Annual Meeting - SCEC.org

Poster Abstracts | Group 2 – USR
relatively simple (vertical and planar) or more complex (non-vertical and non-planar)--have been
proposed.
2-004
TERA3D: A TERA-SCALE FULL-3D WAVEFORM TOMOGRAPHY (F3DT) IN
SOUTHERN CALIFORNIA Chen P, Allam AA, and Jordan TH
We are automating our full-3D waveform tomography (F3DT) and seismic source inversion
algorithm based on the scattering-integral (SI) method and applying the automated algorithm to
iteratively improve the 3D SCEC Community Velocity Model Version 4.0 (CVM4) in Southern
California. In F3DT, the starting model as well as the derived model perturbation is 3D in space
and the sensitivity kernels are calculated using the full physics of 3D wave propagation. The SI
implementation of F3DT is based on explicitly constructing and storing the sensitivity kernels for
individual misfit measurements. Compared with other F3DT implementations, the primary
advantages of the SI method are its high computational efficiency and the ease to incorporate 3D
Earth structural models into real-time seismic source parameter inversions. In a previous study, we
have successfully applied the SI method to improve the 3D seismic velocity structure model (SCEC
CVM3) and seismic source models in the Los Angeles region. In this presentation we will report
our recent progresses on automating the complete F3DT workflow, including an automated phasepicking
algorithm, and extending our analysis to a much larger region in Southern California.
2-007
STRONG SOUTHERN CALIFORNIA CRUSTAL HETEROGENEITY REVEALED BY
ADJOINT TOMOGRAPHY Tape CH, Liu Q, Maggi A, and Tromp J
Adjoint tomography utilizes 3D simulations of seismic wave propagation in conjunction with a
tomographic technique based on adjoint methods. We begin with an initial 3D model of shear and
compressional wavespeeds for southern California provided by the Southern California
Earthquake Center (SCEC; model CVM-H), extending to a depth of 60~km. We use the spectralelement
method to simulate 140 good-quality local earthquakes, each recorded by as many as 160
stations. We compute misfits between observed and synthetic seismograms by using a new
automated time-window selection algorithm that picks any time window within which the data
and 3D synthetics are reasonably similar (e.g., P, S, Love, and Rayleigh waves). Within each time
window we measure a frequency-dependent traveltime anomaly. For each record with a
measurement, we compute an adjoint source that is used to create an adjoint wavefield. The
interaction between the adjoint wavefield and the regular wavefield forms the gradient of the
misfit function for one event. These gradients are combined using a source subspace projection
method to compute a model update.
We are presently on the seventh iteration of a southern California crust and upper mantle model.
Thus far we have applied changes in excess of +/- 20 percent from the initial 3D model. With each
iteration, the changes in wavespeeds have improved the data fit, and we are able to include
additional seismograms whose fits to the data for previous model iterations were too poor for
selection. In general, the tomographic results compare well with surface geology, the most striking
features being the low wavespeeds of the southern San Joaquin basin, the high wavespeeds and
depth extent of the Santa Monica Mountains, the low wavespeeds in the Coast Ranges, the low
wavespeeds in the eastern Mojave shear zone, and the sharp contrast at the eastern front of the
Sierra Nevada due to volcanism in the Coso Junction area. Having applied relatively large-scale,
large-amplitude changes, we can now exploit more of the shorter-period measurements to resolve
and reveal km-scale features in the southern California crust.
144 | Southern California Earthquake Center