Earthquakes

General

An earthquake is a shaking or trembling of the crust of the earth caused by underground volcanic action or by the breaking and shifting of rock beneath the surface. The volcanic action and shifting rocks create strain which continues to build to a sudden release of pressure resulting in a shock wave. The vibrations produced in the crust can vary from barely noticeable to enormously destructive. Shock waves can be classified into two broad categories. Waves that send particles oscillating back and forth in the same direction as the waves are traveling are called primary. Primary waves, sometimes called compressional waves, travel through the earth beneath the crust. Secondary waves cause vibrations which move perpendicular to the wave. These waves travel on the surface of the earth and move much slower than primary waves. Thus, when an earthquake occurs, seismic centers throughout the world record primary waves before the secondary waves arrive.

Historical Overview

Ear...