An earthquake is thetrembling or shaking movement of the earth’s surface. Great earthquakes usually begin with slight tremors, rapidly increase to one or more violent shocks, and diminish gradually. 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 the movement of opposing blocks of rock past one another. This causes vibration to pass through and around the earth in waveform. The subterranean origin of an earthquake is its focus; the point on the surface directly above the focus is the epicenter. Waves generated by earthquakes are of several types. Both p, or primary, waves, which are compression and are the fastest, and s, or secondary, waves, which cause vibrations perpendicular to their motion, are body waves that pass through the earth. L, or long, waves travel along the surface and cause damage near the epicenter. Seismologists have deduced the internal structure of the earth by analyzing changes in p and s waves. The magnitude and intensity of earthquakes are determined by the use of scales, e.g., The Richter scale, no longer widely used, which describes the amount of energy released at the focus of an earthquake; another scale that measures the magnitude of an earthquake’s surface waves and is sometimes inaccurately called the Richter scale; and the moment-magnitude scale, the most widely used scale, which is based on the size of the fault on which the earthquake occurs and the amount of the slippage along the fault
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