Evidence for Mogi doughnut behavior in seismicity preceding small earthquakes in southern California

We examine the average space-time behavior of seismicity preceding M 2-5 earthquakes in southern California from 1981 to 2005 using a high-resolution catalog and identify regions of enhanced activity in a 1-day period preceding larger earthquakes at distances comparable to their predicted source radii. The difference in precursory behavior between large and small earthquakes is subtle but statistically significant when averaged over many earthquakes, and it has similarities to the "Mogi doughnut" seismicity pattern observed to occur prior to some M 6 and larger earthquakes. These results indicate that many standard earthquake triggering models do not account for all of the processes involved in earthquake occurrence.