HR: 0800h
AN: S11A-1012    [Abstracts]
TI: How Heterogeneous Stress Biases the Orientations of Focal Mechanisms
AU: * Smith, D E
EM: desmith@gps.caltech.edu
AF: California Institute of Technology, 252-21 Seismolab Caltech, Pasadena, CA 91125 United States
AU: Heaton, T H
EM: heaton_t@caltech.edu
AF: California Institute of Technology, 252-21 Seismolab Caltech, Pasadena, CA 91125 United States
AB: We explore the effect of spatially heterogeneous stress on standard focal mechanism inversions. We ask the following questions: Does the presence of spatially heterogeneous stress, coupled with a stress rate of a particular orientation, bias which points fail as earthquakes? If there is a bias to which points fail and are consequently included in focal mechanism inversions, then will inversions, as they are currently being interpreted, primarily yield the orientation of the stress rate tensor (due to far-field plate motion) regardless of the other components in the stress field? We numerically address these questions by generating synthetic stress fields, bringing points to failure as a function of time under a plastic yield constraint, using the failed points to produce focal mechanisms/earthquake catalogs, and inverting the synthetic catalogs with standard inversion techniques [ Michael, 1984; Michael, 1987]. The total deviatoric stress field is composed of three terms, a spatially and temporally uniform background stress, a spatially uniform secular stress that increases linearly as a function of time and has an orientation different from that of the background stress, and a temporally uniform but spatially heterogeneous stress that is the summed effect of all past earthquakes, modeled statistically. If the heterogeneous stress has no preferred mean orientation and the added secular stress is much smaller than the background stress then one might assume under the current paradigms that the average orientation of the synthetic focal mechanisms to closely align with the orientation of the background stress. Interestingly, this is not what we find. We what we do find is the following: Standard focal mechanism inversions [Michael, 1984; Michael, 1987] accurately reproduce the average orientation of the stress tensors associated with the points which fail. However, in the presence of significant heterogeneous stress, which points fail and produce focal mechanisms are biased toward the orientation of the secular stress rate. We found that the ratio of the standard deviation of the heterogeneous stress to the amplitude of the background stress determines the orientation of the failure focal mechanisms. When the ratio is very small, i.e. the heterogeneity is much smaller than the spatially uniform background stress, the focal mechanisms on average (with very little orientation heterogeneity) are aligned with the background stress. When the ratio is very large, i.e. the heterogeneity is much larger than the spatially uniform background component, the focal mechanisms on average (which significant orientation heterogeneity) are aligned with the secular stress rate. Real earthquake data often displays significant focal mechanism heterogeneity suggesting that the Earth may have a large component of heterogeneous stress that biases the orientations of the focal mechanisms. This suggests that studies based on focal mechanism inversions may need to be reinterpreted. By comparing our simulation results to real data we have begun developing tools to reinterpret focal mechanism data in the context of heterogeneous stress. It is our hypothesis that in the end we may be able to estimate both the spatially uniform background stress component and the degree of stress heterogeneity.
DE: 7260 Theory and modeling
DE: 8100 TECTONOPHYSICS
DE: 8150 Plate boundary--general (3040)
DE: 8164 Stresses--crust and lithosphere
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2004 AGU Fall Meeting