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GEOLOGY
Selected terms used in
the study of earthquakes and Seismology

aftershock

A series of earthquakes, usually of a lower magnitude that the main shock, which follow the first or main shock of an earthquake.

asperity

A rough spot (or "knot") along a fault which stops movement along that section for a period of time. The more pronounced the asperity, the more stress which will develop and the larger the earthquake which will eventually occur.

dilatancy-diffusion model

A model for explaining certain earthquake precursory phenomenon. One example is changes in P-wave velocities.

elastic rebound theory

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 along opposing sides of the fault.

epicenter

The point atthe surface of the earth directly above the focus of an earthquake.

fault

Fracture in the earth along which there has been displacement.

fault creep

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

first motion studies

Seismological study of an earthquake that determines areas of compression and dilation adjascent to the fault plane which indicates the direction of motion on the causing fault.

focus (foci)

The point within the earth where an earthquake originates. The focus is identified by longitude, latitude, and depth beneath the surface.

intensity scale

An earthquake rating scale, ranging from I through XII, based upon subjective reports of human reactions to ground shaking and upon the damage caused by an earthquake.

Isoseismal

Lines on a map which enclose areas of equal earthquake shaking based upon an intensity scale.

longitudinal wave

(Also known as P-waves or compressional waves.) A type of seismic wave, resembling a sound wave, involving alternating particle compression and expansion in the direction of wave propagation.

L-wave

Seismic waves which travel at the surface of the earth. These waves, the most damaging of the earthquake waves, are generated by the unreflected energy of P- and S-waves striking the earth's surface.

magnitude

The measure of the strength or strain energy released by an earthquake at its source.

Modified Mercali Scale (MM)

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

moment magnitude (M)

A scale of seismic energy reveased by an earthquake based on the product of rock rigidity along the fault, the area of rupture on the fault plane, and the amount of slip.

paleoseismicity

The rock record of past earthquake events in displaced beds and liquifaction in trenches or natural outcrops.

precursor

Observable phenomena which occur before an earthquake or other natural event and indicate that an event is soon to occur.

recurrence interval

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

resonance

The tendency of a structure to vibrate with maximum amplitude when the frequency of the applied seismic waves is the same as the vibrating body's natural frequency.

Richter Magnitude scale

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

seismic gap

The segment along an active fault which has been inactive seismically relative to segments at either end. This is the area of a fault most likely to rupture and produce the next earthquake.

seismograph

The instrument used to measure and record seismic waves.

seismogram

The recording from a seismogram.

spontaneous liquefaction (liquifaction)

The process where water-saturated porus materials (sands, clays, and landfills) suddenly become fluid when being shaken as in an earthquake.

stress

The force applied on an object as a function of its unit area.

strain

Deformation of a material resulting from an applied force. Strain may be elastic (recoverable) or ductile (nonrecoverable).

transverse (shear) wave

(Also known as S-wave.) A seismic wave propagated by a shearing motion which involves oscillation perpindicular to the directing of travel. These waves only travel in solids.

tsunami

A large sea wave which is produced by a submarine earthquake, landslide, or volcanic eruption. Commonly called a tidal wave.

wave period (T)

The length of time (in seconds) between passage of two equivalent points on a waveforms (usually peak-to-peak).

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