The vibrations produced by earthquakes are detected, recorded, and measured by instruments call seismographs. From the data expressed in seismograms, scientists can determine the time, the epicenter, the focal depth, and the type of faulting of an earthquake and can estimate how much energy was released. The two general types of vibrations produced by earthquakes are surface waves , which travel along the Earth's surface, and body waves , which travel through the Earth. Surface waves usually have the strongest vibrations and probably cause most of the damage done by earthquakes.
Body waves are of two types, compressional and shear .
Both types pass through the Earth's interior from the focus of an earthquake to distant points on the surface, but only compressional waves travel through the Earth's molten core. P waves push tiny particles of Earth material directly ahead of them or displace the particles directly behind their line of travel. The first indication of an earthquake is often a sharp thud, signaling the arrival of compressional waves. These shock waves continued with a rather long frequency, which gave the observer an impression of a rolling feeling rather than abrupt hard jolts. The severity of an earthquake can be expressed in several ways.