April 12, 2011

On the Recent Japan Earthquake Sequence

Posted by Evelyn Mervine

Note: This is a guest post by my friend, Jean-Arthur Olive. Arthur volunteered to make these figures and write this post in response to my concern about the recent magnitude 6.6 earthquake that prompted an evacuation and knocked out power at the Fukushima Daiichi nuclear power plant facility for the better part of an hour. For more information on recent events at Fukushima, see: 
Arthur’s Post: 
The March 11th (magnitude 9.0 – ‘mainshock’) earthquake followed a 2-day sequence of increased seismicity that included a strong (magnitude 7.2) earthquake. Such an event is commonly referred to as a ‘foreshock’, although its role as a trigger for the mainshock is not clear.
Click figure to view larger.
In the wake of the magnitude-9.0 earthquake, numerous aftershocks were triggered by the readjustment of stresses on the fault. In map view, the location of all these events roughly outlines the region that ruptured on March 11th.
Click figure to view larger.
Aftershocks may keep happening for years, decades, or even more. In fact, after a long time, it becomes less and less clear whether an earthquake can be considered as an aftershock of some prior large event.
In 1894, Japanese seismologist Fusakichi Omori discovered that the number of aftershocks per day following a large earthquake is inversely proportional to the time elapsed since the earthquake. This is termed Omori’s law. The Japanese sequence seems to fit this empirical prediction rather well, which means that aftershocks should become less and less frequent over the next few months. However, we have no way of predicting what the magnitudes of these aftershocks might be.

Click figure to view larger.

Note: All the data plotted in the figures is from the ANSS composite earthquake catalog.