The friction between huge blocks of rock is always building up. When this stress becomes great enough to overcome the friction along a fault, the energy released by the slip of two blocks of rock becomes a violent earthquake.
The energy released by an earthquake is transmitted to other parts of Earth in the form of seismic waves.

Two main types of waves are produced once the stress that produces an earthquake is released

Body waves – Seismic waves that travel through Earth’s interior.

Surface waves – Seismic waves that travel along Earth’s surface.

Surface waves – Travel long or near Earth’s surface, like waves along the surface of the ocean. They travel more more slowly than P and S waves, and they pass around the Earth, rather than through it. Thus, surface waves are the last to be detected by a seismograph.

Body waves are divided into two types:

Compressional wave – A seismic body wave consisting of alternating pulses of compression and expansion in the direction of wave travel; P wave or primary wave.

Shear wave – A seismic body wave in which rock is subjected to side-to-side or up-and-down forces perpendicular to the wave’s direction of travel; S wave or secondary wave.



Brooklyn College - Geology Department