Seismic belt:

An elongated earthquake zone, for example, circum-Pacific, Mediterranean, Rocky Mountain. About 60% of the world's earthquakes occur in the circum-Pacific seismic belt.

Seismic discontinuity:

A surface or thin layer within the Earth across which P-wave and/or S-wave velocities change rapidly.

Seismic gap:

An area in an earthquake-prone region where there is a below-average release of seismic energy.

Seismic moment:

See Moment (of earthquakes).

Seismic Sea Wave:

A tsunami (see below) generated by an undersea earthquake.

Seismic Wave:

Seismic waves are vibrations generated by sudden movements of rock. After earthquakes occur, the seismic waves propagate from the hypocentre to the surface of the Earth. The speed at which the waves propagate is a function of the nature and type of rock traversed, but generally varies from 1 to 10 km/s. Some waves have a high enough frequency to be audible; others have a very low frequency corresponding to periods of several seconds or minutes. Earthquakes generate two main types of waves: compressive (P) waves, and transverse (S) waves. The two types of waves travel through the interior of the Earth from the hypocentre, but only the compressive waves travel through the part of the Earth called the outer core, which is composed of molten matter. Compressive waves travel faster; they are first to arrive at the surface. This is why they're called primary or P waves. Transverse waves do not travel as fast; they are therefore called secondary or S waves. Sometimes the first indication of an earthquake is a a sudden low sound, indicating the arrival of the P waves. Then, the S waves reach the surface and cause a more violent shaking.

Seismic Zone:

A region in which earthquakes are known to occur.

Seismicity:

The occurrence of earthquakes in space and time.

Seismogram:

Recording of ground motions made by a seismograph.

Seismograph:

A very sensitive instrument used to record and measure earthquakes. During an earthquake, vibrations initiated by fracturing of the earth's crust radiate outward from the point of fracture and are detected by seismographs. The visual record produced is called a "seismogram".

Seismologist:

A scientist who studies earthquakes, seismic sources, and wave propagation through the Earth.

Seismology:

The study of earthquakes, seismic sources, and wave propagation through the Earth.