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Meetings: Abstracts |
We examine two apparently unusual events that followed the MW 5.2 earthquake near the town of Anza, California, on June 12, 2005. (1) Although the mainshock fault was only several kilometers long, aftershocks stretched for at least 50 km along the San Jacinto Fault zone; and (2) A MW 4.9 earthquake occurred 4 days later and 72 km away, near the town of Yucaipa. We test the hypotheses that the extended Anza aftershocks were triggered by aseismic slip that followed the mainshock (Agnew and Wyatt, 2005) and that the close space/time proximity of the Anza and Yucaipa earthquakes was a coincidence. To test the aseismic slip triggering hypothesis we measure the density of the Anza aftershocks as a function of distance, r, from the mainshock fault. If a broad region of aseismic slip triggered these earthquakes we might expect a near constant density, whereas if the earthquakes are typical aftershocks we expect the density to decay as r-1.4 (Felzer and Brodsky, 2005). We observe the latter. Stochastic models of the Anza aftershock sequence based on normal aftershock production, local faulting geometry, and excellent local catalog completeness also look very similar to the observed sequence. This suggests that the apparently long spatial extent of the Anza sequence resulted merely from the dense Anza seismic network that provided the unique ability to catalog the very small aftershocks, which are not usually recorded by regional networks. To test whether the Anza mainshock triggered the Yucaipa earthquake we extrapolate local aftershock decay relationships in time and space to estimate the probability of the mainshock triggering a M(4.9 earthquake near Yucaipa after 4 days. We compare this to the probability of the Yucaipa earthquake occurring randomly. We find that the Anza mainshock roughly doubled the probability of a large earthquake occurring at Yucaipa, indicating a 50% chance that the Anza mainshock triggered the Yucaipa earthquake. We conclude that the extended Anza aftershock sequence is not out of the norm, and that there is a 50-50 chance that the Anza mainshock triggered the Yucaipa event. For more information please see http://eqinfo.ucsd.edu/~dkilb/June2005.html
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Last Update: 2005-01-27