HR: 0800h
AN: S51B-0997    [Abstracts]
TI: A New Paradigm for Inferring Stress using Focal Mechanism Orientations
AU: * Smith, D E
EM: desmith@gps.caltech.edu
AF: Caltech, 252-21 Seismological Lab Caltech, Pasadena, CA 91125
AU: Heaton, T H
EM: heaton_t@caltech.edu
AF: Caltech, 252-21 Seismological Lab Caltech, Pasadena, CA 91125
AB: We are extending stress modeling to explicitly include spatially heterogeneous stress in three dimensions. Our first observation is that if there is significant spatial heterogeneity, then popular focal mechanism inversions yield orientations biased towards the stressing rate not the stress. This occurs because when spatial heterogeneity is coupled to a stressing rate, the simulated region no longer becomes an unbiased, completely random sampler of the stress. The points that preferentially fail and are included in the inversions are those with stress orientations that on average align with the stressing rate orientation. This suggests that if stress in the real Earth is also spatially heterogeneous, which we have reason to believe it is, then standard stress inversion studies need to be reinterpreted to include this bias toward the stressing rate. The stressing rate can be any time perturbation to the region. Candidates include: 1) the build-up of stress on locked faults driven from far-field plate-tectonic motions, 2) post-seismic transients after a major earthquake, and 3) slow earthquakes. We define "significant" spatial heterogeneity by comparing the size of the heterogeneous stress to the spatially homogeneous stress (spatial mean). This ratio, which we will call, Heterogeneity Amplitude, describes how heterogeneous the system is. Observations of fault thickness, rupture velocity, and radiated energy indicate average stresses of less than 20 MPa, when averaged over dimensions of 10s of kms; therefore, we can parameterize our spatially homogeneous stress as < 20 MPa. At the same time, rock mechanics experiments indicate that stresses averaged over meters should exceed 100 MPa to nucleate earthquakes; therefore, short wavelength spatially heterogeneous stress could exceed 100 MPa. Using these two observations, it is possible that some regions could have a, Heterogeneity Amplitude of about 5, which yields a 0.65 bias. We measure bias as follows. If the total angular difference between the average stress orientation and the stressing rate orientation is omega (i.e., rotate through an angle omega about some rotation pole), then a bias of 0.65 means that the inverted tensor orientation is a little over half-way between the average stress and stressing rate orientations, a rotation of 0.65 omega from the average stress orientation. In some regions, we estimate the Heterogeneity Amplitude may exceed 10, which yields a bias > 0.9. At this level of bias, one cannot determine the orientation of the average stress, because the inverted tensor orientation is so closely aligned with the stressing rate orientation. This biasing effect also has important implications for the observed "stress rotations" after a mainshock. If heterogeneity is significant, then the orientations of the aftershocks will on average align with the perturbation to the system, not the background stress plus perturbation. We find: 1) Even if the background stress (spatial mean) is large compared to the stress perturbation, one could still find a measurable rotation of average focal mechanism orientations. 2) The average orientation of focal mechanisms will rotate back to the pre-mainshock orientation once the post-seismic transients decrease below the level of the plate-tectonic stressing rate.
UR: http://www.gps.caltech.edu/~desmith
DE: 4440 Fractals and multifractals
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7260 Theory
DE: 8164 Stresses: crust and lithosphere
SC: Seismology [S]
MN: Fall Meeting 2005