When will the aftershocks end? 


How long will the aftershocks of the 2011 Tohoku Earthquake continue? Is it usual following a huge earthquake for aftershocks to occur this much? (Male, 20s, Chiba)


After a gigantic earthquake, there are many small earthquakes in and around the area called aftershocks.

The number of aftershocks will reduce with time. Roughly, after 2 days; 1/2, after 10 days; 1/10, after 20 days; 1/20, after 100 days; 1/100. The bigger the main shock, the more aftershocks over a longer period of time.
For example, the March 11 Earthquake, counting only aftershocks bigger than a magnitude of 5 on the Richter scale, there were 127 aftershocks on March 11, then 77 on March 12, and 33 on March 13. It is supposed from this progression that 3 or 4 aftershocks bigger than M5 could be experienced per day even at the end of March -20 days after main shock. However, it is not clear how they occur, and how long the aftershocks will last.
Here is a comparison list of past earthquakes and their largest aftershocks.

It can be said:
(1) The bigger the main shock, the bigger the aftershock magnitude. (The difference of magnitude between the main shock and the largest aftershock is about M1.) Moreover, the number of large aftershocks are small and the number of small aftershocks are large.
(2) The interval between the main shock and the largest aftershock is likely to be short, when the main shock occurs in the seabed.

Science Communicator: Yuko Okayama

Supervised by Japan Geoscience Union