SCEC Award Number 11034 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Validation of lateral reflectors in southern California using time reversal seismic imaging
Investigator(s)
Name Organization
Carl Tape University of Alaska, Fairbanks
Other Participants John Shaw, Andreas Plesch
SCEC Priorities B4, B3, C SCEC Groups USR, Seismology, CS
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
 The objective of this project was to identify structures in southern California that are responsible for the generation of laterally reflected surface waves. We analyzed three-component waveforms from 32 high-quality crustal earthquakes containing high signal-to-noise ratios for the post-surface-wave unknown waveforms. Our bandpass period range of interest was 3-10 seconds; this corresponds to surface waves sensitive to structures in the uppermost 10 km. We developed and applied two tools to facilitate a search for laterally reflected surface waves: (1) seismic imaging with impedance sensitivity kernels (``imaging kernels'') and (2) polarization analysis of three-component waveforms. Our imaging kernels illuminated several probable lateral reflectors in proximity to: the southernmost San Joaquin basin, the Los Angeles basin, the San Pedro basin, the Ventura basin, the Manix basin, the San Clemente-Santa Cruz-Santa Barbara ridge, and isolated segments of the San Jacinto and San Andreas faults. Our polarization analysis suggests that some exotic seismic phases can be identified from the data alone, and this will help guide the selection of waveforms used in the constructing the imaging kernels. The work marks a step toward identifying seismically relevant structures in California, and also toward using a new class of waveforms for seismological investigations of source and structure.
Intellectual Merit Modeling full waveforms in the bandpass 3-10 seconds, including post-surface-wave phases, is uncharted territory for seismology. There is unequivocal evidence that ample post-surface-wave waveforms are produced by structural effects that are not accurately represented by our three-dimensional models. We demonstrate that a combination of sophisticated seismic imaging and a detailed polarization analysis can begin to reveal both the key reflectors and the corresponding exotic waveforms. This is a new test in the topic of time-reversal seismic imaging.
Broader Impacts The aim of the project is to improve our representation of southern California structure, in order to obtain better predictions of ground motions for future (and past) earthquakes. We have concurrently worked to improve the documentation and distribution of the CVM-H model.
Exemplary Figure Figure 3 of the report is representative of this effort.
Caption: Composite representation of an impedance kernel (a) and a polarization analysis of one of the stations used in constructing the kernel (b). The gray time window in (b) is marked by changes in all polarization indices; we interpret this as a laterally reflected Rayleigh wave.