A GLOSSARY OF SEISMOLOGICAL TERMS

The earthquake that shocked Cebu this week claimed many lives and damaged many structures in different parts of the island. The Philippine Institute of Volcanology and Seismology stated that the quake was the strongest to be recorded in the province, registering nearly intensity 7 near Bogo City, with its effects felt as far as Bicol and Mindanao. With such a strong quake, many have naturally been using seismological terms in talking about the natural disaster. But what exactly are these terms and what do they mean? Here’s a roundup of seismological terms that’re worth one’s while to know.

- 

Accelerograph - a type of seismograph used to measure ground acceleration as a function of time.

Active fault - a fault along which a slip has occurred in recent geological time, or where earthquake foci are located.

Aftershock - an earthquake that occurs after a “mainshock” or larger earthquake.

Aftershocks occur in the same general region as the “mainshock” and result from readjustments of stress at places along the fault zone.

For great earthquakes, aftershocks may occur over hundreds of kilometers.

Depending on the size and depth of the earthquake, aftershocks may occur for many months after the mainshock; however, the rate of aftershock activity dies off quickly with time.

Array - an ordered arrangement of seismometers or geophones, the data from which is fed into a central receiver.

Aseismic - means not associated with an earthquake. Also used to indicate an area with no record of earthquakes: an aseismic zone.

Damping - loss of energy in wave motion due to transfer into heat by frictional forces.

Depth of an earthquake - a given depth value is the depth below the surface of the mean spheroid.

Dispersion (wave) - the spreading out of a wave train due to each wavelength travelling with its own velocity.

Duration (of strong shaking) - the time interval between the first and last peaks of strong ground motion above a specified amplitude.

Earthquake - the sudden release of stored elastic energy caused by the sudden fracture and movement of rocks along a fault. Some of the energy released is in the form of seismic waves, that cause the ground to shake.