The Study of Earthquake Aftershocks

Earthquakes that are large enough to cause damage will usually be followed by one or more aftershocks. As a general rule aftershocks are smaller in magnitude than first or main earthquakes. If an aftershock is larger in magnitude it is then considered a main shock with the first or previous earthquakes being considered foreshocks.

Aftershocks from a large earthquake will decrease in a pattern or rate. The rate of decrease is in proportion to the inverse of the time since the main earthquake occurred. The pattern itself is reasonably predictable but the actual time, place and magnitude is not. Aftershocks can occur years after the main earthquake event.

Aftershocks occur when the main earthquake does not release all the built up stress with the initial event. While the major strain may be released, the plate edges may not have been able to pass each other completely or smoothly. The plates need to adjust their positions and realign themselves. Dynamic waves from a large earthquake can also trigger a series of aftershocks.

Aftershocks can also occur from stress that builds up in areas further away from the epicenter. As one area releases its stress, the stress may build elsewhere. Aftershocks can also occur on completely separate faults that are near enough in distance to build up pressure from the initial earthquake.

Geologists are trying to use the study of aftershocks as a way to predict the locations and sizes of future earthquakes. Seismologists have been unable to monitor the actual build up of stress deep in the earth and are using data from aftershocks and main earthquakes as a way to indirectly estimate the stress. Researchers are also studying seismic waves in an effort to further predict aftershocks.

By studying the patterns and frequency of aftershocks from medium and small size earthquakes researchers are able to determine the stress level of different parts of an earthquake region. Earthquakes from normal faults, thrust faults and strike-slip faults all have different levels of stress required for the event to occur. The aftershock patterns and time decay also vary according to the type of fault.

A model for studying aftershocks is being used by the Collaboratory for the Study of Earthquake Predictability or CSEP. Gathering enough data will take several years but it is hoped that this information will lead seismologists to better predict earthquakes in advance of their actual occurrence.　

References:

　

http://earthquake.usgs.gov/earthquakes/step/explain.php

http://www.sciencecodex.com/quake_prediction_model_developed

http://library.thinkquest.org/C003603/english/earthquakes/aftershocks.shtml