A Unified Approach for Seismic Source Inversion and Full 3D Tomography
CHEN, P., ZHAO, L. and JORDAN, T. H., Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089
A central problem of seismology is the inversion of regional waveform data for models of earthquake sources and 3D Earth structure. In regions such as Southern California, preliminary 3D Earth models are already available, and efficient numerical methods have been developed for solving the point-source forward problem. We describe a unified inversion procedure that utilizes these capabilities to improve 3D Earth models and derive centroid moment-tensor (CMT) or finite moment-tensor (FMT) representations of earthquake ruptures. Our data are time- and frequency-localized measurements of the phase and amplitude anomalies relative to synthetic seismograms computed from the 3D elastic starting model. The procedure relies on the use of receiver-side Green tensors (RGT) and source-side earthquake wavefields (SEW), which comprise the spatial-temporal displacements produced by the three orthogonal unit impulsive point forces acting at the receiver and (approximate) earthquake hypocenters. We have constructed a RGT database for 33 broadband stations in the Los Angeles region using the SCEC Community Velocity Model (CVM) and K. Olsen's finite-difference code. Synthetic seismograms for any earthquake can be simply calculated by extracting a small, source-centered volume from the RGT database and applying the reciprocity principle. The partial derivatives needed for the CMT or FMT inversion can be generated in the same way. To set up the structural inverse problem, we compute the sensitivity kernels for the same data by convolving the SEW's with the RGT's. Using CVM as our starting model and the data and SEW's from 25 small earthquakes (3.0 < ML < 4.5), we have obtained a revised 3D model for the Los Angeles basin area. To our knowledge, this is the first "fully 3D" inversion of waveform data for regional Earth structure.