves marks the beginning of a new series of shocks, often worse than the P-wave tremor.

Surface waves, named for their discoverers, Love and Rayleigh, are of much greater length and period, e.g., 30 seconds or more, versus less than one second for P waves. Love waves are shear in the horizontal dimension, and the Rayleigh wave induces a retrograde, elliptical motion, something like that in wind-driven ocean waves. The speed of the Love wave is about 2.5 mi per second; the Rayleigh

wave is about 10% slower. Despite the large proportion of earthquake energy represented by these waves, their long period smoothes out the motion they impart, reducing their destructiveness.

Wave motion is not considered in describing the travel of seismic waves through the earth. Instead, the P and S body waves, and their large family of reflected, combined, or resonated offspring, are treated as rays. If the planet were homogeneous, like a ball of wax, these rays would be straight lines. But in the heterogeneous Earth, the rays describe concavely spherical paths away from the earthquake source, and from points of reflection at the surface.

Since they travel at different speeds, seismic waves arrive at a given point on the Earth's surface at different times. Near the source, the ground will shake over a slightly longer interval of time than it took the fault to slip. At great distances, the same energy released by a single event may be detected instrumentally for days.

MEASURING AN EARTHQUAKE

Intensity is an indication of an earthquake's apparent severity at a specified location, as determined by experienced observers. Through interviews with persons in the stricken area, damage surveys, and studies of Earth movement, an earthquake's regional effects can be systematically described. For seismologists and emergency workers, intensity becomes an efficient shorthand for describing what an earthquake has done to a given area.

The Modified Mercalli Intensity Scale generally used in the United States grades observed effects into 12 classes ranging from I, felt only under especially favorable circumstances, to XII, damage total. The older Rossi Forel Intensity Scale (RF) has 10 categories of observed effects, and is still used in Europe. Still other intensity scales are in use in Japan and the former Soviet Union.

Rating earthquakes by intensity has the disadvantage of being always relative. In recent years, an "objective" scale of earthquake magnitude has supplemented intensity ratings. Magnitude expresses the amount of energy released by an earthquake as determined by measuring the amplitudes produced on standardized recording instruments. The persistent misconception that the "Richter scale" rates the size of earthquakes on a "scale of 10" is extremely misleading, and has tended to mask the clear distinction between magnitude and intensity.

Earthquake magnitudes are similar to stellar magnitudes in that they describe the subject in absolute, not relative, terms, and that they refer to a logarithmic, not an arithmetic, scale. An earthquake of magnitude 8, for example, represents seismograph amplitudes 10 times larger than those of a magnitude 7 earthquake, 100 times larger than those of a magnitude 6 earthquake, and so on. There is no highest or lowest value, and it is possible here, as with temperature, to record negative values. The largest earthquakes of record were rated at magnitude 8.9; the smallest, about minus 3. Preliminary magnitude determinations may vary with the observatory, equipment, and methods of estimating—the Alaska earthquake of March 1964, for example, was described variously as magnitude 8.4, 8.5, and 8.6 by different stations.

Magnitude also provides an indication of earthquake energy release, which intensity does not. In terms of ergs, (in the centimeter-gram-second system, an erg is the unit of work equal to a force of 1 dyne acting through a distance of 1 cm (0.39 in); a dyne is the force required to accelerate a freestanding gram mass 1 cm/second) a magnitude 1 earthquake releases about one billionth the energy of a magnitude 7 earthquake; a magnitude 5, about one thousandth that of a magnitude 7, etc.

UNDERSTANDING EARTHQUAKES & WHY THEY ARE ALWAYS A SURPRISE

Most natural hazards can be detected before they strike. However seisms (from the Greek seismos, earthquake) have no known precursors, and so they come without warning. For this reason, they continue to kill, in some areas, at a level usually reserved for wars and epidemics.

Natural hazards worldwide, such as those caused by storms, earthquakes, volcanoes, and tsunamis cause $1 billion in damages daily. The years 1995–1998 have seen more destructive tsunamis than any other time period since the beginning of the twentieth century. Recent tsunamis alone have caused approximately $2 billion in damage, over 1,820 deaths, 1,500 serious injuries (not including the New Guinea Tsunami), and left more than 135,000 homeless. In addition, there have been at least 540,000 fatalities in 209 tsunamis from the year 684 to 1998.

It is estimated that there are 500,000 detectable earthquakes in the world each year—100,000 of those can be felt and 100 of them cause damage. Worldwide, 1,741,127 people have been killed in earthquakes during the twentieth century.

WHERE EARTHQUAKES OCCUR

Our planet's most active earthquake-producing feature is the circum-Pacific seismic belt, which trends along the major geologic faults and the deep oceanic trenches of island arcs decorated here and there with the volcanic "Ring of Fire." The mid-Atlantic Ridge, with its fish-skeleton figure of transverse cracks, is also quite active. Other major seismic belts branch from the circum-Pacific system and arc across southeastern and southern Asia into southern Europe, through the Indian Ocean up through the eastern Mediterranean, and up through southern Asia into China.

In an average year, these belts will generate several million tremors, ranging in severity from barely detectable wiggles to great earthquakes of the size that ravaged San Francisco in 1906 and tilted a third of Alaska in 1964. There is always an earthquake in progress somewhere.

ACTIVE FAULTS OF CALIFORNIA

The most earthquake-prone areas in the contiguous United States are those that are adjacent to the San Andreas fault system of coastal California and the fault system that separates the Sierra Nevada from the Great Basin. Many of the individual faults of these major systems are known to have been active during the last 150–200 years, and others are believed to have been active since the wane of the last great ice advance about 10,000 years ago. Parts of these earthquake-prone areas are among the most densely populated and rapidly urbanizing sections of the western states. A knowledge of the location of these active faults and an understanding of the nature of the earthquake activity that is related to them is necessary for people to accommodate themselves and their work to these hazards.

Earthquakes in California are relatively shallow and clearly related to movement along active faults. During historical times, at least 25 California earthquakes have been associated with movements that ruptured Earth's surface along these faults. On the San Andreas fault, eight moderate-to-severe earthquakes have been accompanied by movements on the fault at the earth's surface since 1838, and other faults in the California region have also experienced repeated earthquakes. The magnitude of shallow earthquakes can generally be correlated with the amount and length of the associated fault movement. Thus, the largest episode of fault movement (or fault slip) recorded in California accompanied the three great earthquakes of 1857, 1872, and 1906—all of which had estimated magnitudes that were over 8 on the Richter scale.

Many of the California faults have had one or more episodes of sudden slip or of slow movement, called creep, during historical time or a documented history of shallow earthquakes. For other faults, however, recent activity can only be inferred from geologic and topographic relations, which indicate that they have been active during the past several thousand years. Such activity suggests that some of these faults will, and that any of them might, slip or creep again.

In parts of California where relatively little geologic work has been done, evidence of other recently active faults will undoubtedly be found as research progresses. This is particularly true of large areas in northern California where topographic features by which recent fault movements can be recognized are commonly obscured by dense vegetation and rapid erosion. Further study may also reveal that some of the unknown faults have been recently active, and that some parts of faults thought to be active are actually dead.

Most of the active California faults are vertical or nearly vertical breaks, and movement along these breaks has been predominantly horizontal. If the block on the opposite side of the fault from the observer has moved to the right, the movement is termed right-lateral; movement of the opposite block to the left is termed left-lateral. Most of the faults trend northwesterly, and movement on these faults has been right-lateral. Notable exceptions to the predominantly northwesterly trend of faults are the west-trending Garlock and Big Pine faults; movement on these faults has been left-lateral.

A few reverse faults have also been active in California. The planes of such faults are inclined to the earth's surface, and the rocks above the fault have been thrust upward over the rocks below the fault plane. The magnitude 7.7 Arvin-Tehachapi earthquake of 1952 was associated with such movement along the White Horse reverse fault, and the magnitude 6.6 San Fernando earthquake of 1971 was caused by a sudden rupture along a reverse fault at the foot of the San Gabriel Mountains.

Studies of historical fault movement have shown that they occur in two ways. The first, and better known, is the sudden displacement, or slip, of the ground along a fault. Such displacement is accompanied by earthquakes and occasionally produces spectacular offsets of topographic and even of human-made features. During the 1906 earthquake, the ground was displaced as much as 21 ft along the San Andreas fault in northern California. During the 1857 earthquake, displacement of the ground along this fault was possibly as much as 30 ft in southern California. The second type of fault movement, termed creep, is now taking place on portions of several faults in California. This type of movement was well documented for the first time in 1956, and has since been found to be commonplace. It is characterized by continuous or intermittent slight slip without noticeable earthquakes. Recent fault creep on portions of the Hayward, Calaveras, and San Andreas faults has produced cumulative offsets ranging from a fraction of an inch to almost a foot in curbs, streets, and railroad tracks, and has caused some damage to buildings.

Most of the faults are, in reality, zones made up of a number of subsidiary faults or fault strands. These fault zones range in width from several feet to a mile or more. Slip along them during historical time and the recent geologic past has been found to recur repeatedly on only one or a few of the multiple strands that constitute these zones. Most of the strands commonly show no evidence of recent activity, although slip does at times recur on older strands or on entirely new ones. The strong tendency for fault slip and earthquakes to recur along the most recently active strands makes knowledge of the precise location of these strands essential to land-use planning.

The source of the stresses that cause the Earth's crust to break and slip in the California region is unknown, but the stresses appear to be related to crustal distortion on a global scale. Geologists have found abundant evidence that these stresses have been acting for millions of years. Whatever their source, the result is a continuing history of surface displacements and earthquakes along numerous faults in the California region.

The San Andreas fault

The most important of California's faults is the San Andreas, which is the "master fault" of the intricate network of faults that cuts through rocks of the coastal region of California. It is a fracture in the Earth's crust along which two parts of the crust have slipped with respect to each other.

The presence of the San Andreas fault was dramatically brought to the attention of the world on April 18, 1906, when displacement along the fault resulted in the great San Francisco earthquake and fire. This, however, was but one of many, many earthquakes that have resulted from displacement along the fault throughout its life of possibly 100 million years.

The fault is a huge fracture some 600 mi or more long, extending almost vertically into the earth to a depth of at least 20 mi. In detail, it is a complex zone of crushed and broken rock from a few hundred feet to a mile wide. Many smaller faults branch from and join the San Andreas fault zone, and if almost any road cut in the zone is examined, one will find a myriad of small fractures, fault gouge (pulverized rock), and a few solid pieces of rock.

Where is the San Andreas fault?

The San Andreas fault forms a continuous break from northern California southward to Cajon Pass. From Cajon Pass southeastward the identity of the fault becomes confused, because several branching faults such as the San Jac-into, Mission Creek, and Banning faults have similar characteristics. Nevertheless, the San Andreas type of faulting continues unabated southward to, and under, the Gulf of California.

Over much of its length, a linear trough reveals the presence of the fault; and from an airplane the linear arrangement of the lakes, bays, and valleys appears striking. Undoubtedly, however, many people driving near Crystal Springs Reservoir, along Tomales Bay, through Cajon or Tejon Passes, do not realize they are on the San Andreas fault zone. On the ground, the fault zone can be recognized by long straight escarpments, narrow ridges, and small-undrained ponds formed by the settling of small blocks within the fault zone. Characteristically, stream channels jog sharply along the fault trace.

Fault movement

Essentially, blocks on opposite sides of the San Andreas fault move horizontally, and if one were to stand on one side of the fault and look across it, the block on the opposite side would appear to be moved to the right. Geologists refer to this as a right-lateral strike-slip fault, or wrench fault.

During the 1906 earthquake, roads, fences, and rows of trees and bushes that crossed the fault were offset several

feet, and the road across the head of Tomales Bay was offset 21 ft, the maximum offset recorded. In each case the ground west of the fault moved relatively northward.

Geologists who have studied in detail the fault between Los Angeles and San Francisco have suggested that the total accumulated displacement along the fault may be as much as 350 mi. Similarly, geologic study of a segment of the fault between Tejon Pass and the Salton Sea revealed geologically similar terrains on opposite sides of the fault now separated by 150 mi, indicating that the separation is a result of movement along the San Andreas and branching San Gabriel faults.

It is difficult to imagine this great amount of shifting of Earth's crust; yet the rate represented by these ancient offsets seems consistent with the rate measured in historical time. Precise surveying shows a slow drift at the rate of about 2 in per year. At that rate, if the fault has been uniformly active during its possible 100 million years of existence, over 300 mi of offset is indeed a possibility.

Since 1934, earthquake activity along the San Andreas fault system has been concentrated in the areas of three cities: Eureka, San Francisco, and Los Angeles/San Bernadino. These are areas where historical earthquakes and fault displacements of the Earth's surface have been most common and where fault creep is taking place today. The sections of the state intervening the three areas mentioned above, on the other hand, have had almost no earthquakes or known slip events since the great earthquakes of 1857 in the southernmost segment and 1906 in the segment between Eureka and San Francisco. This implies to some earth scientists that these two segments of the San Andreas fault are temporarily locked, whereas in the other areas stress is being continually relieved by slip, which produces small-to-moderate earthquakes, and by creep. The lack of such activity in the locked segments could mean that these segments are subject to less frequent but larger fault movements and correspondingly more severe earthquakes.

The recorded history of earthquakes along the San Andreas fault is an extremely small sample from which, however, a clear pattern of behavior can be determined. Judging from this short history, great earthquakes seem to occur only a few times a century, but smaller earthquakes recorded only on sensitive seismographs occur much more frequently.

It is a popular misconception that once there has been a small earthquake along a segment of the fault, strain is released and further earthquakes are not to be expected for many years. Seismologists have pointed out, however, that the really great earthquakes have been preceded by numerous strong shocks and that large earthquakes seem to cluster in 10–20 year periods. Furthermore, the energy released during small earthquakes is insignificant compared to that in earthquakes having the same magnitude as the one in 1906.

Different segments of the fault also behave differently. For example, in the vicinity of Hollister, frequent small shocks are recorded, and slow movement at the rate of 0.47 in per year has been recorded. In contrast, the segment near San Francisco, except for an earthquake of magnitude 5.3 in 1957, has been relatively quiet since 1906. Perhaps, as some believe, it is gradually bending or accumulating strain that will be adjusted all at once in one large "snap."

What can be done about the fault?

Much is yet to be learned about the nature and behavior of the San Andreas fault and the earthquakes it generates. Some questions geologists would like to answer are: How old is the fault? Has movement been uniform? What movement has there been on branching faults? What is the fundamental cause of the stresses that produced the San Andreas fault? Until these questions and others have been satisfactorily answered, the question "what can be done about the fault?" is best responded to, according to the U. S. Geological Survey (USGS), in this way: "Though man cannot stop earthquakes from happening, he can learn to live with the problems they cause. Of prime importance are adequate building codes, for experience shows that well-constructed buildings greatly lessen the hazards. In construction projects, greater consideration should be given to foundation conditions. Degree of damage will range widely, between construction on bedrock, water-saturated mud, filled ground, or landslide terrain. For example, in 1906, most buildings on filled or 'made'land near the foot of Market Street in San Francisco suffered particularly intense damage, whereas buildings on solid rock suffered little or no damage. Geologists are horrified to see land developers build rows of houses straddling the trace of the 1906 break."

Maps showing the most recently active strands or breaks along the San Andreas and related active faults are being prepared by the USGS. This governmental agency can be contacted at 804 National Center, Reston, Virginia 20192. The USGS also maintains Public Inquiries Offices in San Francisco and Los Angeles.

RECENT QUAKE EVENTS

Bay Area's 1989 Earthquake Teaches Basic Lessons

There is no substitute for experience in understanding an earthquake. Most of us will never have a significant earthquake experience, but millions had the next thing to it when television was uniquely deployed in San Francisco (for other reasons) and shifted its focus to tell us first-hand how the violent shaking there felt and looked.

At 5:04 p.m., on a quiet, autumnal afternoon, October 17, 1989, the San Andreas fault upset life beyond description in the San Francisco-Oakland Bay area. It heaved its giant breast in the Santa Cruz Mountains and wrought havoc in widening circles that reached throughout the Bay Area and shook buildings as far as Reno, Nevada, 250 mi to the east, and rattled skyscrapers 400 mi south in Los Angeles. The internationally televised third game of baseball's World Series, about to begin, gave the world the word and picture as it was happening, beginning with views of apprehensive players and fans inside Candlestick Park stadium, and combining them with telephoto visuals of the fires and devastation some 8–10 mi to the north. However, virtually no one in the Bay Area needed the ABC television crew to tell them that the "big one" was happening. Fanning out in every direction from the epicenter, shock waves that reached 6.9 on the Richter scale sundered the quiet afternoon of uncounted

thousands of people in the area, bringing fear, destruction, and, in the next minutes, death as only an earthquake can. There were more than 100 fatalities and 3,000 injuries.

As earthquakes go, the intensity, maintained for a short 15 seconds, was great. But it was nowhere near the 9.2 reading of the 1964 Alaskan quake that destroyed with tidal waves as much as with earth shaking. (Each whole number on the Richter scale equals ten times greater intensity than the previous whole number.) The toll in life and property of this 1989 cataclysm (the third most lethal of all time) resulted because the epicenter was so close to very large concentrations of population and technology-laced living styles. In the cities of San Francisco, Oakland, and other edge communities, the lives of millions, densely packed into a few square miles, involves structures of all types and descriptions. It also involves the steel and concrete double-deck freeway connecting Oakland and the mainland to San Francisco's peninsula.

Structures that survived, and some that did not

The safest structures proved to be high-rise office buildings in San Francisco, constructed since a 1971 tremor had spurred new standards; most vulnerable were the restored

Earthquake Safety Rules

An earthquake strikes your area and for a minute or two the "solid" earth moves like the deck of a ship. What you do during and immediately after the tremor may make life-and-death differences for you, your family, and your neighbors. These rules will help you survive.

Before an Earthquake

At home, bolt down water heaters and gas appliances. Place large, heavy objects and fragile items on securely fastened, lower shelves; brace or anchor heavy objects.

Keep a flashlight and battery-powered transistor radio in the home, ready for use.

During an Earthquake

1. Remain calm. Think through the consequences of any action you take. Try to calm and reassure others; prepare them for the certainty of aftershocks.
2. If indoors, watch for falling plaster, bricks, light fixtures, and other objects. Watch for high bookcases, china cabinets, shelves, and other furniture that might slide or topple. Stay away from windows, mirrors, and chimneys. If in danger, get under a table, desk, or bed; in a corner away from windows; or in a strong doorway. Encourage others to follow your example. Usually it is best not to run outside.
3. If in a high-rise building, get under a desk. Do not dash for exits, since stairways may be broken and jammed with people. Power for elevators may fail.
4. If in a crowded store, do not rush for a doorway since hundreds may have the same idea. If you must leave the building, choose your exit as carefully as possible.
5. If outside, avoid high buildings, walls, power poles, and other objects which could fall. Do not run through streets. If possible, move to an open area away from all hazards. If in an automobile, stop in the safest place available, preferably an open area.

After an Earthquake

1. Check for injuries in your family and neighborhood. Do not attempt to move seriously injured persons unless they are in immediate danger of further injury.
2. Check for fires or fire hazards.
3. Wear shoes in all areas near debris or broken glass.
4. Check utility lines and appliances for damage. If gas leaks exist, shut off the main gas valve. Shut off electrical power if there is damage to your house wiring. Report damage to the appropriate utility companies and follow their instructions. Do not use matches, lighters, or open-flame appliances until you are sure no gas leaks exist. Do not operate electrical switches or appliances if gas leaks are suspected. This creates sparks that can ignite gas from broken lines.
5. Do not touch downed power lines or objects touched by the downed wires.
6. Immediately clean up spilled medicines, drugs, and other potentially harmful materials.
7. If water is off, emergency water may be obtained from water heaters, toilet tanks, melted ice cubes, and canned vegetables.
8. Check to see that sewage lines are intact before permitting continued flushing of toilets.
9. Do not eat or drink anything from open containers near shattered glass. Liquids may be strained through a clean handkerchief or cloth if danger of glass contamination exists.
10. If power is off, check your freezer and plan meals to use foods that will spoil quickly.
11. Use outdoor charcoal broilers for emergency cooking.
12. Do not use your telephone except for genuine emergency calls. Turn on your radio for damage reports and information.
13. Check your chimney over its entire length for cracks and damage, particularly in the attic and at the roofline. Unnoticed damage could lead to a fire. The initial check should be made from a distance. Approach chimneys with caution.
14. Check closets and storage shelf areas. Open closets and cupboard doors carefully and watch out for objects failing from shelves.
15. Do not spread rumors. They often do great harm after disasters.
16. Do not go sightseeing immediately, particularly in beach and waterfront areas where seismic sea waves could strike. Keep the streets clear for passage of emergency vehicles.
17. Be prepared for additional earthquake shocks called "aftershocks." Although most of these are smaller than the main shock, some may be large enough to cause additional damage.
18. Respond to requests for help from police, fire fighting, civil defense, and relief organizations, but do not go into damaged areas unless your help has been requested. Cooperate fully with public-safety officials. In some areas, you may be arrested for getting in the way of disaster operations.

There are no rules that can eliminate all earthquake danger. However, damage and injury can be greatly reduced by following these simple rules.

single homes built 60–90 years ago on landfills in an area known as the Marina district of San Francisco. The former coped with the shocks with well-planned engineering provisions, resulting in minimal damage. Meanwhile, more than 50 of the latter (wood and brick structures) collapsed into their foundations, and many lives were lost as scores were trapped inside. The most shocking element of the catastrophe was reserved, however, for the freeway. Due to reasoning that seems strange in retrospect, the lifeline traffic artery known as the Nimitz Freeway (Interstate Route 80) had been constructed years before as a double-decker, with its supports assumed to be—but never tested to be—earthquake resistant. The supports failed this test. Cars and drivers alike were crushed in mid-cruise as the upper deck first undulated with the shock wave then dropped its millions of tons of steel and concrete on the deck below. Drivers were pinned and vehicles crushed as if made of cardboard. The San Francisco-Oakland Bay bridge fared only slightly better, with one end of an upper section falling to meet the lower level, closing it for days.

As terrifying as this quake was, carnage was in one sense light because the early start of the ball game had drained the streets of much rush-hour traffic. That is small consolation to the families of those who died, but a blessing to thousands of others whose route would have placed them directly under the collapsing concrete that repudiated its supports.

Northridge, California, 1994

Early on the morning of January 17, 1994, Martin Luther King Day, Los Angeles area residents were jolted awake by what was to be the most significant urban earthquake to occur in California since 1906. The initial 10 seconds of trembling ground resulted in massive property damage and loss of life.

The powerful quake struck at its epicenter of Northridge at 4:31 a.m. on January 17, reaching a magnitude of 6.8. Residents within an area of approximately 2,192 mi² experienced over 1,000 aftershocks of a magnitude of 1.5 for weeks after the quake.

Nearly 100 deaths and 9,000 injuries resulted from the incident. Over 50,000 people were displaced from their homes; thousands of individuals were forced to move to temporary shelters in schools and churches, or camp in city parks and endure the chilly, less-than-favorable temperatures and rain.

Nearly 114,039 residential and commercial structures were damaged in the area despite the fact that seismic building code provisions and other mitigations had been intensified, especially since the Loma Prieta earthquake in 1989. Total damage was estimated at $17 billion. This most recent seismic event brought to light the difficulties in protecting Californians from their seismically unsafe environment.

The 1999 Colombia Earthquake

On January 25, 1999, at 1:19 p.m., an earthquake shook the Armenia-Calarca-Pereira area of Colombia, causing extensive damage and killing over 2,000. It was followed by a magnitude 4.5 aftershock at 5:40 p.m. that day.

This region has had many earthquakes above magnitude 5.5 since 1973, most recently a magnitude 6.8 event on June 6, 1994, which killed at least 295 and caused extensive damage in Cauca, Tolima, and Valle Departments.

The January 1999 earthquake caused extraordinary damage for a very moderate earthquake, perhaps due to a phenomenon known as soil amplification, in which thick alluvial layers resonate and amplify seismic energy over what it would be in hard rock.

The quake occurred in the center mountain range of Cordillera Central, of three north-trending mountain ranges in western Colombia. This mountain range has active volcanoes and the earthquake occurred near the Ruiz-Tolima volcanic complex. The volcanoes exist here because the Nazca tectonic plate is subducting beneath South America; most earthquakes in this zone are in the subducting plate and thus at least 62 mi beneath the surface. This earthquake, however, had a source depth of about 11 mi, and was caused by some near-surface tectonic adjustment. This earthquake was a strike slip earthquake; that is, there was essentially no vertical motion in its faulting. Its faulting is similar to what typically occurs on the San Andreas fault.

January 13, 2001, El Salvador Earthquake

A major earthquake, 7.6 magnitude, struck about 105 miles south-southwest of San Miguel, El Salvador, in the Pacific Ocean. The death toll in Central America, as of early March 2001, is approximately 1,300 people, with 8,000 injured. An aftershock occurred in the same area a month later, killing hundreds.

January 26, 2001, southern India Earthquake

A major earthquake, 7.9 magnitude, struck about 65 miles north-northeast of Jamnagar, India. Buildings collapsed in the state of Gujarat. The earthquake was felt at Mumbai (Bombay) and Delhi, as well as Karachi and Peshawar, Pakistan, and in parts of Nepal. As many as 20,000 people were killed by this earthquake. On June 16, 1819, an earthquake in this same general area killed 1,500–2,000 people.

The earthquake occurred along an approximately east-west trending thrust fault at shallow (<15.5 mi) depth. Thrust faults occur when one portion of the Earth's crust is pushed up over an adjacent portion. The strain that caused this earthquake is due to the Indian plate pushing northward into the Eurasian plate.

February 28, 2001, western Washington State Earthquake

A major earthquake of 6.8 magnitude occurred in the Seattle-Tacoma area of Washington state on February 28,2001. At least several hundred people were injured, but no deaths were directly connected to the earthquake. Damage is estimated at about $2 billion.

The location of this western Washington state earthquake is very near the locations of a 1949 magnitude 7.1 earthquake and a 1965 magnitude 6.5 earthquake. These events occurred on a normal fault within the Juan de Fuca plate where it subducts (goes under) the North America plate. The 2001 earthquake is called the "Nisqually earthquake," because of the proximity of the earthquake to the Nisqually River delta in Puget Sound. The name Nisqually is taken from a group of Native Americans who live in the area.

November 3, 2002 (UTC), Denali Fault, Alaska

This 7.9 magnitude shock is the largest earthquake on the Denali fault since at least 1912, when an 7.2 magnitude earthquake occurred in the general vicinity of the fault, 50 miles east of this latest epicenter. This 7.9 magnitude shock, one of the largest ever recorded on U.S. soil, occurred on the Denali-Totschunda fault system, which is one of the longest strike-slip fault systems in the world and rivals in size California's famed San Andreas strike-slip fault system that spawned the destructive San Francisco earthquake in 1906.

Earthquake in west central Mexico, January 21, 2003.

A major earthquake occurred in Colima, Mexico, about 30 miles southeast of Manzanillo, Colima, or about 310 miles west of Mexico City at 7:06 p.m. MST, January 21, 2003 (8:06 p.m. CST in Mexico). A preliminary magnitude of 7.8 was computed for this earthquake. The magnitude and location may be revised when additional data and further analysis results are available. There were at least 28 deaths, 300 injured, and considerable damage in the states of Colima, Michoacan, and Jalisco. The earthquake was felt strongly in Mexico City.

This shallow earthquake occurred in a seismically active zone near the coast of central Mexico. The earthquake occurred near the juncture of three tectonic plates: the North American Plate to the northeast, the Rivera Plate to the northwest, and the Cocos Plate to the south. Both the Rivera Plate and the Cocos Plate are being consumed beneath the North American Plate. The slower moving Rivera Plate is moving northwest at about 2 cm per year relative to the North American Plate, and the faster moving Cocos plate is moving in a similar direction at a rate of about 4.5 cm per year.

There have been several significant earthquakes near the recent event. In 1932, a magnitude 8.4 thrust earthquake struck about 100 km to the north-northwest. More recently, on October 9, 1995, a magnitude 8.0 earthquake struck about 50 km to the northwest killing at least 49 people and leaving 1,000 homeless. The most deadly earthquake in the region occurred about 170 km to the southeast on September 19, 1985. This magnitude 8.0 earthquake killed at least 9,500 people, injured about 30,000, and left 100,000 people homeless.

TSUNAMIS

What is a tsunami?

The phenomenon we call "tsunami" is a series of traveling ocean waves of great length and long period, generated by disturbances associated with earthquakes in oceanic and coastal regions. As the tsunami crosses the deep ocean, its length from crest to crest may be 100 mi or more, its height from trough to crest only a few feet. It cannot be felt aboard ships in deep water, and cannot be seen from the air. But in deep water, tsunami waves may reach forward speeds exceeding 600 mph.

As the tsunami enters the shoaling water of coastlines in its path, the velocity of its waves diminishes and wave height increases. It is in these shallow waters that tsunamis become a threat to life and property, for they can crest to heights of more than 100 ft, and strike with devastating force.

The tsunami of the century: Papua New Guinea, 1998

On the evening of Friday, July 17, at 7:30 p.m., a massive tsunami swept across the sandbar that forms the outer margin of Sissano Lagoon, West Sepik, Papua New Guinea, striking four villages west of the town of Aitape. The wave was reported to be 22.8–33 ft high; up to 3,000 persons were reported killed or missing. This was an unusually damaging tsunami, given the size of the earthquake (a magnitude 7) associated with it.

As of late 1998, scientists were continuing to examine this event, in an attempt to explain the unusually high runups, with the ultimate hope of mitigating such disasters in the future.

Scientists from the USGS participated in the second International Tsunami Survey Team to study the sedimentary deposits left by this tsunami. Animations of the tsunami have also been developed to graphically display how the tsunami evolved from an earthquake source.

Since 1992, the international community has responded to nine major tsunami disasters (Nicaragua, 1992; Flores, 1992; Okushiri, 1993; East Java, 1994; Mindoro, 1994; Kuril Islands, Russia, 1994; Manzanillo, 1995; Irian Jaya, Indonesia, 1996; and Peru, 1996) by dispatching this team of scientists, which has come to be known as the International Tsunami Survey Team (ITST), with more than 30 scientists and 20 students from Indonesia, Korea, Japan, Mexico, Peru, Russia, the United Kingdom, and the United States. The Papua New Guinea survey team was joined by scientists from Australia and New Zealand.

Largest Quakes in the United States (with Magnitudes and Dates)

Prince William Sound, AK (9.2; March 28, 1964)

Andreanof Islands, AK (8.8; March 9, 1957)

Rat Islands, AK (8.7; February 4, 1965)

east of Shumagin Islands, AK (8.3; November 10, 1938)

Lituya Bay, AK (8.3; July 10, 1958)

Yakutat Bay, AK (8.2; September 10, 1899)

Cape Yakataga, AK (8.2; September 4, 1899)

Andreanof Islands, AK (8.0; May 7, 1986)

New Madrid, MO (7.9; February 7, 1812)

Fort Tejon, CA (7.9; January 9, 1857)

Ka'u District, HI (7.9; April 3, 1868)

Kodiak Island, AK (7.9; October 9, 1900)

Gulf of Alaska (7.9; November 30, 1987)

Denali Fault, Alaska (7.9; November 3, 2002)

Owens Valley, CA (7.8; March 26, 1872)

Largest Quakes in the Contiguous United States

New Madrid, MO (7.9; February 7, 1812)

Fort Tejon, CA (7.9; January 9, 1857)

Owens Valley, CA (7.8; March 26, 1872)

Imperial Valley, CA (7.8; February 24, 1892)

New Madrid, MO area (7.7; December 16, 1811)

San Francisco, CA (7.7; April 18, 1906)

Pleasant Valley, NV (7.7; October 3, 1915)

New Madrid, MO (7.6; January 23, 1812)

Landers, CA (7.6; June 28, 1992)

Kern County, CA (7.5; July 21, 1952)

west of Lompoc, CA (7.3; November 4, 1927)

Dixie Valley, NV (7.3; December 16, 1954)

Hebgen Lake, MT (7.3; August 18, 1959)

Borah Peak, ID (7.3; October 28, 1983)

Widely differing magnitudes have been computed for some of these earthquakes; the values differ according to the methods and data used. For example, some sources list the magnitude of the 8.7 Rat Islands earthquake as low as 7.7. On the other hand, some sources list the magnitude of the February 7, 1812, New Madrid quake as high as 8.8. Similar variations exist for most events on this list, although generally not so large as for the examples given.

In general, the magnitudes given in the list above have been determined from the seismic moment, when available. For very large quakes, the moment magnitude is considered to be a more accurate determination than the traditional amplitude magnitude computation procedures. Note that all of these values can be called "magnitudes on the Richter scale," regardless of the method used to compute them.

Source: Stover, C.W., and J.L. Coffman. Seismicity of the United States, 1968-1989. Revised. U.S. Geological Survey Prof. Paper 1527, 1993.

The tsunami warning system

Development of the National Oceanic and Atmospheric Administration (NOAA) Coast and Geodetic Survey's Pacific Tsunami Warning System was impelled by the disastrous waves of April 1946, which surprised Hawaii and took a heavy toll in life and property. The locally disastrous tsunami caused by the March 1964 Alaska earthquake impelled the development of another type of warning apparatus—the Regional Tsunami Warning System in Alaska.

The Regional Tsunami Warning System is headquartered at the Coast and Geodetic Survey's Seismological Observatory at Palmer, Alaska. This is the nerve center for an elaborate telemetry network linking Palmer with remote seismic and tidal stations along the Alaska coast and in the Aleutian Islands. Seismograph stations in the network are at Palmer Observatory and its two remote stations 25 mi south and west, and at Biorka, Sitka, Gilmore Creek, Kodiak, and Adak. Tide stations are at Seward, Sitka, Kodiak, Cold Bay, Unalaska, Adak, Yakutat, and Shemya. Data from these stations are recorded continuously at Palmer, where a 24-hour watch is kept.

When an earthquake occurs in the Alaska-Aleutian area, seismologists at Palmer Observatory rapidly determine its epicenter (the point on the Earth's surface above the underground source of the earthquake) and magnitude. If the epicenter falls in the Aleutian Island arc or near the Alaskan coastal area, and if the earthquake magnitude is great enough to generate a tsunami, Palmer Observatory issues a tsunami warning through the Alaska Disaster Office, Alaska Command, and Federal Aviation Administration (FAA) covering the area near the epicenter. A tsunami watch is issued for the rest of the Alaskan coastline, alerting the public to the possibility of a tsunami threat. If tide stations detect a tsunami, Palmer Observatory extends the tsunami warning to cover the entire coastline of Alaska. If no tsunami is observed, both the watch and warning bulletins are canceled.

Subsidiary warning centers have been established at Sitka and Adak Observatories. These facilities operate small seismic arrays and have a limited warning responsibility for local areas.

The Pacific Tsunami Warning System has its headquarters at the Coast and Geodetic Survey's Honolulu Observatory. There, seismologists monitor data received from seismic and tidal instruments in Hawaii and around the Pacific Ocean, and provide ocean-wide tsunami watches and warnings. The Pacific system works very closely with its regional counterpart in Alaska. Potentially tsunami-generating earthquakes in the Alaska-Aleutian area are detected and evaluated at Palmer Observatory, and the data relayed directly to the Honolulu Observatory. Where there is tidal evidence of a tsunami, the warning is extended by Honolulu to cover the Pacific Ocean basin. For tsunamis generated elsewhere in the Pacific area, tsunami watch and warning bulletins are prepared at the Honolulu Observatory and disseminated in Alaska by the Alaska Disaster Office, the military, and Federal Aviation Administration (FAA).

FREQUENTLY ASKED QUESTIONS ABOUT EARTHQUAKES

What is the biggest earthquake ever?

Since 1900, the earthquake in Chile on May 22, 1960, is the biggest in the world with magnitude 9.5 Mw.

What is the biggest earthquake in the United States?

Since 1900, the earthquake in Alaska on March 28, 1964, is the biggest earthquake in the United States, with magnitude 9.2 Mw. This earthquake is also the second biggest earthquake in the world.

Tsunami Safety Rules

Tsunamis are generated by some earthquakes. When you hear a tsunami warning, you must assume a dangerous wave is on its way. History shows that when the great waves finally strike, they claim those who have ignored the warning.

REMEMBER:

1. Not all earthquakes cause tsunamis, but many do. When you hear that an earthquake has occurred, stand by for a tsunami emergency.
2. A strong earthquake felt in a low-lying coastal area is a natural warning of possible, immediate danger. Keep calm and move to higher ground, away from the coast.
3. A tsunami is not a single wave, but a series of waves. Stay out of danger areas until an "all-clear" is issued by competent authority.
4. Approaching tsunamis are sometimes heralded by a noticeable rise or fall of coastal water. This is nature's tsunami warning and should be heeded.
5. A small tsunami at one beach can be a giant a few miles away. Do not let the modest size of one make you lose respect for all.
6. All tsunamis—like hurricanes—are potentially dangerous, even though they may not damage every coastline they strike.
7. Never go down to the beach to watch for a tsunami. When you can see the wave you are too close to escape it.
8. During a tsunami emergency, your local Civil Defense, police, and other emergency organizations will try to save your life. Give them your fullest cooperation.

Stay tuned to your radio or television stations during a tsunami emergency—bulletins issued through Civil Defense and NOAA offices can help save your life.

Which states in the United States have the most earthquakes?

Alaska and California.

Which state has the most damaging earthquakes?

California.

Which states have the smallest number of earthquakes?

Florida and North Dakota.

What region has the fewest earthquakes?

Antarctica has the fewest earthquakes of any continent, but small earthquakes can occur anywhere in the world.

When will California slide into the ocean?

There is no scientific reason that indicates that California will ever fall into the ocean.

What is the difference between magnitude and intensity?

Magnitude measures the energy released at the source of the earthquake. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded on a seismogram at a certain period. Intensity measures the strength of shaking produced by the earthquake at a certain location. Intensity is determined from effects on people, human structures, and the natural environment. Intensity does not have a mathematical basis, but is based on observed effects.

Where can I buy a Richter scale?

The Richter scale is not a physical device, but a mathematical formula. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded on a seismogram at a certain period.

What is an aftershock?

Smaller earthquakes following the largest earthquake of a series, concentrated in a restricted crustal column.

How long can an earthquake shake?

Two to three minutes.

What is a fault?

A fracture or zone of fractures in rock along which the two sides have been displaced relative to each other parallel to the fracture.

What is liquefaction of soil?

The process of soil and sand behaving like dense fluid rather than a wet solid during an earthquake.

What is the Moho?

The Moho is the abbreviated form of Mohorovicic (pronounced Mo-ho-ro-vish-ich) discontinuity. This is a boundary surface or the sharp seismic velocity discontinuity that separates the Earth's crust from the underlying mantle. Its depth varies from about 3–6 mi beneath the ocean floor to about 20 mi below the continents. The discontinuity probably represents a change in chemical composition. It is named after its Croatian discoverer Andrija Mohorovicic.

When did the first instrument actually record an earthquake?

Probably the earliest seismoscope was invented by the Chinese philosopher Chang Heng in a.d. 132. This was a large urn on the outside of which were eight dragon heads facing the eight principal directions of the compass. Below each dragon head was a toad with its mouth opened toward the dragon. When an earthquake occurred, one or more of the eight dragon-mouths would release a ball into the open mouth of the toad sitting below. The direction of the shaking determined which of the dragons released its ball. The instrument is reported to have detected an earthquake 400 mi away that was not felt at the location of the seismoscope. The inside of the seismoscope is unknown: most speculations assume that the motion of some kind of pendulum would activate the dragons.

Where do earthquakes occur?

Earthquakes can strike any location at any time. But history shows they occur in the same general patterns year after year, principally in three large zones of the earth.

The world's largest earthquake belt, the circum-Pacific seismic belt, is found along the rim of the Pacific Ocean, where about 81% of the world's largest earthquakes occur. The belt extends from Chile, northward along the South American coast through Central America, Mexico, the West Coast of the United States, and the southern part of Alaska, through the Aleutian Islands to Japan, the Philippine Islands, New Guinea, the islands groups of the southwest Pacific, and to New Zealand. This earthquake belt was responsible for 70,000 deaths in Peru in May 1970, and 65 deaths and one billion dollars of damage in California in February 1971.

Why do so many earthquakes originate in this belt?

This is a region of young, growing mountains and deep ocean trenches that invariably parallel mountain chains. Earthquakes necessarily accompany elevation changes in mountains, the higher part of the Earth's crust, and changes in the ocean trenches, the lower part.

The second important belt, the Alpide, extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. This belt accounts for about 17% of the world's largest earthquakes, including some of the most destructive, such as the Iran shock that took 11,000 lives in August 1968, and the Turkey tremors in March 1970 and May 1971 that each killed over 1,000. All were near magnitude 7 on the Richter scale.

The third prominent belt follows the submerged mid-Atlantic Ridge. The remaining shocks are scattered in various areas of the world.

Earthquakes in these prominent seismic zones are taken for granted, but damaging shocks occur occasionally outside these areas. Examples in the United States are New Madrid, Missouri, and Charleston, South Carolina. Many years, however, usually elapse between such destructive shocks.

Can earthquakes be predicted?

It is not possible for scientist to predict earthquakes now and it may never be possible. Some people believe that animals and psychics can predict earthquakes, but that has not been proven.

PROTECTING THE PUBLIC FROM EARTHQUAKE HAZARDS—ADVANCED NATIONAL SEISMIC SYSTEM COMES TO MEMPHIS

October 2002 marked a new milestone in the installation of modern seismic stations in seismically active urban areas across the country. These cities include Memphis, San Francisco, Seattle, Salt Lake City, Anchorage, and Reno. These new instruments are part of a nationwide network of sophisticated ground shaking measurement systems, both on the ground and in buildings, called the Advanced National Seismic System (ANSS). ANSS will become the first line of defense in the war on earthquake hazards—with the ultimate victory being public safety, lives saved, and major losses to the economy avoided.

ANSS stations will assist emergency responders within minutes of an event showing not only the magnitude and epicenter, but where damage is most likely to have occurred.

Ten new ANSS instruments have recently been installed in the Memphis area, 20 have been installed across the mid-America region, and more than 175 have been installed in other vulnerable urban areas to provide real-time information on how the ground responds when a strong earthquake happens.

The ultimate goal of ANSS is to save lives and ensure public safety, said Dr. John Filson, U. S. Geological Survey (USGS) Earthquake Program Coordinator. "This information, already available in Southern California, is generated by data from seismic instruments installed in urban areas and has revolutionized the response time of emergency managers to an earthquake, but its success depends on further deployment of instruments in other vulnerable cities."

In 1997, during the reauthorization of the National Earthquake Hazards Reduction Program, Congress asked for an assessment of the status and needs of earthquake monitoring. The result was the authorization of ANSS to be implemented by the USGS. The system, when implemented, would integrate all regional and national networks with 7,000 new seismic instruments, including 6,000 strong-motion sensors in 26 at-risk urban areas. To date, approximately 350 instruments have been installed.

Earthquakes pose one of the greatest risks for casualties and costly damage in the United States. California's Northridge earthquake in 1994, a magnitude 6.7 quake, took 57 lives when it struck a modern urban environment generally designed for seismic resistance. With losses estimated at $20 billion, this was the most expensive earthquake in U.S. history. During the 1989 World Series, as more than 62,000 fans filled Candlestick Park, a magnitude 7.1 earthquake struck about 60 miles south of San Francisco. The effects of the 20-second quake caused as much as $10 billion in damage. Sixty-two people died.

In March 1964, a magnitude 9.2 earthquake near Anchorage took 125 lives and caused about $311 million in property losses. Thirty blocks of dwellings and commercial buildings were damaged or destroyed in the downtown area of Anchorage. Landslides caused heavy damage, and an area of 130 acres broke the ground into blocks that were collapsed and tilted at all angles.

In 1811 and 1812, the central Mississippi Valley was struck by three of the most powerful earthquakes in U.S. history. Consider what the impact would be if these events happened today in this region that has more earthquakes than any area east of the Rocky Mountains.

The goal of USGS earthquake monitoring is to mitigate risk—using better instruments to understand the damage that shaking causes and to help engineers create stronger and sounder structures that ensure vital infrastructures, and keep utility, water, and communication networks operating safely and efficiently.

The ANSS "strong motion" instruments are critical in giving emergency response personnel real-time maps of severe ground shaking and providing engineers with information about building and site response.

ANSS provides the USGS with the capability to create tools to process earthquake information faster; for example, Shake Map, a rapidly generated computer map that shows the location, severity, and extent of strong ground shaking within minutes after an earthquake. As it modernizes seismic networks, the USGS hopes to be able to provide the ANSS-generated Shake Map capability for every seismically active urban area. A possibility USGS scientists have been keenly aware of throughout the development of ANSS is that an early warning of even a few seconds would give children enough time to get under their desks; could stop trains and subways; shut off pipelines; shut down nuclear facilities; and suspend medical procedures. Another new tool is the "Did you feel it?" website (http://pasadena.wr.usgs.gov/shake/). This allows citizens with internet access to record their observations of shaking. The result is a community intensity map (coded by zip code) across the region.

Earthquake

Causes of earthquakes

Seismic waves

Collapse of buildings

Earthquake-triggered landslide

Liquefaction of soil

Subsidence

Tsunamis

Secondary hazards: fire, disease, famine

Historical incidence of earthquakes

Resources

An earthquake is the shaking or vibration of Earth’s surface as the result of sudden movement along a fault, the movement of molten rock within the Earth, or human activities. The terms temblor and seism are often used as synonyms for earthquake. The location of an earthquake source within the Earth is known as its focus, and the point on the Earth’s surface directly above the focus is known as the epicenter.

Earthquakes are common events. The United States Geological Survey estimates that more than three million earthquakes occur on Earth each year, which is equivalent to more than 9, 000 earthquakes per day. Virtually all of these are too small to be noticed by humans and many occur in remote areas far from seismometers. Since 1900, there has been on average about 1 magnitude 8 earthquake, 18 magnitude 7.0 to 7.9 earthquakes, 120 magnitude 6.0 to 6.9 earthquakes, and 800 magnitude 5.0 to 5.9 earthquakes on Earth each year.

Earthquakes can range in severity from small events that are imperceptible to humans to devastating shocks that level cities and kill thousands. The world’s most destructive earthquake, which occurred in China during the year 1556, killed 830, 000 people. Twenty other earthquakes in Europe, Asia, and the Middle East are known to have resulted in more than 50, 000 deaths each. The most devastating earthquake to strike the United States was the 1906 San Francisco earthquake, which killed about 3, 000 people as a result of shaking and resulting fires. Modern engineering and construction methods have significantly reduced the danger posed by earthquakes in developed countries.

In the United States, for example, only five earthquakes since 1950 have killed more than 60 people. The great Alaskan earthquake of 1964, the second largest earthquake ever recorded by seismologists, killed only 15 people. An additional 110 perished, however, in earthquake triggered tsunamis that struck coastal Alaska, Oregon, and California. Most of the fatal earthquakes occurring in the United States since 1950 have killed only one or two people, and the vast majority of earthquakes do not kill anyone.

The size of an earthquake is described by its magnitude, which reflects the amount of energy released by the temblor. There are many different ways of calculating earthquake magnitude, the most famous of which was proposed in the 1930s by the American seismologist Charles Richter (1900–1985). The Richter magnitude is the base 10 logarithm of the largest seismic wave amplitude recorded on a particular kind of seismograph located 62 mi (100 km) from the earthquake epicenter. Adjustments must be made if other kinds of seismographs are used or if they are located at a different distance from the epicenter. An earthquake of a given magnitude will produce waves 10 times as large as those from an earthquake of the next smaller magnitude. The energy released increases by a factor of about 30 from one magnitude to the next. The Richter scale is open-ended, meaning that it has no mathematical upper or lower limits. In reality, however, there are no faults on Earth large enough to produce a magnitude 10 earthquake. The two largest recorded earthquakes were the magnitude 9.5 Chilean earthquake of 1956 and the magnitude 9.2 Prince William Sound, Alaska, earthquake of 1964.

The effects of an earthquake are measured by its intensity. Unlike magnitude, earthquake intensity varies from place to place. The most common measure of intensity is the modified Mercalli scale, which ranges from an intensity of I (not felt except by a few people under especially favorable circumstances) to XII (total destruction, with objects thrown in the air and lines of sight distorted). Surveys and interviews after a large earthquake can be used to create an iso-seismic map, which shows the distribution of reported earthquake intensities. Most isoseismic maps show a distorted bull’s eye pattern of concentric rings of equal intensity area centered around the epicenter.

Causes of earthquakes

Tectonic plate movements

Some earthquakes occur in areas where the tectonic plates comprising Earth’s lithosphere move horizontally past each other along large faults or zones of faults. Examples of this type include earthquakes along the San Andreas and Hayward faults in California. Earthquakes also occur in places where a continental plate subducts an oceanic plate, for example along the western coast of South America, the northwest coast of North America (including Alaska), and in Japan. If two continental plates collide but neither is subducted, as in Europe and Asia from Spain to Vietnam, earthquakes occur as the rocks are lifted to form mountain ranges.

In other parts of the world, for example the Basin and Range physiographic province of the western United States and the East African Rift, continental plates are being stretched apart by tectonic forces. The result is that some parts of the Earth’s crust are lifted to form mountain ranges while neighboring blocks subside to form basins that collect sediment eroded from the mountains. Earthquakes can occur when movement occurs along faults developed as a result of the stretching.

Faults are planes of weakness, across which rock has moved in opposite directions, within the Earth’s crust. They can range in size from continental scale features such as the San Andreas fault in California to small features across which only a few millimeters or centimeters of movement has occurred. Tectonic plate motions increase the level of stress within Earth’s crust, which is accommodated as elastic strain energy, until the stress exceeds the strength of the fault. Then, the energy is suddenly released as the rocks on each side of the fault slip past each other to create an earthquake. This process is analogous to a rubber band snapping when it was been stretched to the breaking point. Because there is a frictional resistance to movement along faults, rapid seismic slip can generate enough heat to melt the adjacent rocks and form a glassy rock known as pseudotachylyte. In other cases, the elastic strain energy is slowly and quietly dissipated through a process known as aseismic creep.

Magma movement

Rhythmic earthquakes known as harmonic tremors, which are caused by magma and volcanic gas moving through conduits in the Earth’s crust just as air moves through a pipe organ, can foreshadow or accompany volcanic eruptions. Recent studies have also suggested that very large earthquakes, such as the magnitude 9.0 earthquake that affected the west coast of the United States in 1700, may trigger volcanic activity for several decades after their occurrence as the Earth’s crust slowly adjusts to the initial movement. Seismologists can also use earthquake activity to infer the presence of magma that has not yet erupted and formed a volcano. Swarms of small earthquakes near Socorro, New Mexico, for example have helped scientists to locate a mass of molten rock about 12 mi (20 km) beneath the Earth’s surface. Detailed measurements have shown that the surface is being lifted by about 0.08 in (2 mm) per year in that area, but there are no obvious signs that a pool of molten rock lies beneath the surface.

Human activity

Explosions, especially from underground nuclear bomb testing, can produce small earthquakes. Earthquakes caused by explosions produce vibrations different than those caused by movement along faults, and seismic monitoring is an important part of nuclear test ban treaty verification. The implosive demolition of the Kingdome, a sports stadium in Seattle, in the year 2000 produced a magnitude 2.3 earthquake. Seismologists were able to deploy seismometers before the demolition and use the manmade earthquake to learn more about the geology of the area by studying how seismic waves were reflected and refracted beneath Earth’s surface. Another well-known example of earthquakes due to human activity occurred at the Rocky Mountain Arsenal near Denver, Colorado, during the 1960s. The pressure of hazardous waste being injected deep into the Earth through disposal wells was large enough to trigger a series of earthquakes. A subsequent experiment in an oilfield near Rangely, Colorado, showed that earthquakes could be triggered at will be injecting water under pressure.

Seismic waves

Rapid slip along a fault generates waves in much the same way as does a pebble falling into a pool of water, and waves moving outward from an earthquake focus are reflected and refracted each time they encounter a different rock type. There are four different kinds of seismic waves, two of which are known as body waves and two of which are known as surface waves. Body waves travel deep through the Earth, whereas surface waves travel along the Earth’s surface and generally cause the most damage.

The two types of body waves are P-waves and S-waves. P-waves, also known as primary waves, travel the fastest of the four types. They move by alternately compressing and stretching the rock through which they pass. P-wave velocity depends on the rock type and density, but it is generally about 4 mi/s (6 km/s). S-waves, also known as secondary waves, move by shearing or moving from side to side the rock through which they pass. S-waves move more slowly than P-waves and, depending on the type of rock, have a velocity of about 2 mi/s (3 km/s).

The two types of surface waves are known as Rayleigh and Love waves. They travel more slowly than either P- or S-waves, but often cause more damage than body waves because they travel along the Earth’s surface and have a greater effect on buildings.

Seismologists can determine the epicenter of an earthquake by noting the times that seismic waves arrive at three or more different seismometers. Multiplication of the wave velocity by the travel time gives the distance to the epicenter, which is the radius of a circle with its center at the seismometer. The radii from at least three circles will intersect at a point that is Earthquake epicenter. In practice, seismologists first make a rough estimate of the epicenter and then refine their estimate as additional data become available, for example by using velocities corresponding to specific rock types rather than a general estimate.

Collapse of buildings

To construct a house or building under static conditions, the materials need only to be stacked up, attached to each other, and balanced. These kinds of buildings are not designed to accelerate rapidly and change directions like cars or airplanes. Buildings in seismically active areas, however, must be designed and built to withstand the dynamic acceleration that can occur during an earthquake. Large buildings and structures such as bridges, in particular, must be designed so that vibrations arising from earthquakes are damped and not amplified.

Because noticeable earthquakes are rare in most areas, people may not recognize that the objects and buildings around them represent potential hazards. It is not movement of the ground surface alone that kills people. Instead, deaths from earthquakes result from the collapse of buildings and falling objects in them, fires, and tsunamis. The type of construction that causes the most fatal injuries in earthquakes is unrein-forced brick, stone, or concrete buildings that tend not to be flexible and to collapse when shaken.

The most earthquake-resistant type of home is a low wooden structure that is anchored to its foundation and sheathed with thick plywood. Some of the traditional architecture of Japan approximates this shock-resistant design, including wooden buildings that are more than a thousand years old. Unfortunately, wood and paper houses can be easily ignited in the fires that are common after large earthquakes. Both unreinforced masonry and shock-resistant wood houses are used by different cultures in areas of high earthquake risk.

Active faults lie under many parts of the world that do not commonly experience earthquakes. The crust under such places as Italy, California, and Central America moves often enough that an earthquake there, although still unpredictable, is not entirely unexpected. But other populated areas, such as the East Coast and Mississippi Valley in the United States, periodically experience earthquakes just as big as those in any earthquake-prone part of the world, although far less frequently.

Earthquake-triggered landslide

Earthquakes can trigger landslides and rock falls many miles (kilometers) from their epicenters. Local governments can enact zoning regulations to prevent development in areas susceptible to landslides during earthquakes or heavy rainstorms. In other cases, potentially hazardous slopes can be excavated and regarded into a configuration that is able to resist the destabilizing effects of a large earthquake.

Seismically-triggered landslides can reshape the landscape. In 1959, an earthquake triggered a landslide that dammed the Madison River in Montana and created Hebgen Lake. To prevent this natural dam from washing out and causing catastrophic floods, the U.S. Army Corps of Engineers built an emergency spillway through the landslide material. This enabled them to control the release of the water from the new lake. Prehistoric landslides have dammed the Columbia River and could be the source of a legend of the Northwest Indians. In this legend, tribes walked across the Columbia River on a bridge of land to meet each other.

Liquefaction of soil

Seismic shaking can transform water-saturated sand into a liquid mass that will not support heavy loads such as buildings. This phenomenon, called liquefaction, causes much of the destruction associated with some earthquakes. Mexico City, for example, rests on the ancient lakebed of Lake Texcoco, which is a large basin filled with liquefiable sand and ground water. In the Mexico City earthquake of 1985, the wet sand beneath tall buildings liquefied and most of the 10, 000 people who died were in buildings that collapsed as their foundations sank into liquefied sand.

Jets of sand sometimes erupt from the ground during an earthquake. These sand geysers or mud volcanoes occur when formations of soft, wet sand is liquefied and forcefully squeezed up through cracks in the ground. Despite these names, they have no relation to real geysers or volcanoes. Although they generally cause little damage, they are indications that more widespread liquefaction may have occurred or may be possible in the next earthquake.

Subsidence

Earthquakes can cause affected areas to increase or decrease in elevation by several feet, which can in turn lead to flooding in coastal areas. Port Royal, on the south shore of Jamaica, subsided several feet in an earthquake in 1692 and suddenly disappeared as the sea rushed into the new depression. Eyewitnesses recounted the seismic destruction of the infamous pirate anchorage as follows: “… in the space of three minutes, Port-Royall, the fairest town of all the English plantations, exceeding of its riches, … was shaken and shattered to pieces, sunk into and covered, for the greater part by the sea. The earth heaved and swelled like the rolling billows, and in many places Earth crack’d, open’d and shut, with a motion quick and fast. in some of these people were swallowed up, in others they were caught by the middle, and pressed to death. The whole was attended with. the noise of falling mountains at a distance, while the sky. was turned dull and reddish, like a glowing oven.” Ships arriving later in the day found a small shattered remnant of the city that was still above the water. Charts of the Jamaican coast soon appeared printed with the words “Port Royall Sunk.” During the New Madrid (Missouri) earthquake of 1811, a large area of land subsided around the bed of the Mississippi River in west Tennessee and Kentucky. The Mississippi was observed to flow backwards as it filled the new depression and created what is now known as Reelfoot Lake. The last great earthquake in the U.S. Pacific Northwest occurred two years before Port Royal sank in 1690. In the 300 years since then, no major earthquake has released the potential energy that has been building under the crust. Geologists have found buried forests and deposits indicating that coastal areas were periodically flooded, probably as the result of major earthquakes.

Tsunamis

An earthquake can create a large wave known as a tsunami (the Japanese term) or, seismic sea wave. A tsunami is barely detectable as it moves through deep water. Where the ocean becomes shallow near the shore, however, the fast-moving tsunami becomes a large wave that rises out of the sea and strikes the shore with unstoppable force. In a small, mountain-ringed bay, a tsunami can rush hundreds of meters up a sea-facing mountainside. A wall of water forms when a large tsunami enters a shallow bay or estuary, and it can move upriver for many miles. Sometimes tsunamis are mistakenly referred to as tidal waves, because they resemble a tide-related wave called a tidal bore.

The most destructive tsunamis in history have killed tens of thousands of people, many of them located great distances from Earthquake epicenter. The tsunami produced by a 1946 earthquake in the Aleutian Islands, Alaska, killed a total of 165 people. Of that number, 159 were in Hawaii, 5 were in Alaska, and 1 was in California. Coastal towns affected by tsunamis often have no topographic barriers between them and the sea and had no warning of the impending disaster. Building a breakwater to divert a tsunami and expend its energy is sometimes an option for otherwise unprotected coastal towns.

In 2004, a 9.4 magnitude earthquake under the Indian Ocean caused large tsunamis to go ashore in India, Indonesia, and elsewhere in the region. Over a quarter of million people were killed. Most could have

KEY TERMS

Active fault— A fault where movement has been known to occur in recent geologic time.

Aftershock— A subsequent earthquake (usually smaller in magnitude) following a powerful earthquake that originates at or near the same place.

Epicenter— The location where the seismic waves of an earthquake first appear on the surface, usually almost directly above the focus.

Fault— A fracture in Earth’s crust accompanied by a displacement of one side relative to the other.

Focus— The location of the seismic event deep within Earth’s crust that causes an earthquake. Also called Earthquake’s hypocenter.

Foreshock— A small earthquake or tremor that precedes a larger earthquake shock.

Modified Mercalli scale— A scale used to evaluate earthquake intensity based on effects felt and observed by people during Earthquake.

Richter scale— A scale used to compare earthquakes based on the energy released by Earthquake.

Seismic wave— A disturbance produced by compression or distortion on or within Earth, which propagates through Earth materials; a seismic wave may be produced by natural (e.g., earthquakes) or artificial (e.g., explosions) means. P waves, S waves, and surface waves are vibrations in rock and soil that transfer the force of Earthquake from the focus into the surrounding area.

Subsidence— A sinking or lowering of Earth’s surface.

been saved by retreating to higher ground if they had had warning, or understood that withdrawal of water to an unusual. In response, a regional warning system has been installed. As of late 2006, its effectiveness had yet to be tested after a major earthquake.

Secondary hazards: fire, disease, famine

Cities depend on networks of lifeline structures to distribute water, power, and food and to remove sewage and waste. These networks, whether power lines, water mains, or roads, are easily damaged by earthquakes. Elevated freeways collapse readily, as demonstrated by a section of the San Francisco Bay Bridge in 1989 and the National Highway Number 2 in Kobe, Japan, in 1995. The combination of several networks breaking down at once multiplies the hazard to lives and property. Live power lines fall into water from broken water mains, creating an electric shock hazard. Fires may start at ruptured gas mains or chemical storage tanks, but many areas may not be accessible to fire trucks and other emergency vehicles. Even if areas are accessible, there may not be water for fire-fighting. The great fire that swept San Francisco in 1906 could not be stopped by regular firefighting methods and entire blocks of buildings had to be demolished to halt the fire. Most of the 143, 000 people killed in Tokyo and Yokohama because of the 1923 Kwanto perished in fires.

Famine and epidemic disease can quickly strike large displaced populations deprived of their usual food distribution system, sanitation services, and clean water. Furthermore, collapsed hospitals may be of no use to a stricken community that urgently needs medical services. After an earthquake, relief operations commonly offer inoculation against infectious diseases. In countries that do not have sufficient organization, trained personnel, or resources to handle an earthquake-generated refugee population, more people may die of secondary causes than the direct effects of seismic shaking. Even in the most prepared countries, the disruption of networks may prevent relief operations from working as planned. In the aftermath of the January 1995, earthquake in Kobe, Japan, plans for emergency relief made before the disaster did not work as well as planned. Local residents, wary of the danger of aftershocks, had to live outdoors in winter without food, water, or power.

Historical incidence of earthquakes

Catastrophic earthquakes happened just as often in the past as they do today. Earthquakes shattered stone-walled cities in the ancient world, sometimes hastening the demise of civilizations. Knossos, Chattusas, and Mycenae, ancient capitals of countries located in tectonically active mountain ranges, fell to pieces and were eventually deserted. Scribes have documented earthquakes in the chronicles of ancient realms. An earthquake is recorded in the Book of Zachariah, and the apostle Paul wrote that he got out of jail when the building fell apart around him in an earthquake. In the years before international news services, few people heard about distant earthquakes. Only a few handwritten accounts have survived, giving us limited knowledge of earthquakes in antiquity. Because of limited and lost data, earthquakes may seem to have been less common in ancient times. In China, home of the first seismometer, the Imperial government has recorded earthquakes for over a thousand years. Their frequency has not changed through the ages.

See also Continental drift; Mass wasting; Plate tectonics.

Resources

BOOKS

Hough, Susan Elizabeth. Earthshaking Science: What We Know (and Don’t Know) about Earthquakes. Princeton, NJ: Princeton University Press, 2004.

Hough, Susan Elizabeth. Richter’s Scale: Measure of an Earthquake, Measure of a Man. Princeton, NJ: Princeton University Press, 2006.

PERIODICALS

Cochran, Elizabeth S., et al. “Earth Tides Can Trigger Shallow Thrust Fault Earthquakes.” Nature. 306 (2004): 1164-1166.

Hirn, Alfred and Mireille Laigle. “Silent Heralds of Megathrust Earthquakes?” Nature. 305 (2004): 1917-1918.

OTHER

Spall, Henry. “NEIC: An Interview with Charles F. Richter.” July 8, 2002. <http://neic.usgs.gov/neis/seismology/people/int_richter.html> (November 8, 2002).

U.S. Geological Survey. “Earthquake Hazards Program.” 2006. <http://earthquake.usgs.gov/> (accessed October 25, 2006).

Bill Hanneberg

Earthquake

An earthquake is the shaking or vibration of Earth's surface as the result of sudden movement along a fault , the movement of molten rock within the Earth , or human activities. The terms temblor and seism are often used as synonyms for earthquake. The location of an earthquake source within the Earth is known as its focus, and the point on the Earth's surface directly above the focus is known as the epicenter.

Earthquakes are common events. The United States Geological Survey estimates that more than three million earthquakes occur on Earth each year, which is equivalent to more than 9,000 earthquakes per day. Virtually all of these are too small to be noticed by humans and many occur in remote areas far from seismometers. Since

1900, there has been on average about 1 magnitude 8 earthquake, 18 magnitude 7.0 to 7.9 earthquakes, 120 magnitude 6.0 to 6.9 earthquakes, and 800 magnitude 5.0 to 5.9 earthquakes on Earth each year.

Earthquakes can range in severity from small events that are imperceptible to humans to devastating shocks that level cities and kill thousands. The world's most destructive earthquake, which occurred in China during the year 1556, killed 830,000 people. Twenty other earthquakes in Europe , Asia , and the Middle East are known to have resulted in more than 50,000 deaths each. The most devastating earthquake to strike the United States was the 1906 San Francisco earthquake, which killed about 3,000 people as a result of shaking and resulting fires. Modern engineering and construction methods have significantly reduced the danger posed by earthquakes in developed countries. In the United States, for example, only five earthquakes since 1950 have killed more than 60 people. The great Alaskan earthquake of 1964, the second largest earthquake ever recorded by seismologists, killed only 15 people. An additional 110 perished, however, in earthquake triggered tsunamis that struck coastal Alaska, Oregon, and California. Most of the fatal earthquakes occurring in the United States since 1950 have killed only one or two people, and the vast majority of earthquakes do not kill anyone.

The size of an earthquake is described by its magnitude, which reflects the amount of energy released by the temblor. There are many different ways of calculating earthquake magnitude, the most famous of which was proposed in the 1930s by the American seismologist Charles Richter (1900–1985). The Richter magnitude is the base 10 logarithm of the largest seismic wave amplitude recorded on a particular kind of seismograph located 62 mi (100 km) from the earthquake epicenter. Adjustments must be made if other kinds of seismographs are used or if they are located at a different distance from the epicenter. An earthquake of a given magnitude will produce waves 10 times as large as those from an earthquake of the next smaller magnitude. The energy released increases by a factor of about 30 from one magnitude to the next. The Richter scale is open-ended, meaning that it has no mathematical upper or lower limits. In reality, however, there are no faults on Earth large enough to produce a magnitude 10 earthquake. The two largest recorded earthquakes were the magnitude 9.5 Chilean earthquake of 1956 and the magnitude 9.2 Prince William Sound, Alaska, earthquake of 1964.

The effects of an earthquake are measured by its intensity. Unlike magnitude, earthquake intensity varies from place to place. The most common measure of intensity is the modified Mercalli scale, which ranges from an intensity of I (not felt except by a few people under especially favorable circumstances) to XII (total destruction, with objects thrown in the air and lines of sight distorted). Surveys and interviews after a large earthquake can be used to create an isoseismic map , which shows the distribution of reported earthquake intensities. Most isoseismic maps show a distorted bull's eye pattern of concentric rings of equal intensity area centered around the epicenter.

Causes of earthquakes

Tectonic plate movements

Some earthquakes occur in areas where the tectonic plates comprising Earth's lithosphere move horizontally past each other along large faults or zones of faults. Examples of this type include earthquakes along the San Andreas and Hayward faults in California. Earthquakes also occur in places where a continental plate subducts an oceanic plate, for example along the western coast of South America , the northwest coast of North America (including Alaska), and in Japan. If two continental plates collide but neither is subducted, as in Europe and Asia from Spain to Vietnam, earthquakes occur as the rocks are lifted to form mountain ranges.

In other parts of the world, for example the Basin and Range physiographic province of the western United States and the East African Rift, continental plates are being stretched apart by tectonic forces. The result is that some parts of the Earth's crust are lifted to form mountain ranges while neighboring blocks subside to form basins that collect sediment eroded from the mountains . Earthquakes can occur when movement occurs along faults developed as a result of the stretching.

Faults are planes of weakness, across which rock has moved in opposite directions, within the Earth's crust. They can range in size from continental scale features such as the San Andreas fault in California to small features across which only a few millimeters or centimeters of movement has occurred. Tectonic plate motions increase the level of stress within Earth's crust, which is accommodated as elastic strain energy, until the stress exceeds the strength of the fault. Then, the energy is suddenly released as the rocks on each side of the fault slip past each other to create an earthquake. This process is analogous to a rubber band snapping when it was been stretched to the breaking point. Because there is a frictional resistance to movement along faults, rapid seismic slip can generate enough heat to melt the adjacent rocks and form a glassy rock known as pseudotachylyte. In other cases, the elastic strain energy is slowly and quietly dissipated through a process known as aseismic creep.

Magma movement

Rhythmic earthquakes known as harmonic tremors, which are caused by magma and volcanic gas moving through conduits in the Earth's crust just as air moves through a pipe organ , can foreshadow or accompany volcanic eruptions. Recent studies have also suggested that very large earthquakes, such as the magnitude 9.0 earthquake that affected the west coast of the United States in 1700, may trigger volcanic activity for several decades after their occurrence as the Earth's crust slowly adjusts to the initial movement. Seismologists can also use earthquake activity to infer the presence of magma that has not yet erupted and formed a volcano . Swarms of small earthquakes near Socorro, New Mexico, for example have helped scientists to locate a mass of molten rock about 12 mi (20 km) beneath the Earth's surface. Detailed measurements have shown that the surface is being lifted by about 2 mm per year in that area, but there are no obvious signs that a pool of molten rock lies beneath the surface.

Human activity

Explosions, especially from underground nuclear bomb testing, can produce small earthquakes. Earthquakes caused by explosions produce vibrations different than those caused by movement along faults, and seismic monitoring is an important part of nuclear test ban treaty verification. The implosive demolition of the Kingdome, a sports stadium in Seattle, in the year 2000 produced a magnitude 2.3 earthquake. Seismologists were able to deploy seismometers before the demolition and use the manmade earthquake to learn more about the geology of the area by studying how seismic waves were reflected and refracted beneath Earth's surface. Another well-known example of earthquakes due to human activity occurred at the Rocky Mountain Arsenal near Denver, Colorado, during the 1960s. The pressure of hazardous waste being injected deep into the Earth through disposal wells was large enough to trigger a series of earthquakes. A subsequent experiment in an oilfield near Rangely, Colorado, showed that earthquakes could be triggered at will be injecting water under pressure.

Seismic waves

Rapid slip along a fault generates waves in much the same way as does a pebble falling into a pool of water, and waves moving outward from an earthquake focus are reflected and refracted each time they encounter a different rock type. There are four different kinds of seismic waves, two of which are known as body waves and two of which are known as surface waves. Body waves travel deep through the Earth, whereas surface waves travel along the Earth's surface and generally cause the most damage.

The two types of body waves are P-waves and S-waves. P-waves, also known as primary waves, travel the fastest of the four types. They move by alternately compressing and stretching the rock through which they pass. P-wave velocity depends on the rock type and density , but it is generally about 6 km/s (4 mi/s). S-waves, also known as secondary waves, move by shearing or moving from side to side the rock through which they pass. S-waves move more slowly than P-waves and, depending on the type of rock, have a velocity of about 2 mi/s (3 km/s).

The two types of surface waves are known as Rayleigh and Love waves. They travel more slowly than either P- or S-waves, but often cause more damage than body waves because they travel along the Earth's surface and have a greater effect on buildings.

Seismologists can determine the epicenter of an earthquake by noting the times that seismic waves arrive at three or more different seismometers. Multiplication of the wave velocity by the travel time gives the distance to the epicenter, which is the radius of a circle with its center at the seismometer. The radii from at least three circles will intersect at a point that is the earthquake epicenter. In practice, seismologists first make a rough estimate of the epicenter and then refine their estimate as additional data become available, for example by using velocities corresponding to specific rock types rather than a general estimate.

Collapse of buildings

To construct a house or building under static conditions, the materials need only to be stacked up, attached to each other, and balanced. These kinds of buildings are not designed to accelerate rapidly and change directions like cars or airplanes. Buildings in seismically active areas, however, must be designed and built to withstand the dynamic acceleration that can occur during an earthquake. Large buildings and structures such as bridges , in particular, must be designed so that vibrations arising from earthquakes are damped and not amplified.

Because noticeable earthquakes are rare in most areas, people may not recognize that the objects and buildings around them represent potential hazards. It is not movement of the ground surface alone that kills people. Instead, deaths from earthquakes result from the collapse of buildings and falling objects in them, fires, and tsunamis. The type of construction that causes the most fatal injuries in earthquakes is unreinforced brick , stone, or concrete buildings that tend not to be flexible and to collapse when shaken.

The most earthquake-resistant type of home is a low wooden structure that is anchored to its foundation and sheathed with thick plywood. Some of the traditional architecture of Japan approximates this shock-resistant design, including wooden buildings that are more than a thousand years old. Unfortunately, wood and paper houses can be easily ignited in the fires that are common after large earthquakes. Both unreinforced masonry and shock-resistant wood houses are used by different cultures in areas of high earthquake risk.

Active faults lie under many parts of the world that do not commonly experience earthquakes. The crust under such places as Italy, California, and Central America moves often enough that an earthquake there, although still unpredictable, is not entirely unexpected. But other populated areas, such as the East Coast and Mississippi Valley in the United States, periodically experience earthquakes just as big as those in any earthquake-prone part of the world, although far less frequently.

Earthquake-triggered landslide

Earthquakes can trigger landslides and rock falls many kilometers from their epicenters. Local governments can enact zoning regulations to prevent development in areas susceptible to landslides during earthquakes or heavy rainstorms. In other cases, potentially hazardous slopes can be excavated and regarded into a configuration that is able to resist the destabilizing effects of a large earthquake.

Seismically-triggered landslides can reshape the landscape. In 1959, an earthquake triggered a landslide that dammed the Madison River in Montana and created Hebgen Lake. To prevent this natural dam from washing out and causing catastrophic floods, the U.S. Army Corps of Engineers built an emergency spillway through the landslide material. This enabled them to control the release of the water from the new lake . Prehistoric landslides have dammed the Columbia River and could be the source of a legend of the Northwest Indians. In this legend, tribes walked across the Columbia River on a bridge of land to meet each other.

Liquefaction of soil

Seismic shaking can transform water-saturated sand into a liquid mass that will not support heavy loads such as buildings. This phenomenon, called liquefaction, causes much of the destruction associated with some earthquakes. Mexico City, for example, rests on the ancient lakebed of Lake Texcoco, which is a large basin filled with liquefiable sand and ground water. In the Mexico City earthquake of 1985, the wet sand beneath tall buildings liquefied and most of the 10,000 people who died were in buildings that collapsed as their foundations sank into liquefied sand.

Jets of sand sometimes erupt from the ground during an earthquake. These sand geysers or mud volcanoes occur when formations of soft, wet sand is liquefied and forcefully squeezed up through cracks in the ground. Despite these names, they have no relation to real geysers or volcanoes. Although they generally cause little damage, they are indications that more widespread liquefaction may have occurred or may be possible in the next earthquake.

Subsidence

Earthquakes can cause affected areas to increase or decrease in elevation by several feet, which can in turn lead to flooding in coastal areas. Port Royal, on the south shore of Jamaica, subsided several feet in an earthquake in 1692 and suddenly disappeared as the sea rushed into the new depression. Eyewitnesses recounted the seismic destruction of the infamous pirate anchorage as follows: "... in the space of three minutes, Port-Royall, the fairest town of all the English plantations, exceeding of its riches,... was shaken and shattered to pieces, sunk into and covered, for the greater part by the sea.... The earth heaved and swelled like the rolling billows, and in many places the earth crack'd, open'd and shut, with a motion quick and fast... in some of these people were swallowed up, in others they were caught by the middle, and pressed to death.... The whole was attended with... the noise of falling mountains at a distance, while the sky... was turned dull and reddish, like a glowing oven." Ships arriving later in the day found a small shattered remnant of the city that was still above the water. Charts of the Jamaican coast soon appeared printed with the words "Port Royall Sunk." During the New Madrid (Missouri) earthquake of 1811, a large area of land subsided around the bed of the Mississippi River in west Tennessee and Kentucky. The Mississippi was observed to flow backwards as it filled the new depression and created what is now known as Reelfoot Lake. The last great earthquake in the U.S. Pacific Northwest occurred two years before Port Royal sank in 1690. In the 300 years since then, no major earthquake has released the potential energy that has been building under the crust. Geologists have found buried forests and deposits indicating that coastal areas were periodically flooded, probably as the result of major earthquakes.

Tsunamis

An earthquake can create a large wave known as a tsunami (the Japanese term) or, seismic sea wave. A tsunami is barely detectable as it moves through deep water. Where the ocean becomes shallow near the shore, however, the fast-moving tsunami becomes a large wave that rises out of the sea and strikes the shore with unstoppable force . In a small, mountain-ringed bay, a tsunami can rush hundreds of meters up a sea-facing mountainside. A wall of water forms when a large tsunami enters a shallow bay or estuary, and it can move upriver for many miles. Sometimes tsunamis are mistakenly referred to as tidal waves, because they resemble a tide-related wave called a tidal bore.

The most destructive tsunamis in history have killed tens of thousands of people, many of them located great distances from the earthquake epicenter. The tsunami produced by a 1946 earthquake in the Aleutian Islands, Alaska, killed a total of 165 people. Of that number, 159 were in Hawaii, 5 were in Alaska, and 1 was in California. Coastal towns affected by tsunamis often have no topographic barriers between them and the sea and had no warning of the impending disaster. Building a breakwater to divert a tsunami and expend its energy is sometimes an option for otherwise unprotected coastal towns.

Secondary hazards: fire, disease, famine

Cities depend on networks of lifeline structures to distribute water, power, and food and to remove sewage and waste. These networks, whether power lines, water mains, or roads, are easily damaged by earthquakes. Elevated freeways collapse readily, as demonstrated by a section of the San Francisco Bay Bridge in 1989 and the National Highway Number 2 in Kobe, Japan, in 1995. The combination of several networks breaking down at once multiplies the hazard to lives and property. Live power lines fall into water from broken water mains, creating an electric shock hazard. Fires may start at ruptured gas mains or chemical storage tanks, but many areas may not be accessible to fire trucks and other emergency vehicles. Even if areas are accessible, there may not be water for fire-fighting. The great fire that swept San Francisco in 1906 could not be stopped by regular firefighting methods and entire blocks of buildings had to be demolished to halt the fire. Most of the 143,000 people killed in Tokyo and Yokohama because of the 1923 Kwanto perished in fires.

Famine and epidemic disease can quickly strike large displaced populations deprived of their usual food distribution system, sanitation services, and clean water. Furthermore, collapsed hospitals may be of no use to a stricken community that urgently needs medical services. After an earthquake, relief operations commonly offer inoculation against infectious diseases. In countries that do not have sufficient organization, trained personnel, or resources to handle an earthquake-generated refugee population, more people may die of secondary causes than the direct effects of seismic shaking. Even in the most prepared countries, the disruption of networks may prevent relief operations from working as planned. In the aftermath of the January, 1995, earthquake in Kobe, Japan, plans for emergency relief made before the disaster did not work as well as planned. Local residents, wary of the danger of aftershocks, had to live outdoors in winter without food, water, or power.

Historical incidence of earthquakes

Catastrophic earthquakes happened just as often in the past as they do today. Earthquakes shattered stonewalled cities in the ancient world, sometimes hastening the demise of civilizations. Knossos, Chattusas, and Mycenae, ancient capitals of countries located in tectonically active mountain ranges, fell to pieces and were eventually deserted. Scribes have documented earthquakes in the chronicles of ancient realms. An earthquake is recorded in the Book of Zachariah, and the Apostle Paul wrote that he got out of jail when the building fell apart around him in an earthquake. In the years before international news services, few people heard about distant earthquakes. Only a few handwritten accounts have survived, giving us limited knowledge of earthquakes in antiquity. Because of limited and lost data, earthquakes seem to have been less common in ancient times. In China, home of the first seismometer, the Imperial government has recorded earthquakes for over a thousand years. Their frequency has not changed through the ages.

See also Continental drift; Mass wasting; Plate tectonics.

Resources

books

Reiter, L. Earthquake Hazard Analysis. New York: Columbia University Press, 1990.

periodicals

Hill, D. P., F. Pollitz, and C. Hewhall. "Earthquake-Volcano Interactions." Physics Today 55, no. 11 (November 2002): 41–47.

Sykes, L. R. "Four Decades of Progress in Seismic Identification Help Verify the CTBT." EOS, Transactions, American Geophysical Union 83, no. 44 (October 29, 2002): 497–500.

other

Spall, Henry. "NEIC: An Interview with Charles F. Richter." July 8, 2002 [cited November 8, 2002]. <http://neic.usgs.gov/neis/seismology/people/int_richter.html>.

U.S. Geological Survey. "Earthquake Image Glossary." July 29, 2002 [cited November 8, 2002]. <http://earthquake.usgs.gov/image_glossary/>.

U.S. Geological Survey. "EQ Facts and Lists." September 5, 2002 [cited November 8, 2002]. <http://earthquake.usgs.gov/bytopic/>.

U.S. Geological Survey. "USGS Earthquake Hazards Program." November 8, 2002 [cited November 8, 2002]. <http://earthquakes.usgs.gov/>.

Bill Hanneberg

KEY TERMS

Active fault

—A fault where movement has been known to occur in recent geologic time.

Aftershock

—A subsequent earthquake (usually smaller in magnitude) following a powerful earthquake that originates at or near the same place.

Epicenter

—The location where the seismic waves of an earthquake first appear on the surface, usually almost directly above the focus.

Fault

—A fracture in the earth's crust accompanied by a displacement of one side relative to the other.

Focus

—The location of the seismic event deep within the earth's crust that causes an earthquake. Also called the earthquake's hypocenter.

Foreshock

—A small earthquake or tremor that precedes a larger earthquake shock.

Modified Mercalli scale

—A scale used to evaluate earthquake intensity based on effects felt and observed by people during the earthquake.

Richter scale

—A scale used to compare earthquakes based on the energy released by the earthquake.

Seismic wave

—A disturbance produced by compression or distortion on or within the earth, which propagates through Earth materials; a seismic wave may be produced by natural (e.g., earthquakes) or artificial (e.g., explosions) means. P waves, S waves, and surface waves are vibrations in rock and soil that transfer the force of the earthquake from the focus into the surrounding area.

Subsidence

—A sinking or lowering of the earth's surface.

Earthquakes

Introduction

An earthquake is the shaking or vibration of part of Earth’s bulk as the result of sudden movement of landmasses along a fault (crack) or of molten rock within the earth. The terms “temblor” and “seism” are sometimes used as synonyms for earthquake. The location of an earthquake’s source within the earth is known as its focus, and the point on the earth’s surface directly above the focus is known as the epicenter.

Earthquakes are common events. The U.S. Geological Survey (USGS) estimates that more than three million earthquakes occur on Earth each year, which is equivalent to more than 8,000 earthquakes per day. Almost all of these are too small to be noticed by humans and many occur in remote areas. Earthquake strengths are assigned a rating called a magnitude: higher magnitudes signify stronger quakes. Since 1900, there has been on average about one earthquake of magnitude 8, 18 of magnitude 7.0 to 7.9, 120 of magnitude 6.0 to 6.9, and 800 of magnitude 5.0 to 5.9 each year. Earthquakes also occur on some other planets and moons of the solar system, especially on geologically active bodies such as Jupiter’s moon Io.

Earthquakes rarely have lasting effects on ecosystems. Tsunamis, local landslides, and other effects of a quake may kill local populations of animals or plants, but the effects tend to be temporary. Earthquakes are primarily a threat to human beings who are dependent on quake-vulnerable objects, such as bridges, huts, large buildings, pipelines, and the like. Releases of chemical or radioactive pollutants from industrial facilities could damage ecosystems as a secondary effect of an earthquake.

Historical Background and Scientific Foundations

Earthquakes can range in severity from small events that are imperceptible to humans to devastating shocks that level cities and kill thousands. The world’s most destructive earthquake to date, which occurred in China in1556, killed about 830,000 people. Twenty other earthquakes in Europe, Asia, and the Middle East are known to have resulted in more than 50,000 deaths each. The most devastating earthquake to strike the United States was the 1906 San Francisco earthquake, which killed about 3,000 people as a result of structural collapses and resulting fires.

Modern engineering and construction methods have significantly reduced the danger posed by earthquakes in developed countries. In the United States, for example, only five earthquakes since 1950 have killed more than 60 people. The great Alaskan earthquake of 1964, the second largest earthquake ever recorded by seismologists, killed only 15 people—mostly due to sparse population, however, not to quake-resistant building practices. In that quake, an additional 110 perished in earthquake-triggered tsunamis (massive ocean waves) that struck coastal Alaska, Oregon, and California. Most fatal earthquakes occurring in the United States since 1950 have killed only one or two people; the vast majority of earthquakes do not kill anyone.

The power of an earthquake is described by its magnitude, which reflects the amount of energy released. There are several different ways of calculating earthquake magnitude, the most famous of which was proposed in the 1930s by the American seismologist Charles Richter (1900–1985). The Richter magnitude of a quake is the base 10 logarithm of the largest seismic wave amplitude recorded on a particular kind of seismograph located 62 mi (100 km) from the earthquake’s epicenter. Adjustments must be made if other kinds of seismographs are used or if they are located at a different distance from the epicenter.

The logarithmic nature of Richter magnitudes means simply that an earthquake of a given magnitude (say, 6) produces seismic vibrations 10 times more intense than does an earthquake of the next smaller magnitude (say, 5). The energy released increases by a factor of about 30 from one magnitude to the next. The Richter scale is open-ended, meaning that it has no mathematical upper or lower limits. In reality, however, there are no faults on Earth large enough to produce a magnitude 11 earthquake. The two largest recorded earthquakes were the magnitude 9.5 Chilean earthquake of 1956 and the magnitude 9.2 Prince William Sound, Alaska, earthquake of 1964.

The effects of an earthquake are characterized as its intensity. Unlike magnitude, earthquake intensity varies from place to place. The most common measure of intensity is the modified Mercalli scale, which ranges from an intensity I (not felt except by a few people under especially favorable circumstances) to XII (total destruction, with objects thrown in the air and lines of sight distorted). Surveys and interviews after a large earthquake can be used to create an isoseismic map, which shows the distribution of reported earthquake intensities. Most isoseismic maps show a distorted bull’s eye pattern of concentric rings of equal intensity area centered around the epicenter.

Catastrophic earthquakes happened just as often in the past as they do today. Earthquakes shattered stonewalled cities in the ancient world, sometimes hastening the demise of civilizations. Knossos, Chattusas, and Mycenae, ancient capitals of countries located in tectonically active mountain ranges, fell to pieces and were eventually deserted. Scribes have documented earthquakes in the chronicles of ancient realms. However, in the years before international news services, few people heard about distant earthquakes. Only a few handwritten accounts of pre-modern quakes have survived, giving us limited knowledge of earthquakes in antiquity. Because of limited data, earthquakes seem to have been less common in ancient times. In China, where the first seismometers were invented, the imperial government recorded earthquakes for over a thousand years. Their frequency has not changed through the ages.

Tectonic Plate Movements

Some earthquakes occur in areas where the tectonic plates comprising Earth’s lithosphere move horizontally past each other along large faults or zones of faults. Examples of this type include earthquakes along the San Andreas and Hayward faults in California. Earthquakes also occur in places where a continental plate subducts an oceanic plate, for example along the western coast of South America, the northwest coast of North America (including Alaska), and in Japan. If two continental

WORDS TO KNOW

ACTIVE FAULT: A fault where movement has been known to occur in recent geologic time.

AFTERSHOCK: A subsequent earthquake (usually smaller in magnitude) following a powerful earthquake that originates at or near the same place.

FAULT: A fracture in the continuity of a rock formation resulting from tectonic movement.

FORESHOCK: A tremor that precedes a much larger earthquake.

MODIFIED MERCALLI SCALE: A scale used to compare earthquakes based on the effects they cause.

RICHTER SCALE: A scale used to compare earthquakes based on the energy released by the earthquake.

SEISMIC WAVE: A wave of energy that travels through the earth as the result of an earthquake or explosion.

SUBSIDENCE: A sinking or lowering of a part of Earth’s surface.

plates collide but neither is subducted, as occurs where Europe and Asia meet along the line roughly from Spain to Vietnam, earthquakes occur as the rocks are lifted to form mountain ranges.

In other parts of the world, for example the Basin and Range physiographic province (area) of the western United States and the East African Rift, continental plates are being stretched apart by tectonic forces. The result is that some parts of Earth’s crust are lifted to form mountain ranges while neighboring blocks subside to form basins that collect sediment eroded from the mountains. Earthquakes can take place when movement occurs along faults developed as a result of the stretching.

Faults are planes of weakness, across which rock has moved in opposite directions, within Earth’s crust. They can range in size from continental scale features such as the San Andreas Fault in California to small features across which only a few millimeters or centimeters of movement has occurred. Tectonic plate motions increase the level of stress within Earth’s crust, which is accommodated as elastic strain energy, until the stress exceeds the strength of the fault. Then, the energy is suddenly released as the rocks on each side of the fault slip past each other to create an earthquake. This process is analogous to a rubber band snapping when it was been stretched to the breaking point. Because there is a frictional resistance to movement along faults, rapid seismic slip can generate enough heat to melt the adjacent rocks and form a glassy rock known as pseudotachylyte. In other cases, the elastic strain energy is slowly and quietly dissipated through a process known as aseismic creep.

Magma Movement

Rhythmic earthquakes known as harmonic tremors are caused by magma and volcanic gas moving through conduits in Earth’s crust just as air moves through a pipeorgan, and can foreshadow or accompany volcanic eruptions. Recent studies have also suggested that very large earthquakes, such as the magnitude 9.0 earthquake that hit the west coast of the United States in 1700, may trigger volcanic activity for several decades after their occurrence as Earth’s crust slowly adjusts to the initial movement.

Seismologists can also use earthquake activity to infer the presence of magma that has not yet erupted and formed a volcano. Swarms of small earthquakes near Socorro, New Mexico, for example, have helped scientists to locate a mass of molten rock about 12 mi (20 km) beneath Earth’s surface. Detailed measurements have shown that the surface is being lifted by about 0.04 in (2mm) per year in that area, but there are no obvious signs that a pool of molten rock lies beneath the surface.

Human Activity

Explosions, especially from underground nuclear bomb testing, can produce small earthquakes. Earthquakes caused by explosions produce vibrations different than those caused by movement along faults, and seismic monitoring is an important part of nuclear test ban treaty verification. The implosive demolition of the Kingdome, a sports stadium in Seattle, Washington, in the year 2000 produced a magnitude 2.3 earthquake. Seismologists were able to deploy seismometers before the demolition and use the humanmade earthquake to learn more about the geology of the area by studying how seismic waves were reflected and refracted beneath Earth’s surface.

Another well-known example of earthquakes due to human activity occurred at the Rocky Mountain Arsenal near Denver, Colorado, during the 1960s. The pressure of hazardous waste being injected deep into the earth through disposal wells was large enough to trigger a series of earthquakes. A subsequent experiment in an oil field near Rangely, Colorado, showed that earthquakes could be triggered at will by injecting water under pressure.

Seismic Waves

Rapid slippage along a fault generates outward-radiating waves in much the same way as does a pebble falling into a pool of water. Waves moving outward from an earthquake’s focus are reflected and refracted each time they encounter different rock types. There are four different kinds of seismic waves, two of which are known as body waves and two of which are known as surface waves. Body waves travel deep through the earth, whereas surface waves travel along the earth’s surface and generally cause the most damage.

The two types of body waves are P-waves and S-waves. P-waves, also known as primary waves, travel the fastest of the four types. They move by alternately compressing and stretching the rock through which they pass. P-wave velocity depends on the rock type and density, but it is generally about 4 mi/s (6 km/s). S-waves, also known as secondary waves, move by shearing or moving from side to side the rock through which they pass. S-waves move more slowly than P-waves and, depending on the type of rock, have a velocity of about 2 mi/s (3 km/s).

The two types of surface waves are known as Rayleigh and Love waves. They travel more slowly than either P- or S-waves, but often cause more damage than body waves because they travel along the earth’s surface and have a greater effect on buildings.

Seismologists can determine the epicenter of an earthquake by noting the times that seismic waves arrive at three or more different seismometers. Multiplication of the wave velocity by the travel time gives the distance to the epicenter, which is the radius of a circle with its center at the seismometer. The radii from at least three circles will intersect at a point that is the earthquake epicenter. In practice, seismologists first make a rough estimate of the epicenter and then refine their estimate as additional data become available, for example by using velocities corresponding to specific rock types rather than a general estimate.

Impacts and Issues

Collapse of Buildings

Buildings are not usually designed to accelerate rapidly, like cars or airplanes, but rapid, brief acceleration is exactly what earthquakes supply. To be safe, buildings in seismically active areas must therefore be designed and built to withstand the dynamic acceleration that can occur during an earthquake. Large buildings and structures such as bridges, in particular, must be designed so that vibrations arising from earthquakes are damped and not amplified.

Because noticeable earthquakes are rare in most areas, people may not recognize that the objects and buildings around are hazards. It is not movement of the ground surface that usually kills people during earthquakes. Instead, most deaths result from the collapse of buildings and falling objects in them, fires, and tsunamis. The type of construction that causes the most fatal injuries in earthquakes is unreinforced brick, stone, or concrete buildings that tend not to be flexible and to collapse when shaken.

The most earthquake-resistant type of home is a low wooden structure that is anchored to its foundation and

sheathed with thick plywood. Some of the traditional architecture of Japan approximates this shock-resistant design, including wooden buildings that are more than a thousand years old. However, wood and paper houses can be easily ignited in the fires that are common after large earthquakes. Both unreinforced masonry and shock-resistant wood houses are used by different cultures in areas of high earthquake risk.

Active faults lie under many parts of the world that do not commonly experience earthquakes. The crust under such places as Italy, California, and Central America moves often enough that an earthquake there, although still unpredictable, is not entirely unexpected. But other populated areas, such as the East Coast and Mississippi Valley in the United States, periodically experience earthquakes just as big as those in any earthquake-prone part of the world, although far less frequently.

Earthquake-triggered Landslides

Earthquakes can trigger landslides and rock falls many miles/kilometers from their epicenters. Local governments can enact zoning regulations to prevent development in areas susceptible to landslides during earthquakes or heavy rainstorms. In other cases, potentially hazardous slopes can be excavated and regraded into a configuration that is able to resist the destabilizing effects of a large earthquake.

Seismically triggered landslides can reshape the landscape. In 1959 an earthquake triggered a landslide that dammed the Madison River in Montana and created Hebgen Lake. To prevent this natural dam from washing out and causing catastrophic floods, the U.S. Army Corps of Engineers built an emergency spillway through the landslide material. This enabled them to control the release of the water from the new lake. Prehistoric landslides have dammed the Columbia River and could be the source of a legend of the Northwest Indians. In this legend, tribes walked across the Columbia River on a bridge of land to meet each other.

Liquefaction of Soil

Seismic shaking can transform water-saturated sand into a liquid mass that will not support heavy loads such as buildings. This phenomenon, called liquefaction, causes much of the destruction associated with some earthquakes. Mexico City, for example, rests on the ancient lakebed of Lake Texcoco, which is a large basin filled with liquefiable sand and groundwater. In the Mexico City earthquake of 1985, the wet sand beneath tall buildings liquefied and most of the 10,000 people who died were in buildings that collapsed as their foundations sank into liquefied sand.

Jets of sand sometimes erupt from the ground during an earthquake. These sand geysers or mud volcanoes occur when formations of soft, wet sand is liquefied and forcefully squeezed up through cracks in the ground. Despite these names, they have no relation to real geysers or volcanoes. Although they generally cause little damage, they are indications that more widespread liquefaction may have occurred or may be possible in the next earthquake.

Subsidence

Earthquakes can cause affected areas to increase or decrease in elevation by several feet, which can in turn lead to flooding in coastal areas. Port Royal, on the south shore of Jamaica, subsided several feet in an earthquake in 1692 and suddenly disappeared as the sea rushed into the new depression. Eyewitnesses recounted the seismic destruction of the infamous pirate anchorage as follows: “In the space of three minutes, Port-Royall, the fairest town of all the English plantations, exceeding of its riches, was shaken and shattered to pieces, sunk into and covered, for the greater part, by the sea. The earth heaved and swelled like the rolling billows, and in many places the earth crack’d, open’d and shut, with a motion quick and fast. In some of these people were swallowed up, in others they were caught by the middle, and pressed to death. The whole was attended with the noise of falling mountains at a distance, while the sky was turned dull and reddish, like a glowing oven.” Ships arriving later in the day found a small shattered remnant of the city that was still above the water. Charts of the Jamaican coast soon appeared printed with the words “Port Royall Sunk.”

During the New Madrid (Missouri) earthquake of 1811, a large area of land subsided around the bed of the Mississippi River in west Tennessee and Kentucky. The Mississippi was observed to flow backward as it filled the new depression and created what is now known as Reelfoot Lake. The last great earthquake in the U.S. Pacific Northwest occurred two years before Port Royal sank in 1690. In the 300 years since then, no major earthquake has released the potential energy that has been building under the crust. Geologists have found buried forests and deposits indicating that coastal areas were periodically flooded, probably as the result of major earthquakes.

Tsunamis

An earthquake can create a large wave known as a tsunami (the Japanese term) or seismic sea wave. A tsunami is barely detectable as it moves through deep water. Where the ocean becomes shallow near the shore, however, the fast-moving tsunami becomes a large wave that rises out of the sea and strikes the shore with unstoppable force. In a small, mountain-ringed bay, a tsunami can rush hundreds of yards/meters up a sea-facing mountainside. A wall of water forms when a large tsunami enters a shallow bay or estuary, and it can move upriver for many miles. Sometimes tsunamis are mistakenly referred to as tidal waves, because they resemble a tide-related wave called a tidal bore.

The most destructive tsunamis in history have killed tens of thousands of people, many of them located great distances from the earthquake epicenter. Coastal towns affected by tsunamis often have no topographic barriers between them and the sea and usually have no warning of the impending disaster. In December 2004, a 9.2-magnitude earthquake under the Indian Ocean caused large tsunamis to go ashore in India, Indonesia, and elsewhere in the region. Over a quarter of million people were killed. Most could have been saved by retreating to higher ground if they had received a warning, or had understood that the withdrawal of water to an unusual distance from the shore was a sign of an approaching tsunami. In response, a regional warning system (the Indian Ocean Tsunami Warning System) has been installed. As of early 2008, its effectiveness had yet to be tested.

Secondary Hazards: Fire, Disease, Famine

Cities depend on networks of lifeline structures to distribute water, power, and food and to remove sewage and waste. These networks, whether power lines, water mains, or roads, are easily damaged by earthquakes. Elevated freeways collapse readily, as shown by the collapse of a section of the San Francisco Bay Bridge in 1989 and the National Highway Number 2 in Kobe, Japan, in 1995. The combination of several networks breaking down at once multiplies the hazard to lives and property. Live power lines fall into water from broken water mains, creating an electric shock hazard. Fires may start at ruptured gas mains or chemical storage tanks, but many areas may not be accessible to fire trucks and other emergency vehicles. Even if areas are accessible, there may not be water for firefighting. The great fire that swept through San Francisco in 1906 could not be stopped by regular firefighting methods, so entire blocks of buildings had to be demolished to halt the fire. Most of the 143,000 people killed in Tokyo and Yokohama because of the 1923 Kwanto earthquake perished in fires.

Famine and epidemic disease can quickly strike large, displaced populations deprived of their usual food distribution system, sanitation services, and clean water. Furthermore, collapsed hospitals may be of no use to a stricken community that urgently needs medical services. After an earthquake, relief operations commonly offer inoculation against infectious diseases. In countries that do not have sufficient organization, trained personnel, or resources to handle an earthquake-generated refugee population, more people may die of secondary causes than the direct effects of seismic shaking. Even in the most prepared countries, the disruption of networks may prevent relief operations from working as planned. In the aftermath of the January 1995 earthquake in Kobe, Japan, plans for emergency relief made before the disaster did not work as well as planned. Local residents, wary of the danger of aftershocks, had to live outdoors in winter without food, water, or power.

See Also Tsunami Impacts; Volcanoes

BIBLIOGRAPHY

Books

Hough, Susan Elizabeth. Earthshaking Science: What We Know (and Don’t Know) about Earthquakes. Princeton, NJ: Princeton University Press, 2004.

Hough, Susan Elizabeth. Richter’s Scale: Measure of an Earthquake, Measure of a Man. Princeton, NJ: Princeton University Press, 2006.

Periodicals

Cochran, Elizabeth S., et al. “Earth Tides Can Trigger Shallow Thrust Fault Earthquakes.” Nature 306 (2004): 1164–1166.

Hirn, Alfred, and Mireille Laigle. “Silent Heralds of Megathrust Earthquakes?” Nature 305 (2004): 1917–1918.

Web Sites

National Earthquake Information Center, U.S. Geological Survey. “NEIC: An Interview with Charles F. Richter.” http://neic.usgs.gov/neis/seismology/people/int_richter.html (accessed April 7, 2008).

U.S. Geological Survey. “Earthquake Hazards Program.” http://earthquake.usgs.gov/ (accessed April 7, 2008).

Bill Hanneberg