Full 3D Tomography for the Crustal Structure of the Los Angeles Region

Abstract

We apply full 3D tomography (f3dt) to a regional dataset in the Los Angeles area. In f3dt, the starting model as well as the model perturbation is three- dimensional and the sensitivity (Frechet) kernels are computed using numerical schemes that account for the full physics of 3D wave propagation. f3dt can account for the nonlinearity of structural inverse problem through iteration, thus providing the most efficient means for assimilating seismic observations into dynamic ground- motion models. We have successfully applied a scattering-integral (si) formulation of f3dt to improve a 3D elastic structure model, the Southern California Earthquake Center (scec) Community Velocity Model version 3.0 (CVM3.0) with lowest shear velocity clamped at 1 km/sec, in the Los Angeles region. Our data are time- and frequency-localized measurements of phase-delay anomalies relative to synthetics computed from the 3D elastic starting model. The Frechet kernels for our measurements were computed by convolving the earthquake wave fields generated by point earthquake sources with the receiver Green tensors (rgts), which are the spatial- temporal fields produced by three orthogonal unit impulsive point forces acting at the receiver locations. We inverted 7364 phase-delay measurements of P and S body waves by using the lsqr method. The revised 3D model, LAF3D, provides substantially better fit to the observed waveform data than the 3D starting model. To our knowledge, this study is the first successful application of f3dt using real data in structural seismology.