Earthquakes

Earthquakes occur when rocks are subjected to strain and rupture, moving past each other suddenly along a fault plane, forming seismic waves that move through the earth. What is

called the focus of the earthquake is the place below the surface where the slippage occurred; the epicenter is the place on the surface of the earth, directly above the focus. When severe earthquakes occur, they can destroy buildings and other structures and cause great loss of life.

A record of an earthquake tremor, November 29, 1959 (USGS)

The shaking caused by earthquakes may trigger landslides, or cause certain types of clay to liquefy and flow down slope, causing damage to structures on its surface.

The magnitude of an earthquake is the amount of energy released; it is read directly from the seismogram record. On the Richter scale the magnitude is registered logarithmically, meaning that an earthquake of magnitude 3 is 10 times greater than one of magnitude 2; an earthquake of magnitude 4 is 100 times greater than a magnitude 2; a magnitude 5 is 1,000 times greater—and so on.

Another commonly used scale measures the intensity of the earthquake based upon its destructive power, as noticed by people and by observations of the effects on buildings, dams, and other structures. For example, a IV has occurred when dinner plates rattle on a shelf, and a VI when plaster falls. When buildings shift on their foundations, it is a IX.

Tens of thousands of earthquakes occur each year, most of them so small that only the most sensitive seismographs can detect them. Every few years, however, a severe earthquake occurs, killing, unfortunately, thousands of people. A Peruvian earthquake in 1970 killed 50,000 people when their adobe buildings collapsed. In mountainous regions like the Andes, earthquakes cause huge amounts of rock debris to move down slope, burying towns with little warning, as occurred in Chile in 1939, when 40,000 people died. Seismicity below the sea can generate large sea waves called tsunamis (or tidal waves), which cause heavy coastal destruction when they strike land.

Because of the loss of life and property caused by earthquakes, geologists and seismologists are trying to find a way to predict them. Chinese scientists have utilized the sudden change in the behavior of animals to predict earthquakes. In the United States and elsewhere, geologists monitor uplift and tilting of the land surface, changes in groundwater flow and level, and other physical characteristics. Most of the most severe earthquakes occur in narrow bands along plate boundaries; the goal is to find an accurate way to predict the timing and magnitude of an earthquake.

—Sidney Horenstein

See also: Geology, Geomorphology, and Geography; Mountains; Plate Tectonics

Bibliography

Bolt, Bruce A. 1993. Earthquakes, 3rd ed. New York: W. H. Freeman and Company; Keller, Edward A., and Nicholas Pinter. 1996. Active Tectonics: Earthquakes, Uplift, and Landscape. Upper Saddle River, NJ: Prentice Hall; Laing, David. 1991. The Earth System: An Introduction to Earth Science. Dubuque, IA: Wm. C. Brown; Press, Frank, and Raymond Siever. 2000. Understanding Earth, 3rd ed. New York: W. H. Freeman and Company.

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