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 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. Hope I helped :-)!
