When rock masses suddenly move deep within the earth in response to tectonic stress, energy in the form of
seismic waves moves outward through the rock from the point of origin, called the
focus. The initial movement occurs at the focus. The
epicenter is the point on the surface directly above the focus. There are three types of seismic waves: P and S body waves and surface waves.
Body waves. Body waves radiate outward from the focus in all directions and travel through solid rock. A P body wave (primary body wave) is a compressional (longitudinal) wave that induces the particles in the rock to vibrate back and forth in the same direction the wave moves. P. waves move at speeds up to 15,000 miles per hour (7 kilometers/second) (Figure 1).
P Body Waves
An S body wave (secondary body wave) is only about half as fast as a P wave and causes the rock to vibrate at right angles to the direction of wave travel (Figure 2).
S Body Waves
Both P and S waves can travel through solid rock, but only P waves can pass through a fluid medium.
Surface waves. Surface waves are the slowest seismic waves and travel outward on the earth's surface from the epicenter much like ripples do from a stone thrown into water (Figure 3). They create most of the damage at the surface because they are the waves that produce the most ground movement and pass through an area the most slowly.