An earthquake, one of the most destructive natural phenomena, consists of rapid vibrations of rock near the surface of the earth. It is the most terrifying of all natural phenomena and has brought fear since ancient times because of its sudden unpredictable occurrence and enormous capacity of destruction. A single shock usually last no more than a few seconds, although several quakes may last for as much as a couple of minutes. The quake as felt on the surface is always the result never the cause of some geologic process although the damage done may be immense. The Greek word for "shaking,” and (when it applies to the earth) "earthquakes,” is seismos. The science of earthquakes, therefore, is called seismology The scientific study of earthquakes is comparatively new. Until around the 18th century, few accurate descriptions of earthquakes were recorded, and the natural cause of earthquakes was little understood. Although earthquake can be the result of sea-floor spreading, volcano eruptions and plate tectonics, the great majority of earthquakes are caused by sudden movement or the large blocks of the earth's crust near the surface along lines called faults. When the stress that develops in the crust becomes too big for the rock to support, one side of vibrations that transmit waves which travel for long distances through the interior of the earth or along surface. The energy released by an earthquake moves in the form of waves through the rock which act as an elastic body. These waves make the ground vibrate, and are what we call earthquakes. The two main types of earthquake waves are P waves and S waves. P waves or primary waves are the fastest and can travel through solid and liquid materials. In other words, P wave is a fast body wave that travels through the interior of Earth and arrives first at the seismograph. On the other hand, S waves (secondary or shear waves) are slower body waves than P waves. Due to the slow sp...
