

Time Limit on Earthquake Aftershocks
Name: Michael S.
Status: other
Grade: other
Country: Japan
Date: Spring 2011
Question:
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)?
Replies:
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
GutenbergRichter Law.
Please refer to the follow URL for detailed descriptions of these three
laws:
http://en.wikipedia.org/wiki/Aftershock
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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Update: June 2012

