Earthquake Correlations

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Haiti, January 12, 2010, magnitude 7.0.  Chile, February 27, 2010, magnitude 8.8.  Baja, April 4, 2010, magnitude 7.0.  Sumatra, April 6, 2010, magnitude 7.7.   Are they related?  Although it is rather unlikely, the simple answer is that no one knows for sure, because it is a very difficult proposition to unequivocally prove. 

In 1977, a famous seismologist by the name of Don Anderson at Caltech proposed a model of “accelerated plate tectonics” that might account for large earthquakes highly correlated in time.  Anderson’s idea was essentially that the motions of the tectonic plates might not be steady as is generally supposed, but might instead be episodic in time.  In this idea, the convective forces that drive the plates might produce a kind of jerky motion that would explain the intermittent nature of large earthquakes.  Whether this would allow for worldwide correlation of earthquake events was a point not addressed by his model.

Another more recent idea is that earthquake occurrence can be described mathematically as equivalent to a kind of “phase transition”, like the sudden boiling of water, or the spontaneous magnetization of a bar of iron.  A major consequence of this idea is that earthquakes in a large region are correlated in space and time over distances that are several times larger than the largest earthquake in the region.  For Sumatra, this might be thousands of kilometers.  For California, perhaps 2000 km. 

Examples of these effects were seen in aftershocks of the 1906 San Francisco earthquake.  In a paper published in the Bulletin of the Seismological Society of America in 1997, Steeples and Steeples documented earthquake aftershocks that occurred within hours of the San Francisco earthquake in locations as far-flung as Paisley, Oregon, and Phoenix, Arizona. 

Global scale earthquake correlations need global scale earthquake interactions over many earthquake cycles, and examples of these do exist.  The 27 March 1964 Alaska magnitude 9.3 Alaska earthquake produced a measurable strain signal in Hawaii.  The recent Maule, magnitude 8.8 Chile earthquake produced a small, but measurable change in the length of day.  And all earthquakes larger than 6.0 produce earthquake waves and dynamic stress changes that are observed world-wide.

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