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An earthquake is a geological event inside the earth that generates strong vibrations. When the vibrations reach the surface, the earth shakes, often causing damage to natural and manmade objects, and sometimes killing and injuring people and destroying their property. Earthquakes can occur for a variety of reasons; however, the most common source of earthquakes is movement along a fault.

Some earthquakes occur when tectonic plates, large sections of Earth's crust and upper mantle, move past each other. Earthquakes along the San Andreas and Hayward faults in California occur because of this. Earthquakes also occur if one plate overruns another, as on the western coast of South America , the northwest coast of North America , and in Japan. If plates collide but neither is overrun, as they do crossing Europe and Asia from Spain to Vietnam, earthquakes result as the rocks at the abutting plates compress into high mountain ranges. In all three of these settings, earthquakes result from movement along faults.

A fault block may also move due to gravity , sinking between other fault blocks that surround and support it. Sinking fault blocks and the mountains that surround them form a distinctive topography of basins and mountain ranges. This type of fault block configuration is typified by the North American Basin and Range topographic province. In such places, elevation losses by the valleys as they sink between the mountains are accompanied by tremors or earthquakes. Another kind of mountain range rises because of an active thrust fault. Tectonic compression (tectonic, meaning having to do with the forces that deform the rocks of planets) shoves the range up the active thrust fault, which acts like a natural ramp.

Molten rock called magma moves beneath but relatively close to the earth's surface in volcanically active regions. Earthquakes sometimes accompany volcanic eruptions as huge masses of magma move underground.

Nuclear bombs exploding underground cause small local earthquakes, which can be felt by people standing within a few miles of the test site. The earthquakes caused by nuclear bombs are tiny compared to natural earthquakes; but they have a distinctive "sound," and their location can be pinpointed. This is how nuclear weapons testing in one country can be monitored by other countries around the world.

Earth is covered by a crust of solid rock, which is broken into numerous plates that move around on the surface, bumping, overrunning, and pulling away from each other. One kind of boundary between rocks within a plate, as well as at the edges of the plates, is a fault. Faults are large-scale breaks in Earth's crust, in which the rock on one side of the fault has been moved relative to the rock on the other side of the fault by tectonic forces. Fault blocks are giant pieces of crust that are separated from the rocks around them by faults.

When the forces pushing on fault blocks cannot move one block past the other, potential energy is stored up in the fault zone. This is the same potential energy that resides in a giant boulder when it is poised, motionless, at the top of a steep slope. If something happens to overcome the friction holding the boulder in place, its potential energy will convert into kinetic energy as it thunders down the slope. In the fault zone, the potential energy builds up until the friction that sticks the fault blocks together is overcome. Then, in seconds, all the potential energy built up over the years turns to kinetic energy as the rocks surge past each other.

The vibrations of a fault block on the move can be detected by delicate instruments (seismometers and seismographs) in rocks on the other side of the world. Although this happens on a grand scale, it is remarkably like pushing on a stuck window or sliding door. Friction holds the window or door tight in its tracks. After enough force is applied to over-come the friction, the window or door jerks open.

Some fault blocks are stable and no longer experience the forces that moved them in the first place. The fault blocks that face each other across an active fault, however, are still influenced by tectonic forces in the ever-moving crust. They grind past each other along the fault as they move in different directions.

Fault blocks can move in a variety of ways, and these movements define the different types of faults. In a vertical fault, one block moves upward relative to the other. At the surface of the earth, a vertical fault forms a cliff, known as a fault scarp. The sheer eastern face of the Sierra Nevada mountain range is a fault scarp. In most vertical faults, the fault scarp is not truly vertical, and one of the fault blocks "hangs" over the other. This upper block is called the hanging wall and the lower block, the foot wall.

In horizontal faults, the blocks slide past one another without either block being lifted. In this case, the objects on the two sides of the fault simply slide past one another; for example, a road that straddles the fault might be offset by a number of feet. Complex faults display movements with both vertical and horizontal displacements.

Any one of the following fault types can generate an earthquake:

  • Normal faultA vertical fault in which the hanging wall moves down compared to the foot wall.
  • Reverse faultA vertical fault in which the hanging wall moves up in elevation relative to the foot wall.
  • Thrust faultA low-angle (less than 30°) reverse fault, similar to an inclined floor or ramp. The lower fault block is the ramp itself, and the upper fault block is gradually shoved up the ramp. The "ramp" may be shallow, steep, or even curved, but the motion of the upper fault block is always in an upward direction. A thrust fault caused the January 1994 Northridge earthquake near Los Angeles, California.
  • Strike-slip (or transform) faultA fault along which one fault block moves horizontally (sideways), past another fault block, like opposing lanes of traffic. The San Andreas fault in Northern California is one of the best known of this type.

When a falling rock splashes into a motionless pool of water , waves move out from the point of impact. These waves appear at the interface of water and air as circular ripples. However, the waves occur below the surface, too, traveling down into the water in a spherical pattern. In rock, as in water, a wave-causing event makes not one wave, but a number of waves, moving out from their source, one after another, like an expanding bubble.

Tectonic forces shift bodies of rock inside the earth, perhaps displacing a mountain range several feet in a few seconds, and they generate tremendous vibrations called seismic waves. The earthquake's focus (also called the hypocenter) is the point (usually deep in the subsurface) where the sudden sliding of one rock mass along a fault releases the stored potential energy of the fault zone. The first shock wave emerges at the surface at a point typically directly above the focus; this surface point is called the epicenter. Seismometers detect seismic waves that reach the surface. Seismographs (devices that record seismic phenomena) record the times of arrival for each group of vibrations on a seismogram (either a paper document or digital data).

Like surfaces in an echoing room that reflect or absorb sound, the boundaries of rock types within the earth change or block the direction of movement of seismic waves. Waves moving out from the earthquake's focus in an ever-expanding sphere become distorted, bent, and reflected. Seismologists (geologists who study seismic phenomena) analyze the distorted patterns made by seismic waves and search through the data for clues about the earth's internal structure.

Different kinds of earthquake-generated waves, moving at their own speeds, arrive at the surface in a particular order. The successive waves that arrive at a single site are called a wave train. Seismologists compare information about wave trains that are recorded as they pass through a number of data-collecting sites after an earthquake. By comparing data from three recording stations, they can pinpoint the map location (epicenter) and depth within the earth's surface (focus or hypocenter) of the earthquake.

These are the most important types of seismic waves:

  • P-wavesThe fastest waves, these compress or stretch the rock in their path through Earth, moving at about 4 mi (6.4 km) per second.
  • S-wavesAs they move through Earth, these waves shift the rock in their path up and down and side to side, moving at about 2 mi (3.2 km) per second.
  • Rayleigh waves and Love wavesThese two types of "surface waves" are named after seismologists. Moving at less than 2 mi (3.2 km) per second, they lag behind P-waves and S-waves but cause the most damage. Rayleigh waves cause the ground surface in their path to ripple with little waves. Love waves move in a zigzag along the ground and can wrench buildings from side to side.

The relative size of earthquakes is measured by the Richter scale , which measures the energy an earthquake releases. Each whole number increase in value on the Richter scale indicates a 10-fold increase in the energy released and a

thirty-fold increase in ground motion. An earthquake measuring 8 on the Richter scale is ten times more powerful, therefore, than an earthquake with a Richter Magnitude of 7, which is ten times more powerful than an earthquake with a magnitude of 6. Another scalethe Modified Mercalli Scale uses observations of damage (like fallen chimneys) or people's assessments of effects (like mild or severe ground shaking) to describe the intensity of a quake.

Violent shaking changes water bearing sand into a liquid-like mass that will not support heavy loads, such as buildings. This phenomenon, called liquefaction, causes much of the destruction associated with an earthquake in liquefaction-prone areas. Downtown Mexico City rests on the old 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 turned to slurry, as if the buildings stood on the surface of vibrating gelatin in a huge bowl. Most of the 10,000 people who died as a result of that earthquake were in buildings that collapsed as their foundations sank into liquefied sand.

In the sudden rearrangement of fault blocks in the earth's crust that cause an earthquake, the land surface on the dropped-down side of the fault can fall or subside in elevation by several feet. On a populated coastline, this can wipe out a city. 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 seaThe 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 fastin some of these people were swallowed up, in others they were caught by the middle, and pressed to deathThe whole was attended withthe noise of falling mountains at a distance, while the skywas 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.

In 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, to create what is now known as Reelfoot Lake.

Cities depend on networks of so-called "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 a deadly electric shock hazard. Fires may start at ruptured gas mains or chemical storage tanks. Although emergency services are needed more than ever, many areas may not be accessible to fire trucks and other emergency vehicles. If the water mains are broken, there will be no pressure at the fire hydrants, and the firefighters' hoses are useless. The great fire that swept San Francisco in 1906 could not be stopped by regular firefighting methods. Only dynamiting entire blocks of buildings halted the fire's progress. Both Tokyo and Yokohama burned after the Kwanto earthquake struck Japan in 1923, and 143,000 people died, mostly in the fire.

Popular doomsayers excite uncomprehending fear by saying that earthquakes happen more frequently now than in earlier times. It is true that more people than ever are at risk from earthquakes, but this is because the world's population grows larger every year, and more people are living in earthquake-prone areas.

Today, sensitive seismometers "hear" every noteworthy earth-shaking event, recording it on a seismogram. Seismometers detect earthquake activity around the world, and data from all these instruments are available on the Internet within minutes of the earthquake. News agencies can report the event the same day. People have ready access to information about every earthquake that happens anywhere on Earth. And the earth experiences a lot of earthquakesthe planet never ceases to vibrate with tectonic forces, although the majority of them are not strong enough to be detected except with instruments. Earth has been resounding with earthquakes for more than 4 billion years. Earthquakes are a way of knowing that the planet beneath us is still experiencing normal operating conditions, full of heat and kinetic energy.

Ultrasensitive instruments placed across faults at the surface can measure the slow, almost imperceptible movement of fault blocks, which tell of great potential energy stored at the fault boundary. In some areas, foreshocks (small earthquakes that precede a larger event) may help seismologists predict the larger event. In other areas, where seismologists believe seismic activity should be occurring but is not, this seismic gap may be used instead to predict an inevitable large-scale earthquake.

Other instruments measure additional fault-zone phenomena that seem to be related to earthquakes. The rate at which radon gas issues from rocks near faults has been observed to change before an earthquake. The properties of the rocks themselves (such as their ability to conduct electricity ) have been observed to change, as the tectonic force exerted on them slowly alters the rocks of the fault zone between earthquakes. Peculiar animal behavior has been reported before many earthquakes, and research into this phenomenon is a legitimate area of scientific inquiry, even though no definite answers have been found.

Techniques of studying earthquakes from space are also being explored. Scientists have found that ground displacements cause waves in the air that travel into the ionosphere and disturb electron densities. By using the network of satellites and ground stations that are part of the global positioning system (GPS ), data about the ionosphere that is already being collected by these satellites can be used to understand the energy releases from earthquakes, which may help in their prediction.

Scientists have presumed that tides do not have any influence on or direct relationship to earthquakes. New studies show that tides may sometimes trigger earthquakes on faults where strain has been accumulating; tidal pull during new or full moons has been discounted by studies of over 13,000 earthquakes of which only 95 occurred during these episodes of tidal stress. Attention is also being directed toward the types of rock underlying areas of earthquake activity to see if rock types dampen (lessen the effects) or magnify earthquake motions.

Seismologists must make a hard choice when their data interpretations suggest an earthquake is about to happen. If they fail to warn people of danger they strongly suspect is imminent, many might die needlessly. But, if people are evacuated from a potentially dangerous area and no earthquake occurs, the public will lose confidence in such warnings and might not heed them the next time.

As more is discovered about how and why earthquakes occur, that knowledge can be used to prevent the conditions that allow earthquakes to cause harm. The most effective way to minimize the hazards of earthquakes is to build new buildings or retrofit old ones to withstand the short, high-speed acceleration of earthquake shocks.

See also Convergent plate boundary; Earth, interior structure; Faults and fractures; Mid-plate earthquakes; Plate tectonics; Tsunami

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The comedian Earthquake, in conversation with Kevin Aldridge of the Cincinnati Enquirer, described his style of comedy as "up front, straight to the point, and without limitations." One of the quickest wits in the business, Earthquake rarely prepared material in advance. He could be outrageous at times, but shock was never the point of his routines. Instead, he has specialized in sharp one-liners and brutally honest observations on various topics, often drawn on his own experiences. After several years of success in urban comedy venues, Earthquake began to break into the wider worlds of television and film in the early 2000s.

Born in 1963, Earthquake was a native of Washington, D.C. His birth name was Nathaniel Stroman, and his longtime friends still call him Nate. He took the stage name Earthquake because it was easier to say than his real name. Earthquake grew up poor in a tough southeast Washington neighborhood and often didn't have enough to eat. "You can't have dreams when you're hungry," he pointed out to Aldridge. "That's why when I was in school I used to get F's in my first four classes before lunch." Earthquake was a class clown but had no real idea that he could make comedy a career. He later bemoaned the fact that no teacher or advisor had ever pointed him in the direction of performing.

Joined Air Force

Steering clear of crime and drugs, he enlisted in the United States Air Force the day after he graduated from high school. "Shoot, my mother argued with me, hollered at me, and I wasn't getting a check, so how hard could basic training be?" he explained to Matt Ehlers of the Raleigh News & Observer. He spent eleven years in the Air Force, spending time at bases in Florida, California, and the Japanese island of Okinawa and rising to the rank of sergeant. At one point he entered a talent show called Tops & Blues and discovered more of his gift for standup comedy. Earthquake's military career came to an end, however, during the Gulf War of 1991 after he refused to participate in fighting in Kuwait and Iraq. "I didn't want to go over there and fight for oil," he told Daniel Neman of the Richmond Times-Dispatch.

Discharged from the military, he moved to Atlanta. For a while, at the suggestion of a military doctor, he saw a psychiatrist. "I paid $50 an hour to tell him my problems," Earthquake told Neman. But Atlanta's open mic nights offered him another outlet for his feelings: "I could go to a comedy club and they would pay me $50 an hour to tell my problems." So, he explained to Neman, the beginning of his comedy career "was economic." In the early 1990s, Earthquake honed his skills in small clubs in Atlanta and elsewhere. By 1993 he was not only performing but also booking shows at the Uptown Comedy Corner in Atlanta's vibrant Buckhead entertainment district. Later he opened a club of his own, Earthquake's Comedy Corner II.

The early years of Earthquake's career involved some hard times, like the night when the comedian tried out a racially themed routine in front of an all-white audience in a small south Georgia town, only to look out and see an audience member who had put on a white hood. He kept going, finished his routine, and collected his pay. In 1997, Earthquake got his national break when he was asked to join the Russell Simmons Def Comedy Jam Tour, a spinoff from a popular though controversial comedy program on the HBO cable television channel.

Relocated to Los Angeles

Things grew from there. At an appearance at the U.S. Comedy Arts Festival in Aspen, Colorado, the veteran film comedy star Whoopi Goldberg picked him out of the crowd as a rising star. Earthquake moved to Los Angeles to be closer to the center of television comedy. Appearing frequently on the BET channel's Comic View program in the late 1990s, he rubbed elbows with rising stars like D.L. Hughley and Cedric the Entertainer during what some saw as a new golden age of African-American comedy.

Earthquake did his own one-hour special on BET and then began to take on gigs for which the audiences weren't predominantly African American. He appeared on Comedy Central's Premium Blend and then in his own special on the channel, on VH1's The List, and on HBO's Real Time with Bill Maher, where he became the first performer in the show's history to receive a standing ovation. A fixture on the national comedy club circuit by this time, Earthquake drew large audiences with repeat appearances in such markets as Houston and Cincinnati.

Encouraged by Steve Harvey

His comedy was quick and fully improvised; Neman likened him to a tightrope walker working without a net. Earthquake agreed, telling Neman that "I never say the same thing the same way. We don't go that way. I think true comedians come straight from the heart, complete with imperfections…. You have general ideas of jokes, but you don't put it in no order. I don't have jokes. I have visions of what I want to talk about." He credited his improvisational style to the influence of comic Steve Harvey. "He showed me how to do it," Earthquake told an HBO interviewer. "I was like, I'm writing this joke. And he said, Man, why are you writing that joke? It was come out already…. Just express your views. You're a comic."

In 2002 Earthquake married, telling the Florida Sun-Sentinel that marriage was like "having cable with just one channel." He and his wife and young son made their home in Los Angeles. Marriage and family played a large part in his routines, but some of his material was political in nature. The former war resister now backed the Iraq war and had become a conservative, drawing some of his themes from the Fox television network's O'Reilly Factor talk show hosted by commentator Bill O'Reilly. Earthquake attributed the poor showing of Republicans among African-American voters to "a bad image. It's like if the KKK had a bake sale. No matter how good the cupcakes, black people still wouldn't go," he told Mekeisha Madden of the Detroit News. He was an equal-opportunity satirist, however, frequently poking fun at President George W. Bush and criticizing him as the chaos in Iraq deepened. As for terrorist mastermind Osama bin Laden, Earthquake told National Public Radio interviewer Tavis Smiley, "Child support'll find him. That's who you need to put on the case."

The next step for Earthquake was to follow his late-1990s contemporaries into television and movies. Signed by the ABC television network to develop a comedy, he came up with Earthquake, in which he was set to star as a struggling father of four. The sitcom didn't make it onto the air in the fall of 2004, but Earthquake kept at it, working on another series deal with HBO and performing in his own 30-minute One Night Stand HBO special in August of 2005. He appeared with Arnez J and other comics that summer as part of a Super Stars of Comedy tour, and he ventured into film (Getting Played, opposite Vivica Fox) and theater, playing a principal in a play called Listen to Your Woman. He had, he told Kevin Aldridge, big plans for the future: "Hopefully, my TV show gets picked up and then I get caught in a scandal where I cheat on my wife with Beyoncé and Janet Jackson, only to get left by both of them and marry Oprah. But after I slap down Stedman [Graham, Oprah's boyfriend] and change my name to Harpo."

At a Glance …

Born Nathaniel Stroman in 1963 in Washington, DC; married Robin Goings, 2002 (divorced 2005); children: one daughter. Military Service: Served in U.S. Air Force, ca. 1980–1991.

Career: Comedian, 1980s–.

Address: Office—Good-Laff Productions, 16430 Ventura Blvd., Suite £304, Encino, CA 91346. Web—

Selected works


Getting Played, 2005.


Listen to Your Woman, 2005.


Earthquake Live, 2005.
It's About Got Damm Time! (DVD), 2005.


The Night B4 Christmas, 2003.
One Night Stand, 2005.
Earthquake has also appeared on various television comedy shows on HBO and Comedy Central.



Atlanta Journal and Constitution, May 20, 1993, p. N12; January 30, 1994, p. D1.

Cincinnati Enquirer, June 14, 2003, p. B6; March 19, 2004, p. E2.

Detroit News, November 21, 2003, p. E1.

Florida Times Union, March 7, 1997, p. D6.

Houston Press, February 26, 2004, Calendar section.

Knoxville News-Sentinel, May 14, 2004, Preview section, p. 24.

News & Observer (Raleigh, NC), June 6, 2003, p. WUP33.

Richmond Times-Dispatch, March 13, 2003, p. D29.

Sun-Sentinel (Fort Lauderdale, FL), April 4, 2003, Showtime section, p. 34.


Quakes House, (October 4, 2005).

"Earthquake Interview," HBO, (October 4, 2005).


Interview with Earthquake, Tavis Smiley Show, National Public Radio, October 31, 2003.

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EARTHQUAKES occur when the lithospheric plates that compose the surface of the earth shift in relation to one another. Earthquakes are happening constantly all over the world, but major quakes seem to occur only once every two or three years. The size of an earthquake is generally described in terms of intensity and magnitude. The Modified Mercalli scale gauges earthquake intensity by assessing the effect of the quake on the inhabitants of an area. Intensity assessments do not depend on seismographic instruments, but are subjective appraisals of (1) human and animal reaction to shaking and, (2) damage to structures of human origin and to the ground surface. Seismologists use the scale to assign to each earthquake an intensity ranking from I (felt by only a few people under favorable conditions) to XII (total damage).

Magnitude of energy released by an earthquake at its point of origin is a strictly quantitative measure based upon data from seismographs that record maximum wave amplitude (the extreme range of vibrations—or shock waves—caused by the sudden movement of the earth's crust). Charles Richter developed the first magnitude scale in 1935, but a variety of magnitude scales are used today. The Richter magnitude scale has no upper or lower numerical limits; some very small earthquakes are actually given negative numbers. The scale is logarithmic, meaning that each increase of one Richter number represents

a tenfold increase in the magnitude of the earthquake. An earthquake of magnitude 5 releases energy equivalent to that released by 1,000 tons of TNT. Recently, seismologists and earthquake engineers have begun to use a measure called "seismic moment" to estimate the size of seismic sources. Moment magnitude measures the leverage of the forces (couples) across the whole area of the fault slip rather than just wave motion, which is affected by fracture and friction in the rocks.

Scientists have used intensity and magnitude data to prepare seismic risk maps of the United States. One map places locales in one of four zones: Zone 0, such as Florida, is an area where no damage is expected; Zone 3 is one in which a quake intensity of VIII and higher is expected, as in parts of California. The western United States exhibits the greatest seismic activity in the country—especially Alaska, California, Nevada, Utah, and Montana—although the upper part of the Mississippi embayment, southwest Kentucky, southern Illinois, and southeastern Missouri are also seismically active.

The historical record of earthquakes in the United States goes back to 1638 in New England and to about 1800 in California. One of the earliest major earthquakes to affect the colonies occurred in the Three Rivers area north of Quebec, along the lower Saint Lawrence River, on 5 February 1663. It caused chimneys to break as far away as Massachusetts Bay. In the early nineteenth century, the Midwest was hit with a series of earthquakes that began in New Madrid, Missouri. The largest of the shocks from these quakes, which occurred in 1811 and 1812, were felt over an area of about 950,250 square miles. Nor has the southern part of the United States been spared. An unpredicted earthquake occurred near Charleston, South Carolina, on 31 August 1886 that did considerable damage in Charleston (much of which was built on filled land) and killed, by some estimates, more than one hundred people. It was the largest seismic event in recorded history on the eastern seaboard. Tremors were felt as far away as New York, Boston, Cuba, and Bermuda. The most notorious earthquake in U.S. history was the one that hit San Francisco on 18 April 1906. It was associated with a rupture of the San Andreas fault from the vicinity of Point Delgada to a point in San Benito County near San Juan, a distance of more than 250 miles. The shock hit at 5 a.m. and, almost instantly, building after building crumbled to the ground. Thousands of fires ignited and burned out of control for three days fed by severed electrical wires, overturned coal burners, ruptured gas mains, broken water lines that prevented fighting the fires, and bungled efforts of troops trying to create backfires with

dynamite. The earthquake and fire caused extensive damage throughout northern California, but in San Francisco it obliterated 500 city blocks, caused nearly $500 million in damages, and killed more than 3,000 people.

California was hit again by major earthquakes in 1925 and 1933, but it was almost sixty years before the United States experienced another quake of the magnitude of the 1906 San Francisco earthquake. That event occurred during the late afternoon of 27 March 1964, at 5:36 p.m. local time. An earthquake of magnitude 8.6 on the Richter scale occurred in the sparsely inhabited mountainous area of northern Prince William Sound in south central Alaska. It caused serious damage within an area of approximately 7,500 square miles, creating large changes in land levels and vertical displacements of nearly thirty-six feet in places along the continental margin. Three hundred people were killed, some from the effects of the quake itself and others by drowning in the seismic sea-wave (tsunami, or tidal wave) caused by the quake.

During the last third of the twentieth century, California again rocked from seismic activity. On 9 February 1971, an earthquake of magnitude 6.5 on the Richter scale struck the San Fernando Valley. This earthquake demonstrated the extent of damage that can occur from a moderate shock centered in a large metropolitan area (the Los Angeles Basin, with a population of 5 million). It caused sixty-five deaths, and damage was estimated to exceed $500 million. Southern California experienced an earthquake measuring 6.4 on the Richter scale in 1979. Eight years later, another quake in the area measured 5.9. In October 1989, the Loma Prieta earthquake struck the San Francisco Bay area, killing at least sixty-three people and collapsing several elevated highways, including a section of the bridge between San Francisco and Oakland. Damages from this earthquake, that registered 7.1 on the Richter scale, reached $6–7 billion. In 1992, a quake measuring 7.4 on the Richter scale struck the desert east of Los Angeles, with one fatality. That same year, a quake of 6.9 struck northern California, with no fatalities. And in 1994, a major quake struck the Los Angeles area, with its epicenter in the city's Northridge section. This quake, measuring 6.6 on the Richter scale, damaged many structures in the city, including freeways, and killed at least fifty-one people. Property losses exceeded $4 billion. Scientists have not yet determined how to predict the precise onset of an earthquake; however, since the 1960s, engineers have developed earthquake-resistant building techniques that can reduce the impact of ground shaking. Regardless, public acceptance of earthquake probability estimates and mandated hazard abatement measures often has been slow.


Bolt, Bruce A. Earthquakes. New York: Freeman, 1999.

Bolt, Bruce A. Earthquakes and Geological Discovery. New York: Scientific American Library, 1993.

Coffman, Jerry L., and Carl A. von Hake, eds. Earthquake History of the United States. Boulder, Colo.: Environmental Data Service, 1973.

Geschwind, Carl-Henry. California Earthquakes: Science, Risk, and the Politics of Hazard Mitigation. Baltimore: Johns Hopkins University Press, 2001.

Hansen, Gladys C., and Emmet Condon. Denial of Disaster: The Untold Story and Photographs of the San Francisco Earthquake and Fire of 1906. San Francisco: Cameron, 1989; 1990.

Steinberg, Theodore. Acts of God: The Unnatural History of Natural Disaster in America. New York: Oxford University Press, 2000.

Bruce A.Bolt

Nancy M.Gordon/c. p.

See alsoCharleston ; Disasters ; Geological Survey, U.S. ; San Francisco Earthquakes .

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An earthquake is an unpredictable event in which masses of rock shift below Earth's surface, releasing enormous amounts of energy and sending out shock waves that sometimes cause the ground to shake dramatically. Not all earthquakes are enormous, but they can become one of Earth's most destructive forces.

Causes of earthquakes

Earth's crust is composed of many huge, rocky plates known as tectonic plates. These plates constantly move slowly across the surface of Earth, bumping into each other, overrunning each other, and pulling away from each other. When the strain produced by these movements increases beyond a certain level, the pent-up energy ruptures the crust and creates a fracture known as a fault. The released pressure also causes the ground-shaking vibrations associated with an earthquake.

The motion of earthquakes: Seismic waves

The vibrations transmitting the shock of an earthquake are called seismic waves (pronounced SIZE-mik). These waves travel outward in all directions, like ripples from a stone dropped in a pond. The area where energy is first released to cause an earthquake is called the focus. The focus lies underground at a shallow, intermediate, or deep depthdown to about 430 miles (700 kilometers). The epicenter is the point on Earth's surface directly above the focus.

Seismic waves travel both through Earth and along its surface. Waves traveling through Earth are called body waves. The two main types are P waves (primary) and S waves (secondary). P waves stretch and compress the rock in their path through Earth. The fastest waves, they move at about 4 miles (6.4 kilometers) per second. S waves move the rock in their path up and down and side to side. They move at about 2 miles (3.2 kilometers) per second.

Words to Know

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

Fault: A crack running through rock that is the result of tectonic forces.

Focus: The underground location of the seismic event that causes an 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 waves: Classified as body waves or surface waves, vibrations in rock and soil that transfer the force of the earthquake from the focus into the surrounding area.

Seismic waves traveling along Earth's surface are called surface waves or L waves (long). The two main types, Rayleigh waves and Love waves, are named after two prominent seismologists (scientists who study earthquakes). Although surface waves move slower than body wavesless than 2 miles (3.2 kilometers) per secondthey cause greater damage. Rayleigh waves cause the ground surface in their path to ripple with little waves. Love waves move in a zigzag along the ground. Both Rayleigh and Love waves set off avalanches, landslides, and other earthquake damage.

Measuring earthquakes

An earthquake's power can be measured in two ways: by intensity (strength) and magnitude (ground covered). While intensity of an earthquake is usually described through people's perceptions and the amount of property destroyed, magnitude is measured by using seismographs or devices that detect ground movement.

Intensity can be measured using the modified Mercalli scale. First developed by Italian seismologist Guiseppe Mercalli (18501914) in 1902, the scale compares the surface effects of earthquakes to each other. It is divided into 12 levels, from level 1 meaning "felt by few" to level 12 meaning "total damage."


A tsunami is a giant wave created by an underwater earthquake, volcano, or landslide. As part of the seabed (ocean floor) rises or drops, water is displaced or moved, producing a great wave. A tsunami (Japanese for "harbor wave") crosses the deep ocean at speeds up to 500 miles (800 kilometers) per hour, but it is only detectable on the surface as a low swell (a wave with no crest). As the giant wave approaches the shallows near shore, it slows down and rises up dramatically, often as much as 200 feet (60 meters). It then strikes the shore with unstoppable force. A wall of water forms when a large tsunami enters straight into a shallow bay or estuary, and can move upriver for many miles.

Magnitude is measured using the Richter scale, developed by American seismologist Charles F. Richter (19001985) in 1935. The Richter scale compares the energy released by an earthquake to the energy released by other earthquakes. Each whole number increase in value on the

scale indicates a 10-fold increase in the energy released and a 30-fold increase in ground motion. Therefore, an earthquake of 6 on the Richter scale is 10 times more powerful than an earthquake with a value of 5, which is 10 times more powerful than an earthquake with a value of 4. An earthquake that measures 8 or above on the Richter scale causes total damage.

Earthquake occurrence and prediction

Earth experiences more than one million earthquakes a year. The vast majority of these measure 3.4 or below on the Richter scale and cannot be felt by people. The planet never ceases to vibrate with the motion of its tectonic forces. Full of heat and kinetic energy (the energy of an object due to its motion), Earth has been resounding with the violence of earthquakes for more than four billion years. In recorded human history, great earthquakes have been responsible for some of the most horrendous natural disasters. In the past 800 years, 17 earthquakes have each caused 50,000 or more deaths.

At the beginning of the twenty-first century, an estimated 100 million Americans live on or near an active earthquake fault. Hundreds of millions more lived on or near such faults around the world. Knowing

the exact time and place an earthquake will occur still lies beyond the ability of scientists. However, in order to interpret seismic activity and possibly to prevent needless deaths, seismologists 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 seem to precede earthquakes. These include changes in tide and ground-water levels, fluctuations in the magnetic properties of rocks, and the swelling or tilting of the ground. Peculiar animal behavior has also been reported before many earthquakes, and scientific research into this phenomenon has been conducted.

[See also Fault; Plate tectonics ]

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earthquake, trembling or shaking movement of the earth's surface. Most earthquakes are minor tremors. Larger earthquakes usually begin with slight tremors but rapidly take the form of one or more violent shocks, and end in vibrations of gradually diminishing force called aftershocks. The subterranean point of origin of an earthquake is called its focus; the point on the surface directly above the focus is the epicenter. The magnitude and intensity of an earthquake is determined by the use of scales, e.g., the moment magnitude scale, Richter scale, and the modified Mercalli scale.

Causes of Earthquakes

Most earthquakes are causally related to compressional or tensional stresses built up at the margins of the huge moving lithospheric plates that make up the earth's surface (see lithosphere). The immediate cause of most shallow earthquakes is the sudden release of stress along a fault, or fracture in the earth's crust, resulting in movement of the opposing blocks of rock past one another. These movements cause vibrations to pass through and around the earth in wave form, just as ripples are generated when a pebble is dropped into water. Volcanic eruptions, rockfalls, landslides, and explosions can also cause a quake, but most of these are of only local extent. Shock waves from a powerful earthquake can trigger smaller earthquakes in a distant location hundreds of miles away if the geologic conditions are favorable.

See also plate tectonics.

Seismic Waves

There are several types of earthquake waves including P, or primary, waves, which are compressional and travel fastest; and S, or secondary, waves, which are transverse, i.e., they cause the earth to vibrate perpendicularly to the direction of their motion. Surface waves consist of several major types and are called L, or long, waves. Since the velocities of the P and S waves are affected by changes in the density and rigidity of the material through which they pass, the boundaries between the regions of the earth known as the crust, mantle, and core have been discerned by seismologists, scientists who deal with the analysis and interpretation of earthquake waves (see earth). Seismographs (see seismology) are used to record P, S, and L waves. The disappearance of S waves below depths of 1,800 mi (2,900 km) indicates that at least the outer part of the earth's core is liquid.

Damage Caused by Earthquakes

The effects of an earthquake are strongest in a broad zone surrounding the epicenter. Surface ground cracking associated with faults that reach the surface often occurs, with horizontal and vertical displacements of several yards common. Such movement does not have to occur during a major earthquake; slight periodic movements called fault creep can be accompanied by microearthquakes too small to be felt. The extent of earthquake vibration and subsequent damage to a region is partly dependent on characteristics of the ground. For example, earthquake vibrations last longer and are of greater wave amplitudes in unconsolidated surface material, such as poorly compacted fill or river deposits; bedrock areas receive fewer effects. The worst damage occurs in densely populated urban areas where structures are not built to withstand intense shaking. There, L waves can produce destructive vibrations in buildings and break water and gas lines, starting uncontrollable fires.

Damage and loss of life sustained during an earthquake result from falling structures and flying glass and objects. Flexible structures built on bedrock are generally more resistant to earthquake damage than rigid structures built on loose soil. In certain areas, an earthquake can trigger mudslides, which slip down mountain slopes and can bury habitations below. A submarine earthquake can cause a tsunami, a series of damaging waves that ripple outward from the earthquake epicenter and inundate coastal cities.

Major Earthquakes

On average about 1,000 earthquakes with intensities of 5.0 or greater are recorded each year. Great earthquakes (magnitude 8.0 or higher) occur once a year, major earthquakes (magnitude 7.0–7.9) occur 18 times a year, strong earthquakes (magnitude 6.0–6.9) 10 times a month, and moderate earthquakes (magnitude 5.0–5.9) more than twice a day. Because most of these occur under the ocean or in underpopulated areas, they pass unnoticed by all but seismologists. Moderate to strong earthquakes can cause more significant destruction if they occur closer to the earth's surface. Notable earthquakes have occurred at Lisbon, Portugal (1755); New Madrid, Mo. (1811 and 1812); Charleston, S.C. (1886); Assam, India (1897 and 1950); San Francisco (1906); Messina, Italy (1908); Gansu, China (1920); Tokyo, Japan (1923); Chile (1960); Iran (1962); S Alaska (1964); Managua, Nicaragua (1972); Guatemala (1976); Hebei, China (1976); Mexico (1985); Armenia (1988); Luzon, Philippines (1990); N Japan (1993); Kobe, Japan (1995); Izmit, Turkey (1999); central Taiwan (1999); Oaxaca state, Mexico (1999); Bam, Iran (2003); NW Sumatra, Indonesia (2004); Sichuan, China (2008); S Haiti (2010); Chile (2010); South Island, New Zealand (2010, 2011); and NE Japan (2011). The Lisbon, Chilean, Alaskan, Sumatran, and NE Japan earthquakes were accompanied by significant tsunamis.

Twelve of the twenty largest earthquakes in the United States have occurred in Alaska. Most of the largest in the continental United States have occurred in California or elsewhere along the Pacific Coast, but the three New Madrid earthquakes (1811–12) also were among the largest continental events, as was the Charleston, S.C., earthquake (1886). On Good Friday 1964, one of the most severe North American earthquakes ever recorded struck near Anchorage, Alaska, measuring 8.4 to 8.6 in magnitude. Besides elevating some 70,000 sq mi (181,300 sq km) of land and devastating several cities, it generated a tsunami that caused damage as far south as California. Other recent earthquakes that have affected the United States include the Feb., 1971, movement of the San Fernando fault near Los Angeles. It rocked the area for 10 sec, thrust parts of mountains 8 ft (2.4 m) upward, killed 64 persons, and caused damage amounting to $500 million. In 1989, the Loma Prieta earthquake above Santa Cruz shook for 15 seconds at an magnitude of 7.1, killed 67 people, and toppled buildings and bridges. In Jan., 1994, an earthquake measuring 6.6 with its epicenter in N Los Angeles caused major damage to the city's infrastructure and left thousands homeless.


See C. H. Scholz, The Mechanics of Earthquakes and Faulting (1991); C. Lomnitz, Fundamentals of Earthquake Prediction (1994); D. S. Brumbaugh, Earthquakes: Science and Society (1998); B. A. Bolt, Earthquakes (4th ed. 1999). See also bibliography under seismology.

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134. Earthquakes

See also 179. GEOLOGY .

an earthquake occurring at very deep levels of the earth.
the slow upward and downward motion of the earths crust. bradyseismic , adj.
any major disaster, as an earthquake, flood, etc. See also 414. WATER . cataclysmal , adj.
a line drawn about an epicenter through all points affected by the same seismic shock. coseismic , adj.
a point on the earths surface directly above the true center of the seismic disturbance from which the shock waves of an earthquake seem to radiate.
a major earthquake. macroseismic , adj.
a violent earthquake. megaseismic , adj.
an almost imperceptible earth tremor caused by a violent sea storm or an earthquake and detected only by a microseismometer. microseismic , adj.
the intensity, frequency, and distribution of earthquakes in a specific area.
seismism, seism
an earthquake. seismic , adj.
the record of an earthquakes vibrations and intensity made by a seismograph.
any of various devices for measuring and recording the vibrations and intensities of earthquakes. seismographer , n. seismographic, seismographical , adj.
1 . the scientific measuring and recording of the shock and vibrations of earthquakes.
2 . seismology.
the branch of geology that studies earthquakes and their effects. Also seismography . seismologist , n. seismologic, seismological , adj.
a special seismograph equipped to measure the actual movement of the ground. seismometry , n. seismometric , adj.
an earthquake that occurs in a part of the world far away from a recording station. teleseismic , adj.
an instrument for detecting or measuring very slight earth tremors.

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earthquake Tremor below the surface of the Earth which causes shaking to occur in the crust. Shaking lasts for only a few seconds, but widespread devastation can result. According to plate tectonics, earthquakes are caused by the movement of crustal plates, which produces fault lines. The main earthquake regions are found along plate margins, especially on the edges of the Pacific, such as the San Andreas fault. When the shock takes place, three different waves are created: primary/push (P), secondary/shake (S), and longitudinal/surface (L). P and S waves originate from the seismic focus (point of origin), up to 690km (430mi) deep. They travel to the surface and cause shaking. On the Earth's surface, they travel as L waves. The surface point directly above the seismic focus is the epicentre, around which most damage is concentrated. A large earthquake is usually followed by smaller ‘aftershocks’. An earthquake beneath the sea is known as a tsunami. Earthquake prediction is a branch of seismology. Present methods indicate only a probability of earthquake activity, and cannot be used to predict actual events. The world's largest recorded earthquake (1976) at Tangshan, China, killed more than 250,000 people and measured 8.2 on the Richter scale.

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earthquake Motion of the Earth. Tectonic earthquakes result from the release of accumulated strain when brittle failure occurs. This failure coincides with the release of stress on the rocks that actually break. Earthquakes are usually classified in terms of their depth: shallow are less than 70 km depth; intermediate 70–300 km; and deep more than 300 km. No earthquakes are known below 720 km depth. Earthquakes may also be caused by volcanic activity or induced explosions (e.g. A-bombs) to which the elastic model of tectonic earthquakes does not apply. The energy released is not stored kinetic energy, but chemical/physical energy which imposes a sudden stress that locally exceeds the strength of the rocks and no significant accumulated strain is involved as the rocks yield to the imposed stresses.

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earthquake Motion of the Earth, which usually results from the sudden release of stress. Earthquakes are usually classified in terms of their depth: shallow ones are less than 70 km depth; intermediate 70–300 km; and deep more than 300 km. No earthquakes are known below 720 km depth. See earthquake prediction; foreshock; Mercalli scale; and Richter scale.

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earth·quake / ˈər[unvoicedth]ˌkwāk/ • n. a sudden and violent shaking of the ground, sometimes causing great destruction, as a result of movements within the earth's crust or volcanic action. ∎ fig. a great convulsion or upheaval: a political earthquake.

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"earthquake." The Oxford Pocket Dictionary of Current English. . 14 Dec. 2017 <>.

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earthquake in Greek mythology, Poseidon was god of earthquakes, and one of his epithets was ‘earth-shaker’.

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earthquakeache, awake, bake, betake, Blake, brake, break, cake, crake, drake, fake, flake, forsake, hake, Jake, lake, make, mistake, opaque, partake, quake, rake, sake, shake, sheikh, slake, snake, splake, stake, steak, strake, take, undertake, wake, wideawake •bellyache • clambake • headache •backache • pancake • teacake •seedcake • beefcake • cheesecake •fishcake • johnnycake • tipsy cake •rock cake • shortcake • oatcake •oilcake • fruitcake • cupcake •pat-a-cake • cornflake • snowflake •rattlesnake • handbrake • mandrake •heartbreak • airbrake • daybreak •jailbreak • canebrake • windbreak •tiebreak • corncrake • outbreak •footbrake • muckrake • earache •firebreak • namesake • keepsake •handshake • milkshake • heartache •beefsteak • sweepstake • stocktake •out-take • uptake • grubstake •wapentake • toothache • seaquake •kittiwake • moonquake • earthquake

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