Here what the guy on NBC news said about "earthquake clusters" this morning...


Earthquake clusters
John Rundle, an expert on earthquake dynamics at the University of California at Davis, said evidence is mounting that major earthquakes do cluster in space and time, but that the New Zealand quakes were too small to count (magnitude 6.3 for Christchurch) in this current spate of events.

Even taking out the New Zealand events, there are still four major earthquakes at least as big as magnitude 8.6 in the last seven years – the two events in Sumatra in 2004 and 2005, the Chilean earthquake in 2010, and Japan. This is unusual, Rundle said.

The last big event on the San Andreas in southern California was in 1857 and historical analyses show they occur about every 160 years, "which would lead one to think we are pretty much due for one of these magnitude 7 plus events on Carrizo Plain, which could obviously grow into a magnitude 8 if we are unlucky," Rundle said.

In Northern California, the biggest threat is on the Hayward fault, which last ruptured in 1868 and the recurrence interval is about 170 years. "So, we are definitely in the window where we could see a Hayward event," he noted.

Examined in this light, the west coast is due for a major quake whether or not a major event on one side of a tectonic plate can trigger a major event on the other side. Rundle noted that according to his modeling, there is a correlation between the clustering activity and the periodicity of when faults rupture. "There is a definite association," he said.



Here is the rest of it:

http://cosmiclog.msnbc.msn.com/_news/2011/03/14/6268123-is-japans-quake-part-of-a-cluster