SCEC Award Number 21165 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Development and Implementation of Full 3-Dimensional Waveform Tomography Applied to the Greater Los Angeles Region
Investigator(s)
Name Organization
John Vidale University of Southern California Thomas Jordan University of Southern California
Other Participants Brianna Birkel
SCEC Priorities 4a, 4c SCEC Groups CXM, Seismology, GM
Report Due Date 03/15/2022 Date Report Submitted 10/15/2024
Project Abstract
 The deep, soft sedimentary basin surrounding Los Angeles is a region of ongoing scientific interest and study, due to its overlying dense infrastructure and tendency to amplify 3-10s period seismic waves. In this study, we evaluate the accuracy of the latest seismic velocity models – CVM-S4.26.M01 and CVM-H 15.1.0 – by comparing observed seismograms from several recent moderate magnitude earthquakes to their synthetic counterparts.

These synthetic seismograms are computed via forward modeling simulation software using both the octree-based full-3D tomography Hercules toolchain (Taborda et al. 2016) and the finite-difference code developed by Rob Graves. In the Los Angeles basin, we see significant differences between observations and predictions, even at periods longer than 5 seconds and particularly within the 3-5 second period range. These differences are quantified using the Anderson 2004 goodness-of-fit metrics, as well as via direct waveform comparison.

We additionally identify smaller, more specific regions within the Los Angeles Basin that demonstrate the largest misfit and require more detailed study. These results suggest that earthquake hazard estimation in the Los Angeles basin will benefit from specific, focused improvements of the velocity models in this region.
Intellectual Merit Los Angeles mitigation is the primary rationale in the United States for understanding the amplification of shaking by large sedimentary basins. The basins are deep and extensive. The cities in the basin are replete with critical and aging buildings and infrastructure. The earthquakes are frequent. Lessons learned from Los Angeles will have application to similar earthquake-prone cities such as Seattle, Salt Lake City, and places with smaller basins such as the Bay Area and Portland. Structural models are now approaching the fidelity needed to model shaking to estimate seismic hazard down to roughly three seconds period. Engineers need estimates at even shorter period, which requires identification of shortcomings in the current models and paths toward improvement.
Broader Impacts The ultimate goal of this ongoing project is identification of regions within existing seismic velocity models that are in need of further study and improvement, as well as improvement of seismic velocity models that can be incorporated into Cybershake calculations of the seismic hazard for Southern California. Our work thus far identifies significant misfit between empirical and synthetic seismograms, which demonstrates the need for further studies of this kind.
Exemplary Figure Figure 4 shows a goodness-of-fit comparison between data and physics-based models across the LA Basin and includes an overlay of geomorphic provinces in the Los Angeles area.
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