HR: 0830h
AN: S41C-0099    [PDF]
TI: The Apparent Stress Profile for Normal-Fault Earthquakes Across Subduction Zones
AU: * Choy, G L
EM: choy@usgs.gov
AF: U.S.G.S., Box 25046, MS 966, Denver, CO 80225 United States
AU: Kirby, S
EM: skirby@usgs.gov
AF: U.S.G.S., 345 Middlefield Rd, MS 977, Menlo Park, CA 94025 United States
AB: The behavior of apparent stress for normal-fault earthquakes across the various tectonic regimes of a subduction zone complex is derived by examining the apparent stress ($\tau_a = \mu E_S/M_0 $, where $E_S$ is radiated energy and $M_0$ is seismic moment) of 139 large global earthquakes with depth $< $70 km that occurred from 1987 to 2001. Using accurately determined hypocenters to delineate Wadati-Benioff zones in fine detail, we are able to ascribe the hypocenters to specific tectonic features. The highest and most anomalous values of $\tau_a$ (up to 5 MPa) are found in the depth range 35-70 km. These events are characteristically intraslab and in proximity to zones of intense deformation such as a sharp slab bend or the juncture of multiply convergent slab systems. Events with high $\tau_a$ ( $>$ 1 MPa) at shallow depths between 10-35 km are generally intraslab, in cold slabs, and near plate boundaries with oblique convergence or local contortion. Low $\tau_a$ ( $< $1 MPa) is associated with events occurring between the outer rise and the trench axis, as well as with intracrustal events behind the trench axis. The average apparent stress of intraslab normal-fault earthquakes is considerably higher than that of interplate thrust-fault earthquakes. This suggests that lower stress drops are needed to rupture plate interfaces that have been made smoother by cumulative total displacements (from hundreds to thousands of kms). In contrast, intraslab normal-fault earthquakes generally occur in cold and intact lithosphere along which total fault displacements have been much less (from hundreds of meters to a few km). We have identified earthquake pairs in which an interplate-thrust and an intraslab-normal earthquake occurred remarkably close in space and time. The intraslab-normal member of each pair radiated anomalously high amounts of energy compared to its thrust-fault counterpart. These earthquakes are dramatic examples in which $M_e$ (energy magnitude) is a better estimate of the potential for earthquake damage than $M_W$. This discovery may help explain why loss of life due to intraslab earthquakes was greater in the 20th century in Latin America than the fatalities associated with interplate thrust events that represented much higher total moment release.
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting