SCEC Award Number 21082 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Understanding the spatial variation of high frequency radiation from earthquakes in Southern California
Investigator(s)
Name Organization
Rachel Abercrombie Boston University Peter Shearer University of California, San Diego
Other Participants
SCEC Priorities 1d, 2d, 4a SCEC Groups Seismology, GM, FARM
Report Due Date 03/15/2022 Date Report Submitted 07/07/2023
Project Abstract
 Earthquakes of similar moment often radiate very different amounts of high-frequency radiation, and quantifying this variation and resolving its causes are vital to understanding the underlying rupture physics and producing reliable ground motion prediction equations (GMPEs). These energy variations are commonly modeled in terms of earthquake stress drop, a fundamental source parameter implicit in many of the science goals of SCEC5, but challenging to measure. During our SCEC collaboration, the PIs have investigated the origins of the large uncertainties and scatter in stress-drop estimates by comparing different approaches to analysis of P-wave spectra from small to moderate earthquakes in Southern California. We have gained an understanding of the limitations of the methods and how they can be improved, most recently identifying biases in the calculation of the spectra and the correction for path attenuation. One of our insights is that stress-drop estimates rely heavily on certain modeling assumptions, which are difficult to verify from observations even with high-quality regional network data. Thus, we favor measures of high-frequency radiation that are closer to the data and rely less on specific theoretical rupture models or poorly-constrained empirical corrections.
Intellectual Merit Earthquakes of similar moment often radiate very different amounts of high-frequency radiation, and quantifying this variation and resolving its causes are vital to understanding the underlying rupture physics and producing reliable ground motion prediction equations (GMPEs). These energy variations are commonly modeled in terms of earthquake stress drop, a fundamental source parameter implicit in many of the science goals of SCEC5, but challenging to measure. This project has investigated the origins of the large uncertainties and scatter in stress-drop estimates by comparing different approaches to analysis of P-wave spectra from small to moderate earthquakes in Southern California. Our collaborative work matches a research priority of the Ground Motion (GM) Group and involves participation in the community Ridgecrest stress-drop validation study.
Broader Impacts Our SCEC research funds have provided partial support for a UCSD graduate student.
Exemplary Figure Figure 1. Comparison of relative source parameters vs. P velocity, with and without depth correction (Abercrombie et al., 2021). The ratios of re-calculated corner frequencies between the deep and shallow earthquakes from 18 regions (colors and symbols, multiple symbols represent different magnitude bins, and alternative velocity models.), are plotted against the ratio of the average P-wave velocity at the depths of the respective deep and shallow event populations. The black dashed line represents a constant stress drop with depth, if rupture velocity is proportional to P-wave velocity.
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