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Earthquake Terms

July 20, 1986

Here are some terms commonly associated with earthquakes:

Epicenter--The surface point directly above the point in the earth's crust where a rupture begins. The epicenter is determined by a network of seismic stations linked by telephone hookups to monitoring centers, such as those at Cal Tech in Pasadena and USC in Los Angeles.

Intensity--A subjective measure based on the effects of the quake on the surface of the earth and man-made structures. The intensity of a quake will vary with the type of soil. Generally, the harder the soil in an area the milder the intensity of a quake.

Magnitude--The maximum motions of a quake recorded by a seismograph, the Richter scale being the most common measure.

Liquefaction--A process where water-saturated sediment temporarily loses strength, usually because of shaking, and behaves like a liquid. The soil in the South Bay is mostly sediment and the water table has been rising in recent years--due to lack of consumption and high rainfall--making liquefaction an increasing danger.

Strike-slip fault--When the moving side of a fault slides horizontally, generally to the north or northwest. Most of the faults in the South Bay are of this type.

Dip-slip--When one side of a fault is moving vertically compared to the other. There are two types of dip-slip faults: normal, where the lower mass is moving up, and reverse or thrust, where the upper side is moving higher.

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