SCEC Award Number 22085 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Understanding the spatial variation of high frequency radiation from earthquakes in Southern California Continuation Project
Investigator(s)
Name Organization
Peter Shearer University of California, San Diego Rachel Abercrombie Boston University
Other Participants
SCEC Priorities 1d, 2d, 4a SCEC Groups Seismology, GM, FARM
Report Due Date 03/15/2023 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 Our collaborative work matches a research priority of the Ground Motion (GM) Group and involves participation in the proposed stress-drop validation study (Baltay and Abercrombie, 2020; priority of Seismology and GM). Our longer-term aim is to develop an improved approach to estimate more accurate and reliable measures of source radiation (SCEC5 Q1), together with regional attenuation and site effects (SCEC5 Q4).
Broader Impacts Funding for this project partially supported a graduate student at UCSD.
Exemplary Figure Figure 1. Stress-drop estimates for 28,378 M 2 to 4 earthquakes in southern California from 1996 to 2019. Each earthquake has at least 10 M ≤ 1.6 calibration events (assumed to have Brune fc = 30 Hz) within 5 km in horizontal distance and 2 km in depth; thus results are only obtained in regions of relatively dense seismicity. Higher stress drops are plotted in blue, lower stress drops in red. Surface fault traces are plotted.