Update July 19, 2010: Earthquake Forecasts for San Diego and Los Angeles

john's picture

In a blog posted on July 8, 2010 ("A New Type of Forecast') we provided figures showing the time history of magnitude M>7 earthquakes within 150 miles of San Diego and within 1 year of the time indicated on the horizontal axis. 

it seems timely to update that figure, and to provide another such forecast for the Los Angeles area. 

In the first plot below, readers will find plots similar to those shown for San Diego in the earlier blog, and should refer to that blog for a more detailed explanation as to how to read plots like these.  There are slight differences:  we continually try to improve our methods, and in addition, there are continual, small changes to the ANSS seismic catalog that we use to compute the probabilities. 

In this San Diego plot, the current probability is seen to be 6.9%, a relatively rapid increase from the probability of about 3.4% just after the M7.2 Mexicali earthquake on April 4, 2010.  That earthquake evidently relieved a portion of the tectonic stress in the region, but the many small earthquakes indicate that much of the stress has been transferred to other areas in southern California.  Just prior to the Mexicali earthquake, the probability had been calculated to be about 18.0%.

In the second plot below, we provide a similar plot for a circular region of radius 150 miles around Los Angeles.  The red curve in this plot is the probability that a magnitude M>7 earthquake will occur within 1 year from the date indicated on the horizontal axis.  In this figure, the current probability is seen to be about 9.9%, an increase from the value of 8.8% calculated just after the April 4 earthquake.  Prior to the earthquake, the calculated probability had been 12.1%, so the Mexicali earthquake was seen to have less of an effect near Los Angeles, which is farther away from the epicenter, than near San Diego, which is closer.



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