The elastic strain can be released by brittle fracture of the rocks. The released kinetic energy radiates as seismic waves. The point of fracture is called the focus of the earthquake. The point on the surface directly above the focus is called the epicenter . Seismic waves are classified by their mecahnism of propagation.
Body waves travel through the depths of the Earth.
P-waves , or primary waves, are compressional waves like sound waves and travel at the highest velocities. The vibration is parallel to the propagation. P-waves are transmitted by solids, liquids, and gasses. The vibration is perpendicular to propagation. S-waves are transmitted only by solids.