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Comparison of Stress Drop Variation of M3.8-4.2 Earthquakes Between 0-12 km Depth Using Four Independent Methods on 2019 Ridgecrest, CA Data

Arjun Neupane, Christine J. Ruhl, & Rachel E. Abercrombie

Published August 16, 2021, SCEC Contribution #11572, 2021 SCEC Annual Meeting Poster #231

Earthquake static stress drop is an enigmatic and highly uncertain source parameter to measure, but is fundamentally related to the rupture process and is needed to understand observed variations in strong ground motion between different events. Too often, stress drop estimates and inferred trends are the result of simplified modelling and observational data limits (e.g., bandwidth). We use a semi-automatic Empirical Green’s Function (EGF) method with strict objective criteria to estimate corner frequencies of 40 events (M3.8-4.2) from the 2019 Ridgecrest, CA earthquake sequence as part of the community validation efforts. We use earthquakes that are highly similar (cross-correlation coefficient > 0.7), smaller magnitude difference (1.0-2.5 magnitude units), collocated (within three estimated source dimensions to account for location uncertainty, assuming constant stress drop of 3 MPa) with each target event to calculate spectral ratios, thus isolating the earthquake source. We stack the ratios per-station using a limit of 5 EGFs per-station and then stack all available stations. From this spatially averaged spectral ratio, we measure the corner frequency and its uncertainties using a grid-search around the best-fit value and assuming a simple circular source model. We select only the best fit events based on additional manually-applied criteria: visual complexity, number of total ratios, variation of corner frequency estimates using different modelling approaches, criteria, and independent parts of the waveform (P and S). Finally, we calculate stress drop from corner frequency, seismic moment, and depth-varying shear-wave velocities and compare these results with the P wave results from Trugman (2020) and S wave results from Bindi et al. (2020). These studies use the spectral decomposition method to estimate corner frequency for a large number of events. Using uniform source modeling parameters (i.e., velocity, seismic moment, and appropriate k-values), we calculate stress drops and compare their results. The EGF method reduces the trends seen in depth for events from 0 to 12 km in the small magnitude range in which we focused on within this study.

Neupane, A., Ruhl, C. J., & Abercrombie, R. E. (2021, 08). Comparison of Stress Drop Variation of M3.8-4.2 Earthquakes Between 0-12 km Depth Using Four Independent Methods on 2019 Ridgecrest, CA Data. Poster Presentation at 2021 SCEC Annual Meeting.

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