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 Earthquakes have captured the imagination of people living in earthquake prone regions since ancient times. Ancient Greek philosophers thought quakes were caused by subterranean winds, while others blamed them on fires in the bowels of the earth. About AD 130, a Chinese scholar named Chang Heng reasoned that waves must ripple through the earth from the source of an earthquake.