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:
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By 1961, Utsu wanted to change the rule. His rule looked like this:
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where "p" changes the rate, this mainly has a number between 0.7 and 1.5.