Body waves: A seismic wave that travels through the interior of the Earth and is not restricted to any boundary surface.

Earthquake: A sudden motion or trembling in the Earth caused by the abrupt release of slowly accumulated strain.

Elastic energy: The energy stored within the Earth during elastic deformation.

Epicenter: That point on the Earth's surface which is directly above the focus of an earthquake.

Fault: A surface or zone of rock fracture along which there has been displacement.

Focus: That point on a fault at which the sudden break resulting in an earthquake begins.

Plate tectonics: The Earth's surface is composed of large, semirigid sections (plates) about 50km (30 miles) thick that float across the mantle, with seismic activity and volcanism occurring primarily at the junctions of these sections.

Plates: Large, nearly rigid, but still mobile segments of blocks involved in plate tectonics, that include both crust and some part of the upper mantle.

P-waves (or primary waves): That type of seismic body wave which is propagated by alternating compression and expansion of material in the direction of propagation.

Seismic energy: The energy that is released as vibrations during an earthquake.

Seismic waves: Waves produced by an earthquake, including both body waves and surface waves.

Strike-slip fault: A fault for which the movement or slip is parallel to the strike (direction) of the fault.

Surface waves: A seismic wave that travels along the surface of the Earth.

S-waves (or secondary waves): That type of seismic body wave which is propagated by a shearing motion of material perpendicular to the direction of travel.

Tsunami: A sea wave produced by any large-scale disturbance of the sea floor, principally by a submarine earthquake or by submarine earth movement.

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