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Published January 2010 | Published
Journal Article Open

Seismic tomography of the southern California crust based on spectral-element and adjoint methods

Abstract

We iteratively improve a 3-D tomographic model of the southern California crust using numerical simulations of seismic wave propagation based on a spectral-element method (SEM) in combination with an adjoint method. The initial 3-D model is provided by the Southern California Earthquake Center. The data set comprises three-component seismic waveforms (i.e. both body and surface waves), filtered over the period range 2–30 s, from 143 local earthquakes recorded by a network of 203 stations. Time windows for measurements are automatically selected by the FLEXWIN algorithm. The misfit function in the tomographic inversion is based on frequency-dependent multitaper traveltime differences. The gradient of the misfit function and related finite-frequency sensitivity kernels for each earthquake are computed using an adjoint technique. The kernels are combined using a source subspace projection method to compute a model update at each iteration of a gradient-based minimization algorithm. The inversion involved 16 iterations, which required 6800 wavefield simulations. The new crustal model, m_(16), is described in terms of independent shear (V_S) and bulk-sound (V_B) wave speed variations. It exhibits strong heterogeneity, including local changes of ±30 per cent with respect to the initial 3-D model. The model reveals several features that relate to geological observations, such as sedimentary basins, exhumed batholiths, and contrasting lithologies across faults. The quality of the new model is validated by quantifying waveform misfits of full-length seismograms from 91 earthquakes that were not used in the tomographic inversion. The new model provides more accurate synthetic seismograms that will benefit seismic hazard assessment.

Additional Information

© 2010 Wiley. Accepted 2009 October 22; received 2009 October 18; in original form 2009 July 31. We thank Andreas Fichtner and Guust Nolet for reviews that improved this manuscript. This study would not have been possible without either seismic waveforms or parallel computing facilities. Seismic wave forms were provided by the Southern California Earthquake Data Center (SCEDC), the Northern California Earthquake Data Center (NCEDC), the Incorporated Research Institutions for Seismology (IRIS) and the University of Nevada Reno (Glenn Biasi) (see Table 1). All earthquake simulations were performed on the CITerra Dell cluster at the Division of Geological & Planetary Sciences (GPS) of the California Institute of Technology (Caltech). Egill Hauksson, Jeanne Hardebeck, Guoqing Lin, Don Helmberger and Shengji Wei provided unpublished earthquake source parameters. Paul Friberg executed the earthquake source inversions for the initial model. We thank Malcolm Sambridge for discussions and for suggesting the use of a source subspace projection method to compute the model update in the tomographic inversion. We acknowledge many valuable discussions with Albert Tarantola during his extended visits to Caltech (2007) and Princeton (2009). We utilized SAC and GMT software for processing seismograms and for plotting the figures (Goldstein et al. 2002; Wessel & Smith 1991). The Fortran90 software packages SPECFEM3D and FLEXWIN are available for download from the Computational Infrastructure for Geodynamics (www.geodynamics.org). Faria Chowdhury of the Southern California Earthquake Data Center constructed the interactive website (Section 8) for displaying cross-sections of the tomographic model. We acknowledge support by the National Science Foundation under grant EAR-0711177. This research was supported by the Southern California Earthquake Center. SCEC is funded by NSF Cooperative Agreement EAR-0106924 and USGS Cooperative Agreement 02HQAG0008. The SCEC contribution number for this paper is 1297.

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August 21, 2023
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