:  A series of earthquakes following the first or main shock.  They are usually of lower 

     magnitude than the main shock.

Rough spot or “knot” along a fault that stops movement along that section for a period of time.  

    The bigger the asperity, the greater the stress buildup and the larger the earthquake that results. (See also

     seismic gap.)

The theory that movement along a fault and the resulting seismicity is the result of 

     an abrupt release of stored elastic strain energy between two rock masses on either side of the fault.

The gradual slip or motion along a fault without an earthquake.

Point within the earth where an earthquake originates.

An earthquake rating scale (I-XII) based upon subjective reports of human reactions to

     ground shaking and upon the damage caused by an earthquake.

A type of seismic wave involving particle motion that is alternating  expansion and compression in the direction of wave propagation.  Also known as compressional waves,

       they resemble sound waves in their motion.

A measure of the strength or the strain energy released by an earthquake at its

      source.  (See moment magnitude, Richter magnitude).

Earthquake intensity scale from I (not felt) to XII (total destruction)based upon damage and reports of human reactions.

A scale of seismic energy released by an earthquake

based on the product of the rock rigidity along the fault the area of rupture on the fault plane, and the amount of slip.

The rock record of past earthquake events in displaced beds and liquefaction features in

      trenches or natural outcrops.

Observable phenomena that occur before an earthquake and indicate that an

      event is soon to occur.

The return period of an event, such as a flood or earthquake, of a given magnitude.

      For flooding, it is the average interval of time within which a given flood will be equaled or exceeded by

      the annual maximum discharge.

The tendency of a system (a structure) to vibrate with maximum amplitude when the

      frequency of the applied force (seismic waves) is the same as the vibrating body’s natural frequency

The logarithm of the maximum trace amplitude of a particular seismic wave on    a seismogram, corrected for distance to epicenter and type of seismometer.  A measure of the energy     released by an earthquake.

The segment along an active fault that has seismically been quiet relative to segments at   either end.  It is the part of the fault most likely to rupture and generate the next earthquake.  (See also        aperity.)

The recording from a seismograph.

:  Process whereby water-saturated sands, clays, or artificial fill suddenly become fluid upon shaking, as in an earthquake.

The force applied on an object per unit area, expressed in pounds per square fool (lb/ft²) or       kilograms per square meter (kg/m²). 

A seismic wave propagated by a shearing motion that involve oscillation perpendicular to the direction of travel.  It travels only in solids.

The time (in seconds) between passage of equivalent points on two consecutive waveforms.