SCEC Award Number 19163 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Testing and Reconciling Stress Drop and Attenuation Models for Southern California
Investigator(s)
Name Organization
Rachel Abercrombie Boston University Peter Shearer University of California, San Diego
Other Participants 1 UCSD Student
SCEC Priorities 1d, 2d, 4a SCEC Groups Seismology, GM, FARM
Report Due Date 04/30/2020 Date Report Submitted 04/27/2020
Project Abstract
 The aim of our collaborative work is to improve the quality and reliability of stress drop estimates (SCEC5 Q1) and attenuation models (SCEC5 Q4) for southern California, and beyond. This involves developing an improved, multi-scale approach for obtaining more precise, reliable stress drops with known uncertainties, and simultaneously inverting for attenuation.
Earthquake stress drop is a fundamental source parameter, implicit in many of the science goals of SCEC 5. Stress drops are now commonly estimated from seismic data, but are hard to measure reliably and well. The large uncertainties and scatter in results affect strong ground motion prediction, and also limit our understanding of the physics of the earthquake rupture process, including distinguishing induced seismicity from natural seismicity. We are combining the complementary approaches of the two PIs to improve resolution in stress drop estimates, and quantify uncertainties, and also investigate the effects of path dependent attenuation.
Shearer et al. (2019a) use a tight cluster of aftershocks of the 1992 Landers earthquake to show that limited frequency range causes trade-offs between modeling parameters, and consequently relative stress drop measurements are much more reliable than estimates of absolute values. Combination of high frequency recordings from the deep Cajon Pass borehole sensor and the Southern California Seismic Network (SCSN) reveals that high-frequency information can help constrain trade-offs between source and attenuation parameters (Shearer et al., 2019b). In addition, analysis of results from multiple published, consistent studies is showing assuming depth-independent attenuation can result in an apparent depth dependence to earthquake source parameters.
Intellectual Merit Earthquake stress drop is a fundamental source parameter, implicit in many of the science goals of SCEC 5. Stress drops are now commonly estimated from seismic data, but are hard to measure reliably and well. The large uncertainties and scatter in results affect strong ground motion prediction, and also limit our understanding of the physics of the earthquake rupture process, including distinguishing induced seismicity from natural seismicity. Our detailed investigation, and combination, of two complementary methods has revealed:
-Empirical Green's Function (EGF) approaches to resolve earthquake corner frequency suffer from insufficiently acknowledged parameter tradeoffs
-Relative stress drop measurements in compact seismicity clusters are well resolved and reveal changes in average stress drop over short distances
-Including wider frequency range, especially high-frequency observations with minimal attenuation, can help resolve these trade-offs.
-Analysis of results from previous studies in multiple regions confirms that assuming a single EGF for a wide depth range can map structural variation into source variation.
Broader Impacts The aim of our collaborative work is to improve the quality and reliability of earthquake stress drop estimates, which are fundamental to studies of earthquake physics, and also seismic hazard prediction. We have presented our results at SCEC (SCEC Contribution #9629), and also have a peer-reviewed publication in Journal of Geophysical Research – Solid Earth (SCEC Contribution #8916). The results of our comparison are already being implemented as improvements to a number of ongoing analyses, because both PIs are collaborating with multiple groups and analyzing earthquakes in many parts of the world. Our work therefore meets the both the specific and broader goals of SCEC, to use Southern California as a natural laboratory to improve our understanding of earthquake processes in Southern California and beyond.
Exemplary Figure Figure 2. Depth dependence within event spectra from spectral decomposition studies. (a) Synthetic spectra of large earthquake (solid) and small earthquake(dash) Brune-type source spectra affected by more attenuation (shallow, red) or less (deep, blue). (b) Spectral ratios of synthetic source spectra showing that the path-dependent depth effect (seen with one global EGF, black and magenta) cancels out if depth dependent EGFs are used (red and blue), (c) and (d) examples showing spectral ratios of stacked M2.25 event spectra divided by stacked M1.25 event spectra, showing the apparent depth difference assuming no depth-dependent attenuation (black and magenta) cancels, or decreases, when the depth-dependent EGF is used (blue and red).

Credits - the analysis is unpublished to date, but should be cited as Abercrombie et al., (in prep).
Also since the results from two independent studies are included, should also cite:
Hardebeck & Aron (2009) and Trugman and Shearer (2017).