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Name: Michael S.
Status: other
Grade: other
Country: Japan
Date: Spring 2011

At what point in time is an aftershock no longer considered an aftershock to a previous earthquake, but is considered a new earthquake in and of itself (which will have its own aftershocks)?

Michael S.

An aftershock is a smaller earthquake that occurs after a previous large earthquake, in the same area of the main shock. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock.

Aftershocks tend to obey a number of empirical laws concerning magnitude and frequency.

There are three laws that address this aspect of aftershocks:

Omori's Law

Bath's Law, and

Gutenberg-Richter Law.

Please refer to the follow URL for detailed descriptions of these three laws:

Additional pertinent information from Wikipedia::

Aftershocks occur with a pattern that follows Omori's law. Omori's law, or more correctly the modified Omori's law, is an empirical relation for the temporal decay of aftershock rates. In 1894, Fusakichi Omori published his work on the aftershocks of earthquakes, in which he stated that aftershock frequency decreases by roughly the reciprocal of time after the main shock.

According to these equations, the rate of aftershocks decreases quickly with time. The rate of aftershocks is proportional to the inverse of time since the mainshock. Thus whatever the odds of an aftershock are on the first day, the second day will have 1/2 the odds of the first day and the tenth day will have approximately 1/10 the odds of the first day (when p is equal to 1). These patterns describe only the mass behavior of aftershocks; the actual times, numbers and locations of the aftershocks are 'random', while tending to follow these patterns. As this is an empirical law, values of the parameters are obtained by fitting to data after a mainshock has occurred, and they imply no specific physical mechanism in any given case.

Sincere regards, Mike Stewart

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