Testing and Reconciling Stress Drop and Attenuation Models in Southern California

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

Submitted August 15, 2016, SCEC Contribution #6793, 2016 SCEC Annual Meeting Poster #222

Earthquake stress drop is a fundamental source parameter, implicit in many SCEC science goals. It is relatively easy to estimate from seismic data, but hard to measure reliably and well. The large uncertainties and scatter in results affect strong ground motion prediction and limit our understanding of the physics of the earthquake rupture process. In addition, improved stress drop estimates will help in determining if there are any systematic differences in rupture properties between natural and induced earthquakes, and among background seismicity, swarms, foreshocks, and aftershocks. 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. Our goal is to investigate sources of consistency and discrepancies, and to better quantify uncertainties in stress drop estimates. P-wave spectra also contain information about attenuation and other path effects that can be compared to existing models of attenuation in southern California. Our longer-term aim is to develop an improved approach to invert jointly for more accurate and reliable stress drops, together with regional attenuation and site effects.

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 [2013, 2014, 2015a] developed a smaller-scale, more-detailed approach in an attempt to obtain the best possible results for a small number of 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 the Cajon Pass borehole, in which previous results predict spatial variability in both earthquake stress drops and attenuation. Here we present preliminary results for the Landers region, which contains over 1000 aftershocks of the 1992 mainshock. Of these, we focus on several hundred earthquakes for which we were able to obtain P-wave corner frequency and stress drop estimates using both methods.

Eventually we plan to: (1) determine precise (relative) parameters for the best-recorded events, (2) quantify uncertainties in the measurements, (3) update the Shearer et al. [2006] study to include post-2001 events throughout southern California, and (4) investigate azimuthal variations in the derived path-correction functions to determine if they are consistent with the Hauksson and Shearer [2006] attenuation model for southern California.

Key Words
stress drop, corner frequency, attenuation

Shearer, P. M., Abercrombie, R. E., & Trugman, D. T. (2016, 08). Testing and Reconciling Stress Drop and Attenuation Models in Southern California. Poster Presentation at 2016 SCEC Annual Meeting.

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