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

Abstract

S U M M A R YWe 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.

Full‐3‐D tomography for crustal structure in Southern California based on the scattering‐integral and the adjoint‐wavefield methods

“…Distributions of the 160 earthquakes and 258 seismic stations used in this study. Red circles: epicenters of the earthquakes; beach balls: focal mechanisms of the earthquakes; white triangles: permanent broadband stations; white squares: temporary broadband stations; solid line magenta box: region of our F3DT study; dash line magenta box: region of the adjoint tomography conducted in Tape et al [2009Tape et al [ , 2010; background color: regional topography; black solid lines: major faults.…”

“…The summation of all adjoint kernels gave the gradient of the objective function. The gradient was then smoothed by convolving with a three-dimensional spatial Gaussian function [Tape et al, 2007[Tape et al, , 2010. The horizontal width and the vertical width of the Gaussian function were chosen separately to maximize the reduction of the objective function in that iteration.…”

“…[2007], and an upper mantle structure from finite-frequency teleseismic surface-wave tomography [Prindle and Tanimoto, 2006]. Notably, CVM-H11.9.1 incorporated the crustal structure obtained by Tape et al [2009Tape et al [ , 2010 from 16 AW-F3DT iterations of low-frequency earthquake waveform data. The CVM-H series is the velocity model native to SCEC's Unified Structural Representation, which also comprises the SCEC Community Fault Model [Plesch et al, 2007].…”

Global Imaging of the Earth's Deep Interior: Seismic Constraints on (An)isotropy, Density and Attenuation

“…The first regionalto continental-scale seismological applications of full waveform inversion indicate that resolution indeed improves (e.g. Chen et al, 2007;Fichtner et al, 2009;Tape et al, 2010). In particular, the amplitudes of the heterogeneities increase, and more small-scale features are robustly imaged.…”

Acceleration strategies for elastic full waveform inversion workflows in 2D and 3D

“…Previous studies have already presented results of 3D elastic full waveform inversion: [6,10,17,18,34,42,43,46]. In order to validate our approach in 3D, the SEG/EAGE Overhrust model is used.…”