Aftershock

An aftershock is a smaller earthquake which comes after a large earthquake (called the main shock) in the same area. An aftershock is the opposite of a foreshock, which a small earthquake comes before the large earthquake.

Omori's Law

There are many laws which refer to aftershocks, but the most famous is Omori's Law. In 1894, Fusakichi Omori did some works on aftershocks, and firstly he came up with this rule:

[math]\displaystyle{ n(t) = \frac {K} {c+t} }[/math]

• "t" is time.
• "n(t)" is the rate of earthquakes in a time after the main shock
• "c" is a constant
• "K" is the size of the earthquake.

By 1961, Utsu wanted to change the rule. His rule looked like this:

[math]\displaystyle{ n(t) = \frac {k} {(c+t)^p} }[/math]

where "p" changes the rate, this mainly has a number between 0.7 and 1.5.

Aftershock Media

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  Distribution of aftershocks associated with the May 12th 2008 Sichuan earthquake through to 28th May

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  Aftershocks of the 2004 Indian Ocean earthquake, with data up to January 10, 2005.*This is an updated version of :Image:Neic slav fig72.jpg, with more recent data and smaller file size. Note USGS original is a GIF; file size is 60 kB vs 285 kB Source* *

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  Magnitude of the Central Italy earthquake of August 2016 (red dot) and aftershocks (which continued to occur after the period shown here)