Tangshan Earthquake in 1976
An earthquake is a shaking of the ground. It occurs when large masses of rock suddenly change position. Earthquakes and tremors (small earthquakes) are occurring somewhere around the globe all the time. Some cause a little shaking and people barely know what's going on. Other cause catastrophic damage.

Earthquakes have killed 1.8 million between 1900 and 2005. Hundreds of thousands have died in single events. No other natural events have caused as much destruction in human history and no other events occur with such suddenness and capriciousness. They only thing that ranks with them are catastrophic volcanic eruptions and tsunami. The former occur with much less frequency and are easier to predict than earthquakes. The latter are generally caused by earthquakes. If anything the destructive power of earthquakes increases as time goes buy as the number of people living in earthquake-prone areas increase even as technology to help them improves.

Earthquakes usually occur on faults, massive cracks or fractures that usually are located around places that tectonic plates meet. The hypocenter or focus is the center of the energy of an earthquake, or where the earthquake originates below the surface of the earth. The epicenter is point in the earth's surface directly above the hypocenter.

Good Websites and Sources: U.S. Geological Survey (USGS) National Earthquake Information Center ; Wikipedia article on Earthquakes Wikipedia ; Surviving an Earthquake ; Earthquake Pamphlet ; Earthquake severity ; USGS Earthquake Frequently Asked Questions ; Collection of Images from Historic Earthquakes Pacific Earthquake Engineering Researc Center, Jan Kozak Collection ; World Earthquake Map ; Most Recent Earthquakes ; Earthquake Safety Site ; Interactive Earthquake Guide ; USGS Earthquakes for Kids


Seismic Waves

The energy of an earthquake is emitted in the form of shock (seismic) waves. There are two kinds of seismic waves: interior ones, which travel through the earth; and surface ones, which travel along the surface. Surface waves are the most damaging to humans.

Seismic waves that affect the surface are divided into P-waves and S-waves. P-waves, or primary waves, are faster and reach the surface first. They alternately compress and expand the rock which they pass through but pack relatively little energy. S-waves, or secondary waves, come later and move particles and rock from side to side perpendicular to the direction they are traveling. They are much stronger and destructive and can produce intense vibrations that rip buildings off their foundations, churn soils into molasses and cause roofs to fall on people.

Both P and S waves produce heaving and rolling surface waves, which are usually most intense along the ruptured fault and weaken as they move away from the epicenter. The seismic echos of these waves that move through the earth and along the surface. can picked up by instruments around the globe. Scientists can locate the hypocenter and epicenter of a given earthquake by comparing the arrival times of the P-waves and S-waves received at different seismic stations in different locations.

Causes of Earthquakes

Most earthquakes occur along cracks in the earth called faults. Some fault are nice neat lines that are visible from planes and outer space. Other are more complex, resembling a shattered or cracked window. Faults visible from the sky easy to locate. Others are less obvious and require some defective work to locate.

The heaviest concentrations of faults are found in places where tectonic plates meet. Along major faults, tectonic plates moves at rate of about eight or nine feet a century, or 1.3 inches a year, which is roughly about the same speed as fingernail growth.

If the movement of tectonic plates or block of earth along faults was steady and smooth earthquakes would not occur. But that is not what happens. Friction keeps the plates together until enough pressure builds up to cause the plates to suddenly lurch. When this happens energy that causes earthquakes is released.

If a small segment of a tectonic plate moves an earthquake with a magnitude of less than 6 occurs. When a large segment, or several small segments rupture, a major earthquake over 7 often occurs. Similar process are taking place with blocks of earth along smaller fault in the general area of where the tectonic plates meet.

Many earthquakes occur along subduction zone, where oceanic plates meet continental plates. Here the denser ocean plate pushes beneath the continental plate. The constant movement and pressure causes earthquakes. Earthquakes also occur along extension zones, where the crust is stretched by subduction. They have also been caused by volcanic eruptions, atomic bob blasts and even dynamite blasted from geological exploration.

For centuries scholars and thinkers have speculated about their causes and mechanisms of earthquakes. Aristotle likened them to the Earth passing gas while the Japanese speculated they were caused by a giant fish wriggling under the Earth’s surface.

tectonic plates

Causes of Earthquakes in Japan

The four major plates that merge under Japan are: 1) the Eurasian Plate, 2) the Philippine Sea Plate, 3) the Pacific Plate and 4) the North American Plate. Most earthquakes occur along two plate boundaries near Japan's Pacific coast, where the Eurasian Plate and the Philippine Sea Plate meet and the Eurasian Plate and Pacific Plate collide.

Three plates move in three directions three miles beneath the streets of Tokyo. The Pacific Plate is the fastest moving. It subducts about four inches a year below the North American Plate and the Philippine Plate. The stress is taken up by strong shallow earthquakes off the east coast of Japan.

Earthquakes along Eurasian Plate and the Philippine Sea Plate are regarded as particularly dangerous because lots of people live there and a lot of friction build up as the Philippine Sea Plate slides at a rate of 5 millimeters a year under the Eurasian Plate. This boundary passes directly under Shizuoka before making a U-turn at Mt. Fuji and returning to the sea at Sagami Bay off Yokohama, the epicenter of the great 1923 quake.

Many earthquakes that affect Japan originate far out at sea with their epicenters deep under the ocean floor. Other occur deep under the earth’s crust.

Types of Earthquakes

A strike-slip, or transform, fault earthquake is a sudden lateral movement of one rock mass against another at fault in response to plate tectonic forces. During a large earthquake of this sort the rock masses on either side a fault can lurch 10 feet or more in opposite directions. Strike-slip faults are where blocks of earth slide past each other laterally. The plates lock together as pressure builds until the plates surge past each other and cause an earthquake.

A reverse fault earthquake is caused by the compression of a subduction zone. Thrust faults occur where blocks of earth collide at an angle and one block shifts upward. A normal fault earthquake is caused by the normal extension of a crustal extension where plates pull apart.

Aftershocks are earthquakes that occur after a major earthquake. They complete the crustal movement initiated by the original earthquake and are usually much smaller than the original earthquake but that is not always the case. Some are as powerful or almost powerful as the original. Aftershocks can occur minutes, hours, days and even years after original.

Megathrust earthquake are destructive earthquakes that occur when two plates collide. Earthquakes of this type can reach a level of 9 or more on the Richter scale while the largest earthquakes that occur on faults like the San Andreas fault can only reach a level of 8 on the Richter scale. The earthquake that caused the tsunami in Indonesia in 2004 and the 9.2 earthquake in Alaska in 1964 were a megathrust earthquake Subduction faults are usually angled at about 10 to 15 percent. Faults like San Andreas fault are roughly vertical. There is a much larger locked area with subduction zones, creating much more energy when the plates unlock

Measuring Earthquakes

Earthquakes are measured using the Richter scale, named after the American geologist Charles Richter (1900-1985), who invented the scale. The Richter scale is base-10 logarithmic. Earthquakes receive a rating of between 0 to 10. Each whole number increase represents a tenfold increase in ground movement. Thus a 7.0 scale earthquake is ten times stronger than a 6.0 one.

Earthquakes measuring three or four on the Richter scale can be felt. Ones measuring five can cause some damage. Those rating more than six can be destructive. Those measuring more than 7 can cause great damage and many death. Earthquakes above eight are often catastrophic. The most powerful earthquakes ever recorded measured 9 and above. No 10 earthquake has ever been recorded. The destructive power of an earthquake often has more to do with where it occurs than how strong it is.

The magnitude and intensity of earthquakes are measured with a devise called a seismograph, which records the peaks of the seismic waves. A seismograph consist of a stylus on a pendulum and a rotating drum. During an earthquake the drum moves up and down while the stylus on the pendulum remains stable.

There are 140 stations in 89 countries checking for earthquakes. The data is sifted through by 268 experts who look for secret nuclear tests as well as earthquakes. The network was set up in the 1990s and reached full global coverage in 2004. The number of stations is expect to reach 321 by 2010. The budget is $105 million. The clearing house for the data is in Vienna, where not coincidently the main United Nations nuclear watchdog agency is located.

Vibrations from earthquake and nuclear tests can be picked very far away. The instruments use to measure them are so sensitive they picked up the break up of the Columbia space shuttle in 2003 and the undersea collapse of the Russian submarine Kursk in 2000. The network that looks for nuclear explosions has 170 stations that detect underground shock waves, 11 that scan for undersea explosions as well 80 that sniff the air for radioactivity and 60 that listen for loud sounds in the atmosphere. In May 1998, shock waves from India’s nuclear test was picked up at 11 stations.

Earthquake Energy

Richter scale measurement
The energy released by a 6.7 earthquake is roughly equal the energy released by a one megaton hydrogen bomb. Sometimes when scientists looking for secret nuclear tests pick up something they are not sure whether it is a nuclear explosion or an earthquake. One way they can tell is by carefully examining the shock waves recorded on a seismograph. A magnitude 6.7. earthquake begins gently and suddenly becomes more violent. A nuclear explosion of similar magnitude begins with a sharp spike, followed by aftershocks.

Large earthquakes usually create a visible crack or a displacement of surface rocks and soil and cause the fault to shift at a rate of about two miles per second. The San Francisco earthquake in 1906, which measured 7.8 on the Richter scale, caused movement along 270-mile section of fault that ranged from 6 feet to 16 feet.

Shock waves from earthquakes travel farther in places where the crust is made of old rock, but fortunately these are the places where earthquake are least likely to happen. In areas of new crust generation where earthquakes occur more frequently there are larger amounts of sand, clay and gravel. Shock waves don’t travel as far in these materials but they do more damage by concentrating their forces and causing a lot of shaking.

Earthquakes and Geology

Earthquakes originate below the surface of the earth in places called nucleation sites. Shallow earthquakes are more likely to cause extensive damage than deep ones. Most damaging earthquakes have a hypocenter that is less than six miles below the surface. Although deep quakes usually cause less catastrophic damage to a specific area than a shallower ones they often cause damage over a broader area.

In places where there has been a lot of tectonic activity---where rocks have been folded and mix---earthquake waves radiate outward inefficiently and thus earthquakes tend be very destructive near their epicenters but not far way from it. Earthquakes at the center of plates are often more destructive because the rock has not been folded and mixed so much and is more uniform and conducive to transporting waves over long distances.

Rock and soil types can make a big difference in an earthquake. Bedrock offers a measure of stability. It helps to dampen the shaking from a quake. Sediment and loose soils on the other hand amplify the shaking. In a given earthquake the shaking can be ten times stronger in loose soil than it is over bedrock.

Many earthquakes form rising and falling bumps in the soil and sediments called oscillations that can cause sever damage. On reclaimed land or soil saturated with water, the soil can liquify and begin to flow and shake with even more extreme violence. This occurs because during intense shaking solid particles in soft muds, silts and wet land move downward, while water shoots up at great pressure.

20080317-Tangshan Erathquake 5.jpg
Damage from the Tangshan Earthquake in 1976

Earthquake Damage

Earthquakes with a magnitude of more than 7 on the Richter scale strike 20 times a year somewhere in the world. But these are not necessarily the most destructive earthquakes. The biggest factor in determining earthquake damage and destruction is location, location and location. Earthquakes that strike populated area are going to cause more death and destruction than ones that occur in unpopulated areas.

Some of the most damaging and deadly earthquakes---the one that killed 250,000 people in Tangshen China and the ones in Lisbon and Missouri---occurred is places where no one though a major earthquake would happen and the victims were caught totally unprepared.

Earthquake damage is also often more of a matter of architecture and affluence than the strength of the earthquake. In developing countries the houses are often shoddily built. In the worst case they are made of stones held together with weak mortar and topple easy, crushing people when they do. That is why a 6.0 earthquake in Armenia could kill 26,000 people and a 7.1 one in San Francisco could kill 62.

The damage from an earthquake is often very spotty. Neighborhoods that have been totally destroyed often sit next to ones that look untouched. The reason depends on a number of factors including construction methods, maintenance, soil stability, fault lines in the underlying rock, and location of gas lines.

Some deaths are attributed directly to the earthquake. Many more often die in mudslides, landslides, and tsunamis generated by the quake or in the harsh conditions that follow it. Landslides block roads, leaving thousands without food and shelter. Diseases spread; no medical help is available.

Fires often cause the most damage. Sometimes the fires are so hot they can melt steel and ignite fires 40 meters away. Many start when stoves topple over or gas lines break. Often the fires burn out of control because water maina are broken and not enough people can be mobilized to put them out.

Earthquake Damage and Buildings

About 80 percent of all earthquake deaths occur in collapsed building. Many of these would have been avoided if the buildings were properly constructed. One geophysicist told the New York Times, "Buildings kill people not earthquakes."

Structures built on rock suffer high-frequency vibrations that may not topple the building but can cause enough structural faults to make it unsafe. Buildings on softer soils need special braces because the building can resonate with the quake and vibrate apart. Buildings on reclaimed land or soil saturated with water are particularly vulnerable because the soil can liquify and shake extremely violently, causing entire building to lean, sink or topple.

Unreinforced buildings sway from side to side and if the earthquake is powerful enough give way and collapse. The joints where H-shaped steel beams are welded to the main pillars are typically the most vulnerable spots of a building. Seismic energy converges on the joints which can crack under the force.

Old wooden houses are shaken off their foundations, causing roofing and unsecured items to topple on residents. High rises are designed to sway. They often remain standing and keep their occupants alive but suffer from broken windows and cracked girders and sometimes have to be torn down.

Mid-rise buildings---especially ones with unreinforced masonry are often the most dangerous places to be. Their rigid walls tend to crack and crumble rather than sway. After a large earthquake many of the welded joints give way under the stress and the building tilts, and sometimes collapse, sometimes killing most of the people inside.

People trapped inside collapsed buildings without food and water or adequate supplies rarely survive under the rubble for more than three or four days. Without water, especially in the summer heat, people are claimed by dehydration. After earthquakes villagers sometimes spend months sleeping in their beds outdoors until the threat passes.

20080317-TangshanEarthquake1976. 4jpg.jpg
More damage from the Tangshan Earthquake in 1976

Earthquake Damage and Infrastructure

Unprotected electric power lines, waiter mains and gas lines rupture, causing power outages, fires and flooding. Roads and bridges give way as a result of the shaking and swaying. Damage not only causes immediate deaths and casualties but hampers rescue efforts, causing more deaths.

Land line communications are disrupted hampering rescue efforts even more. Hospitals with structural damage and power outages have a hard time maintain a normal functioning level, let alone keeping up with the high numbers of casualties that pour in.

In large cities, subways and trains can become derailed; tunnels and stations can crack, collapse or become flooded. Tunnels are particularly vulnerable where one ground material change into another such as between rock and soil. Damages can be catastrophic if an oil refinery or chemical factory near a populated area explodes or catch fire. Even worse is if a levee system or a dam upstream from a populated area collpases.

Earthquake Damage and Buildings in the Developing World

Many victims of earthquakes in the developing world die because they were caught inside poor housing. Masonry structures with mud brick or concrete blocks walls and clay tile roofs are regarded as death traps. Common in the developing world, they are not built to sway or move in an earthquake and often have floors added in a piecemeal fashion. Even an earthquake of moderate strength is enough to break the brittle walls and supports. The materials are bulky and have trouble absorbing shock and can kill because the materials are so heavy.

Many buildings are little more than unreinforced masonry piles. Stone walls and concrete roofs are a particularly nasty combination. In an earthquake, the stones crumble, the walls give way and the heavy concrete roof collapses, causing serious injury or death to anyone caught underneath.

Mud brick houses are often the most deadly. Unlike houses made of concrete and wood that collapse in jumbles and leave gaps for survivors, mud brick homes tend to crumble in piles. Many people died from being crushed; other suffocate from the weight of the brick on top of them, lack of air and breathing in dust. Mud bricks are a popular building material because they are cheap and warm in the winter and cool in the summer.

Apartments with little or no steel in the concrete pillars are among the deadliest structure of all. Under the heavy weight, the floors collapse, pancake style, and crush or trap the people inside.

Houses with wooden frames are much more vulnerable to collapse than ones with steel frames. But even houses with steel frames are vulnerable to collapse if steel pieces are poorly welded. Even when building codes are followed buildings collapse because material are of poor quality and the walls and ceilings are poorly tied together.

Image Sources: USGS

Text Sources: New York Times, Washington Post, Los Angeles Times, Times of London, National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Page Top

© 2008 Jeffrey Hays

Last updated April 2010

This site contains copyrighted material the use of which has not always been authorized by the copyright owner. Such material is made available in an effort to advance understanding of country or topic discussed in the article. This constitutes 'fair use' of any such copyrighted material as provided for in section 107 of the US Copyright Law. In accordance with Title 17 U.S.C. Section 107, the material on this site is distributed without profit. If you wish to use copyrighted material from this site for purposes of your own that go beyond 'fair use', you must obtain permission from the copyright owner. If you are the copyright owner and would like this content removed from, please contact me.