Toda et al, JGR 2005

Summary. There is growing evidence that large earthquakes can inhibit or promote failure on nearby faults for decades to centuries, and that the transfer of stress plays a governing role in this interaction. Here we attempt to reproduce the distribution of mainshocks, aftershocks and surrounding seismicity observed during 1986-2003 in a 300 x 310 km area centered on the 1992 M=7.3 Landers earthquake. Our approach incorporates the static stress transferred by each M≥6 shock, after which seismicity evolves according to a law developed from laboratory rock mechanics experiments. For this model, Coulomb stress changes amplify the background seismicity, so small stress changes produce large changes in seismicity rate in areas of high background seismicity. Similarly, seismicity rate declines in the stress shadows are evident only in areas with previously high seismicity rates. Thus a key constituent is the background seismicity rate, which we smooth from the 5 years of seismicity preceding the test period. Finally, we offer a M≥5 earthquake forecast for 2005-2015, assigning probabilities to 324 10 x 10-km cells.

There are several ways we assess the model success. The mean correlation coefficient between observed and predicted shocks is 0.52 for 1986-2003, and 0.63 for 1992-2003; a control standard aftershock model yields 0.54 and 0.52 for the same periods. Four M≥6 shocks struck during the test period; three locate at sites where the expected seismicity rate falls above the 92 percentile, and one above the 75 percentile. At heart, we interpret our results to mean that earthquake stress changes do not simply turn on or off seismicity; rather, the background seismicity rate is enhanced by stress increases and suppressed by stress decreases. This, we believe, best explains why seismicity in stress trigger zones is often patchy or discontinuous; why seismicity rate declines in stress shadows are often subtle or absent, and why some aftershock zones expand, migrate or densify. While our model is a far cry from an earthquake prediction, it is, perhaps, on the road to the more useful and accurate earthquake forecasts that we all seek.