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Focal mechanism effects on S/P amplitude ratios in southern California

Janine Buehler, Debi Kilb, Frank L. Vernon, Wei Wang, & Peter M. Shearer

Published August 15, 2017, SCEC Contribution #7796, 2017 SCEC Annual Meeting Poster #267

Ground Motion Prediction Equations (GMPEs) are key in seismic­ hazard analysis and incorporate information about the earthquake source, the source-receiver path, and influences from the near­-station region. Ground motion variations that result from sampling different parts of the earthquake focal sphere are not typically accounted for in GMPEs, and the spatial distribution of observed peak amplitudes often fails to show a clear radiation pattern, especially at higher frequencies and longer ranges. Here we investigate the location and frequency dependence of focal mechanism effects on observed ground motions in the San Jacinto fault zone region in southern California and when it might be important to include the influence of source radiation patterns. We identify clusters of earthquakes with similar focal mechanisms and good azimuthal station coverage recorded by the ANZA network and examine their S/P amplitude ratios. For each source-­station pair we compute the peak absolute S/P amplitude ratio and compare these observed ratios with theoretically derived values. Although there is generally large scatter in the observed data, we find clusters of events with azimuthal variations in their S/P ratios that are correlated with theoretical values. We study these clusters in more detail, examining to what extent the S/P ratios, and their azimuthal patterns, change for different source-­station distances and frequency bandwidths (the radiation-pattern signature generally seems clearest for measurements below 2 Hz). Finally, we explore if the observed amplitude characteristics can be replicated using regional scattering models.

Buehler, J., Kilb, D., Vernon, F. L., Wang, W., & Shearer, P. M. (2017, 08). Focal mechanism effects on S/P amplitude ratios in southern California. Poster Presentation at 2017 SCEC Annual Meeting.

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