Why Earthquakes Occur

Map of Plate Tectonics

The earth's crust is divided into several major plates, some 50 miles thick, which move slowly and continuously over the earth's interior.

Most earthquakes occur as the result of slowly accumulating pressure that causes the ground to slip abruptly along a geological fault plane on or near a plate boundary. The resulting waves of vibration within the earth create ground motion at the surface that vibrates in a very complex manner.

The point where the fault first slips is the "focus" or "hypocenter" of the earthquake. A theoretical point on the earth's surface directly above the focus is the "epicenter" of the earthquake. The epicenter of the January 17, 1994 Los Angeles earthquake was located in the city of Northridge in the San Fernando Valley.

Last Modified: Wednesday, 12-Apr-2006 15:56:00 EDT