Authors

Karen Felzer, USGS, kfelzer -@- gps.caltech.edu
Debi Kilb, IGPP/SIO/UCSD, dkilb -@- ucsd.edu

Project Description
We examine two unusual events that followed the Mw 5.2 earthquake near the town of Anza, California, on June 12, 2005. (1) Aftershocks stretched for at least 50 km along the San Jacinto Fault zone even though the mainshock fault was only ~4.5 kilometers long and (2) There was a Mw 4.9 earthquake 4 days later and 72 km away near the town of Yucaipa, California. In this paper we test the hypothesis that the extended Anza aftershocks were triggered by aseismic slip that followed the mainshock (Agnew and Wyatt, 2005) and determine the probability that the Yucaipa earthquake was triggered by the Anza earthquake. We test the aseismic slip triggering hypothesis by measuring the density of Anza aftershocks as a function of distance, r, from the mainshock fault. If a broad region of aseismic slip triggered these aftershocks we would expect a near constant density of events as a function of distance, whereas if the earthquakes are simply typical aftershocks we expect the density to decay as r^-1.37 (Felzer, 2006). We observe the latter. Based on expected aftershock production, local fault geometry, and a low catalog completeness magnitude threshold of Ml~0.5 we show that stochastic models of the Anza aftershock sequence can reproduce many of the characteristics of the observed Anza sequence. This suggests that the apparently long spatial extent of the Anza sequence simply stems from the unique ability of the dense ANZA seismic network to detect rarely recorded small aftershocks. To test whether the Anza mainshock triggered the Yucaipa earthquake we examine the ratio of the expected rate of Anza aftershocks and background seismicity at the time and location of the Yucaipa mainshock. We find that there is only a 13% chance that the Anza mainshock triggered the Yucaipa earthquake