HR: 1340h
AN: S13A-1034    [Abstracts]
TI: The Role of Stress in Causing High b-Value Regions in Aftershock Zones
AU: * Wiemer, S
EM: stefan@sed.ethz.ch
AF: ETH Zurich, Swiss Seismological Service, Schafmattstr. 30, HPP P5, Zurich, 9093 Switzerland
AU: Toda, S
EM: s-toda@aist.go.jp
AF: Active Fault Research Center, Geological Survey of Japan , Site 7, Higashi, 1-1, Tsukuba, 305-8567 Japan
AU: Woessner, J
EM: woessner@sed.ethz.ch
AF: ETH Zurich, Swiss Seismological Service, Schafmattstr. 30, HPP P5, Zurich, 9093 Switzerland
AB: Aftershock zones present an ideal environment for studying physical mechanism influencing the earthquake size distribution, or b-value, because of the high seismic activity and the sudden changes caused by a mainshock. Several recent studies have documented dramatic temporal and spatial heterogeneity of b within aftershock sequences of recent large events such as Landers, Hector Mines, Denali and Western Tottori, with b-values ranging from 0.5 to above 1.5. To improve our understanding of the link between mainshocks slip, resulting stress changes, aftershocks occurrence and their size distribution, we investigate the fine scale b-value distribution within several aftershock zones and compare it with results from stress tensor inversions. Several recent studies have speculated that areas of high slip during mainshocks subsequently show high b-values and vice versa. A first order observation is also that regions of high slip during the mainshock are also regions of high heterogeneity, in agreement the heterogeneous postseimic stress field hypothesis defined by Michael. We then map the rotations of the stress field near the rupture zone, observing significant rotations which are consistent with the predicted coseismic rotations of the principal stress axes in an elastic half space under an assumption of a 30-bar uniaxial NE-SW compression. In contrast, a region wedged between the Landers and Joshua Tree rupture zones, in which Coulomb stress increases for pre-existing strike-slip faults, show less rotation and low b-values. We propose a conceptual model where stress perturbations caused by the main shocks are on the order of the background regional stress field, thus allowing faults or cracks near the rupture zone to be activated which are in principal unfavorably oriented for rupture given the regional stress field. These events, however, occurring in a heterogeneous stress field, are generally small, leading to high b-values. In contrast, stress transfer to the surrounding areas mainly beyond the edges of the source fault increases differential stress, which promotes ruptures of moderate-to-large scale matured faults that are consistent with the tectonic stress field. This results in low b-value. The recovery with time of the stress field near the rupture zone to a more homogeneous state, which would coincide with a decrease in the b-values, would depend on the local loading rate. For the Landers region this process is still ongoing 14 years after the mainshocks.
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting