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. Because compressional waves travel at great speeds and ordinarily reach the surface first, they are often called “primary waves” or simply “P” waves. P waves push tiny particles of Earth material directly ahead of them or displace the particles directly behind their line of travel. Shear waves do not travel as rapidly through the Earth’s crust and mantle as do compressional waves, and because they ordinarily reach the surface later they are called “secondary” or “S” waves. Instead of affecting material directly behind or ahead of their line of travel, shear waves displace material at right angles to their path and are therefore sometimes called “transverse” waves. [Illustration: Diagram of propagation of seismic waves.] The first indication of an earthquake is often a sharp thud, signaling the arrival of compressional waves. This is followed by the shear waves and then the “ground roll” caused by the surface waves. A geologist who was at Valdez, Alaska, during the 1964 earthquake described this sequence: _The first tremors were hard enough to stop a moving person, and shock waves were immediately noticeable on the surface of the ground. These shock waves continued with a rather long frequency, which gave the observer an impression of a rolling feeling rather than abrupt hard jolts. After about 1 minute the amplitude or strength of the shock waves increased in intensity and failures in buildings as well as the frozen ground surface began to occur.... After about 3½ minutes the severe shock waves ended and people began to react as could be expected._ [Illustration: _Large earthquakes cause more damage east of the Rocky Mountains; this map shows areas that suffered major architectural damage (striped areas) and minor damage (dotted areas) during the magnitude-8 earthquakes in New Madrid and San Francisco and the smaller but still damaging quakes in Charleston and San Fernando._] The severity of an earthquake can be expressed in several ways. The _magnitude_ of an earthquake, usually expressed by the _Richter Scale_, is a measure of the amplitude of the seismic waves. The _moment magnitude_ of an earthquake is a measure of the amount of energy released—an amount that can be estimated from seismograph recordings. The _intensity_, as expressed by the _Modified Mercalli Scale_, is a subjective measure that describes how strong a shock was felt at a particular location.