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A comparison of different methods of calculating source spectra and stress drop in Southern California

Rachel E. Abercrombie, Peter M. Shearer, & Daniel T. Trugman

Published April 19, 2017, SCEC Contribution #7268

The large uncertainties and variability in studies of earthquake stress drop affect strong ground motion prediction and limit our understanding of earthquake physics. With multiple researchers using a variety of different methods to compute and model source spectra, it is hard to compare results, or understand the origin of any discrepancies. To improve the quality and reliability of stress drop measurements in Southern California, we compare in detail two different approaches that analyze P-wave spectra.
Shearer et al. [2006] developed a large-scale, regional approach involving stacking and averaging P-wave spectra to obtain parameters for large catalogs of events. Abercrombie et al. [2017] developed a smaller-scale, EGF approach in an attempt to obtain the best possible results for the best-recorded earthquakes. To compare these methods, we focus on two test regions with dense seismicity, one near the Landers earthquake epicenter and one around Cajon Pass. The Landers region contains over 1000 aftershocks of the 1992 mainshock, and we are able to calculate P-wave corner frequency and stress drop estimates for several hundred of these using both methods. Initially, we applied the two approaches independently. There is a strong correlation between the results, but there is also significant scatter with differences up to a factor of ~1.5 in corner frequency. The results from the smaller-scale approach have a larger range of corner frequency and stress drop compared to the large-scale stacking approach, which is consistent with the averaging involved in the methods. We analyze the results to determine whether the differences are mainly for the less-well recorded events, or ones for which the methods included different data. To investigate the effects of specific steps in the procedures, we repeat the two analyses side by side, to test for differences in data selection criteria, spectral calculations, and EGF modeling.

Abercrombie, R. E., Shearer, P. M., & Trugman, D. T. (2017, 04). A comparison of different methods of calculating source spectra and stress drop in Southern California. Poster Presentation at Annual Meeting of the Seismological Society of America.

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