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How to Measure Variations in High-Frequency Radiation for Small to Moderate Earthquakes

Peter M. Shearer, Rachel E. Abercrombie, & Wei Wang

Published August 14, 2019, SCEC Contribution #9629, 2019 SCEC Annual Meeting Poster #087

Many studies have shown that some small earthquakes radiate much more high-frequency energy than others. This variability is a primary contributor to differences in estimated radiated energy, stress drop, and GMPE residuals. Data mining of large waveform catalogs from local networks is key to understanding this important observable and its relation to earthquake dynamics. For many decades, researchers have used empirical Green’s function (EGF) approaches to correct body-wave spectra for attenuation and estimate corner frequencies and Brune-type stress drops. Recently, however, it has become increasingly clear that there are strong tradeoffs among parameters in the model fits, which may explain why stress-drop estimates of specific earthquakes show relatively poor agreement between different studies. Both synthetic examples and spectral analyses of seismicity clusters in southern California show that empirical approaches have too many degrees of freedom to determine reliably the corner frequency, without imposing constraints on the assumed high-frequency fall-off rate, the degree of scaling with moment, or the average corner frequency of the smallest earthquakes in the data set. We are exploring approaches to reduce these tradeoffs and uncertainties and produce more reliable corner-frequency estimates. These include: (1) New analyses of data from the Cajon Pass Borehole to constrain the spectral properties and high-frequency fall-off rate of small earthquakes to higher frequencies than surface stations can resolve. (2) Experiments in constraining small-earthquake spectral properties to yield more accurate Green’s functions and spectral shapes for the larger earthquakes in the dataset. (3) Performing joint inversions for source properties and Q structure, while limiting the role of empirical station terms, thus forcing the assumed Green’s function to be physically realistic. Our long-term goal is to use insights gained from these studies to devise optimal strategies for spectral analysis and studies of small-earthquake stress-drop variability for large datasets of distributed seismicity.

Key Words
earthquake variability, stress drop, high-frequency radiation

Shearer, P. M., Abercrombie, R. E., & Wang, W. (2019, 08). How to Measure Variations in High-Frequency Radiation for Small to Moderate Earthquakes. Poster Presentation at 2019 SCEC Annual Meeting.

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