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Earthquake Glossary

A Guide to Earthquake Terminology

An earthquake is the sudden, rapid shaking of the earth, caused by the breaking and shifting of subterranean rock as it releases strain that has accumulated. This earthquake glossary outlines other terms.

Aftershock

Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. They are smaller than the main shock and can continue over a period of weeks, months, or years. In general, the larger the main shock, the larger and more numerous the aftershocks, and the longer they will continue.

Epicenter

The epicenter is the point on the earth's surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins.

Fault

A fault is a fracture along which the blocks of crust on either side have moved relative to one another parallel to the fracture.

Hypocenter

The hypocenter, or focus, is the point within the earth where an earthquake rupture starts. The epicenter is the point directly above it at the surface of the earth.

Magnitude

The magnitude is a number that characterizes the relative size of an earthquake. Magnitude is based on measurement of the maximum motion recorded by a seismograph. Several scales have been defined, but the most commonly used are (1) local magnitude (ML), commonly referred to as "Richter magnitude," (2) surface-wave magnitude (Ms), (3) body-wave magnitude (Mb), and (4) moment magnitude (Mw). Scales 1-3 have limited range and applicability and do not satisfactorily measure the size of the largest earthquakes. The moment magnitude (Mw) scale, based on the concept of seismic moment, is uniformly applicable to all sizes of earthquakes but is more difficult to compute than the other types. All magnitude scales should yield approximately the same value for any given earthquake.

Tectonic Plates

The tectonic plates are the large, thin, relatively rigid plates that move relative to one another on the outer surface of the earth.

Tsunami

A tsunami is a sea wave that results from large-scale seafloor displacements associated with large earthquakes, major submarine slides, or exploding volcanic islands.
 

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Did You Know?

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While major earthquakes are sometimes believed to be primarily a West Coast threat, there are 45 states and territories throughout the United States that are at moderate to high risk for earthquakes.

Federal Emergency Management Agency, www.ready.gov