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aftershock: A lesser earthquake that follows a stronger one.

body waves: A group of seismic waves that move through solid rock. See P-wave and S-wave.

density: The quantity of matter (or mass) per unit volume in a substance.

earthquake:A sudden sharp movement within the Earth’s outer layers resulting from the abrupt release of accumulated strain.

epicenter: The point on the surface of the Earth that is directly above the focus of an earthquake.

fault:A crack in rock layers or in the Earth's crust.

focus: The place inside the Earth where an earthquake begins.

geophysicist: A scientist who is trained in the study of the physics of the Earth. A geophysicist uses electrical, gravitational, magnetic, and seismic methods to evaluate and learn more about the physical structure of the Earth. A specialist in geophysics.

geophysics:The study of the physics of the Earth, especially its electrical, gravitational and magnetic fields, and propagation of elastic (seismic) waves within it.

intensity: A measurement of the shaking resulting from an earthquake. Intensity lessens with distance from the epicenter.

long wave:See Surface wave

love wave: A type of surface wave that shakes the ground from side to side.

L-wave:See Surface wave

magnitude: The strength of an earthquake according to the amount of energy released.

mantle: The middle layer of the Earth, located between the crust and the core. The mantle is about 2900 km (1820 miles) thick.

moonquake:Seismic activity on the moon—the Earth has earthquakes, the moon has moonquakes.

pendulum: A weight hung from a fixed point in such a way that it can swing freely back and forth.

primary wave:See P-wave

P-wave: Primary wave: a seismic wave that pushes and pulls rocks, contracting and expanding them as it moves through them. P-waves arrive first during an earthquake.

Rayleigh wave: A rolling surface wave that moves the ground up and down.

reflection: The bouncing back of energy when it hits a surface through which it cannot pass.

refraction: The bending of a wave when it enters a substance of a different density.

Richter scale: A measure of earthquake magnitude. The Richter scale begins with zero but it has no maximum.

salt dome: A mushroom-shaped or plug-shaped formation made of salt.

seismic wave: Wave of energy that travels through the Earth as a result of seismic activity.

seismogram: An image that uses wavy lines to show the intensity of the shaking of the Earth.

seismograph: An instrument that measures and records vibrations within the Earth and of the ground.

seismograph station: Seismic equipment set up more or less permanently to make timed recordings of ground motion on a routine basis. Seismic stations are located all over the world.

seismometer: The sensor part of an instrument that detects motions of the Earth's surface caused by seismic waves produced during an earthquake.

seismologist: Scientist who studies earthquakes and seismic waves.

seismology: The study of earthquakes and seismic waves, both natural and resulting from human activity.

shear wave: See S-wave

sideways wave: See S-wave

surface wave: A seismic wave that travels on the surface of the Earth, moving either up and down or from side to side. It arrives after P-waves and S-waves.

S-wave: Secondary wave: a seismic wave that moves rocks from side to side as it moves through them. S-waves arrive second during an earthquake.

transverse wave: See S-wave

velocity: The rate at which a wave travels through a medium, or the rate at which a body is displaced in a given direction. Its usage in geophysics is as a property of a medium-distance divided by travel time.

vibration: Shaking or moving back and forth very rapidly.

warhead: The part of a torpedo or other ballistic device that contains the explosive or other substance intended to damage the enemy.

wave: A disturbance that moves through a medium, transporting energy from one location to another as it goes.

wave front: The edge of an advancing wave, which includes adjacent points that have the same rotation or variation considered in relation to a reference or assumed instant of starting.