HR: 0800h
AN: S11A-0996    [Abstracts]
TI: Deep Seismicity as a Proxy for Crustal Strength, a Case Study of the 1992 Landers Aftershock Zone
AU: * Raphael, A J
EM: araphael@memphis.edu
AF: Center for Earthquake Research and Information, University of Memphis, 3876 Central Ave., Memphis, TN 38152 United States
AB: Strength is the stress required to cause permanent deformation in a rock. Strength may not be constant, but may vary with time. For example, because crustal strength depends in part on strain rate, the rock strength of a region may be temporarily altered in response to a large earthquake. Are such changes observable? I modeled crustal strength of the 1992 Landers aftershock zone considering the thermal state and changes in strain rate. These models, one for the northern and southern end of the aftershock zone, indicate that a small increase in the strain rate of a region has the potential to lower the depth of the brittle-ductile transition of a region, temporarily allowing earthquakes to occur deeper than before the Landers earthquake. I compared the pre-mainshock seismicity and the aftershock seismicity using the beta statistic of Matthews and Reasonberg (1988). The Southern California Seismic Network (SCSN) "A" quality events were divided into a fine grid for several time periods before and after the 1992 Landers earthquake. Comparing the pre and post-mainshock time periods revealed a small, but significant, increase in the depth of seismicity. Comparing distributions of the beta statistic between several post-shock time windows shows that the seismicity of the Landers region appears to consistently decrease in rate and to shallow over the course of 210 days following the earthquake. However, the northern and southern ends of the Landers aftershock zone behaved differently. The northern end had a faster decrease in the rate of deep aftershocks than the southern end, possibly as a function of an increase heat flow or possibly related to the directivity of the rupture itself.
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2004 AGU Fall Meeting