Know Your Earthquake Terms

Familiarize yourself with these terms to help identify an earthquake hazard:

Aftershock

An earthquake which follows a larger earthquake or main shock and originates at or near the focus of the larger earthquake. Generally, major earthquakes are followed by a larger number of aftershocks, decreasing in frequency with time.

Benioff Zone

A dipping zone containing earthquake hypocentres lying along the top of a subducting plate.

Epicenter

That point on the Earth's surface directly above the hypocenter of an earthquake.

Focus

That point within the Earth from which originates the first motion of an earthquake and its elastic waves.

Focal zone

The rupture zone of an earthquake. In the case of a great earthquake, the focal zone may extend several hundred kilometers in length.

Foreshock

A small tremor that commonly precedes a larger earthquake or main shock by seconds to weeks and that originates at or near the focus of the larger earthquake.

Hypocenter

The calculated location of the focus of an earthquake.

Magnitude

A measure of the strength of an earthquake or strain energy released by it, as determined by seismographic observations. The local body- and surface-wave magnitudes will have approximately the same numerical value.

Modified Mercalli scale

Mercalli scale modified for North American conditions.

Subduction zone

An elongated region along which a block of crusts descends relative to another crustal block, for example, the descent of the Pacific plate beneath the Andean plate along the Andean trench.

Teleseism

An earthquake that is distant from the recording station.

Travel time

The time required for a wave train to travel from its source to a point of observation.

Tsunami

A huge sea wave caused by earthquakes. (Referred to by many as a tidal wave.)