Aftershock definition
The Ramblin' Man
Is it the wave pattern, time of occurrence, focus location, or something
else? A few of us hydro types were chatting, and realized we really
didn't know how they are formally distinguished...
Thanks!
-Larry
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|Lawrence D. Matson | +---+ |
|Dept. of Geosciences | Central PA Glaciation Hazard is |LOW| Today |
|Penn State University | +---+ |
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Lucy Jones
>How is an aftershock defined? What separates it from another main shock?
> Is it the wave pattern, time of occurrence, focus location, or something
> else? A few of us hydro types were chatting, and realized we really
> didn't know how they are formally distinguished...
An aftershock is an earthquake that occurs when and where a seismologist
would expect it, based on our experience with previous aftershock
sequence. A little less irreverently, I recently wrote the following
definition for a general interest publication:
Lucy Jones
for myself
Earthquakes occur in clusters. One of the first things recognized about
earthquakes is that large events hardly ever occur alone. When one earthquake
happens, we usually see another at a nearby or identical location. To be able
to talk about this phenomenon, seismologists coined three terms -- foreshock,
mainshock, and aftershock. In any cluster of earthquakes, the one with the
largest magnitude is called the mainshock; anything before it is called a
foreshock and anything after it is called an aftershock. A mainshock can turn
into a foreshock if a subsequent event comes along with a larger magnitude.
Clustering of earthquakes usually occurs only very near the location of the
mainshock. The rupture surface that moves in the mainshock experiences a great
redistribution of the stress on it during the mainshock and it is that
disrupted surface that produces most of the aftershocks. Sometimes the change
in stress in the mainshock is great enough to trigger aftershocks on nearby
faults. However, the stress change dies off quickly with distance from the
fault so we rarely see aftershocks more than a few kilometers from the main
fault. As a rule of thumb, we say that aftershocks are other earthquakes
triggered at a distance from the mainshock fault no greater than the length of
that fault
.
In general, an earthquake large enough to cause damage will produce several felt
aftershocks within the first hour. The rate of aftershocks dies off quickly
with time so even the second day will have many fewer aftershocks than the
first. The daily rate of aftershocks is proportional to the inverse of time
since the mainshock. Thus the tenth day after the mainshock will have
approximately 1/10 the number of aftershocks that the first day had. We call an
earthquake an aftershock as long as the rate at which earthquakes are occurring
in that region is greater than the rate we saw before the mainshock. How long
that will be de pends on the size of the mainshock (bigger earthquakes have a
higher rate of aftershocks so it stays above background longer) and how active
the region was before the mainshock (if it was quiet, the aftershocks stay
noticeable longer).