HR: 1330h
AN: S52A-0127    [PDF]
TI: Finite Moment Tensors of Southern California Earthquakes
AU: Jordan, T H
EM: tjordan@usc.edu
AF: Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089 United States
AU: * Chen, P
EM: pochen@usc.edu
AF: Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089 United States
AU: Zhao, L
EM: zhaol@usc.edu
AF: Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089 United States
AB: We have developed procedures for inverting broadband waveforms for the finite moment tensors (FMTs) of regional earthquakes. The FMT is defined in terms of second-order polynomial moments of the source space-time function and provides the lowest order representation of a finite fault rupture; it removes the fault-plane ambiguity of the centroid moment tensor (CMT) and yields several additional parameters of seismological interest: the characteristic length $L_{\rm{c}}$, width $W_{\rm{c}}$, and duration $T_{\rm{c}}$ of the faulting, as well as the directivity vector $\mathbf{v}_{\rm{d}}$ of the fault slip. To formulate the inverse problem, we follow and extend the methods of McGuire et al. [2001, 2002], who have successfully recovered the second-order moments of large earthquakes using low-frequency teleseismic data. We express the Fourier spectra of a synthetic point-source waveform in its exponential (Rytov) form and represent the observed waveform relative to the synthetic in terms two frequency-dependent differential times, a phase delay $\delta \tau_{\rm{p}}(\omega)$ and an amplitude-reduction time $\delta \tau_{\rm{q}}(\omega)$, which we measure using Gee and Jordan's [1992] isolation-filter technique. We numerically calculate the FMT partial derivatives in terms of second-order spatiotemporal gradients, which allows us to use 3D finite-difference seismograms as our isolation filters. We have applied our methodology to a set of small to medium-sized earthquakes in Southern California. The errors in anelastic structure introduced perturbations larger than the signal level caused by finite source effect. We have therefore employed a joint inversion technique that recovers the CMT parameters of the aftershocks, as well as the CMT and FMT parameters of the mainshock, under the assumption that the source finiteness of the aftershocks can be ignored. The joint system of equations relating the $\delta \tau_{\rm{p}}$ and $\delta \tau_{\rm{q}}$ data to the source parameters of the mainshock-aftershock cluster is denuisanced for path anomalies in both observables; this projection operation effectively corrects the mainshock data for path-related amplitude anomalies in a way similar to, but more flexible than, empirical Green function (EGF) techniques.
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
DE: 7212 Earthquake ground motions and engineering
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting