Source Complexity of M 3.5 – 5.5 Earthquakes in the 2019 Ridgecrest Earthquake Sequence

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

Submitted September 11, 2022, SCEC Contribution #12508, 2022 SCEC Annual Meeting Poster #059

Stress drop is an important source parameter that is commonly used for estimating ground motion and understanding physics of the earthquake source. However, complexity of earthquake sources often gives rise to a high variability in the measurements of corner frequency and resulting stress drop. These uncertainties affect the reliability of stress drop estimates for the prediction of ground motion. In this study, we estimate corner frequencies for 157 moderate earthquakes (M3.5-5.5) in the 2019 Ridgecrest earthquake sequence using a semi-automatic Empirical Green’s Function (EGF) Method and a simple circular source model. For each target event, we first stack the spectral ratios at each station and then stack all the station stacked ratios to obtain a spatially-averaged spectral ratio for fitting. We obtain the relative source time functions for each target using a similar stacking approach. We observe spatial variation in the estimated corner frequencies and also azimuthal variation and deviation from the simple model indicating complexity in the source process for a significant number of target events. To quantify these complexities we calculate the residuals between the observed spectral ratios and the fit obtained using a simple circular source model. To analyze the stability of the residual spectra, and their dependence on the assumed source model, we repeat the analysis using the lower and upper bounds of the corner frequency. We also investigate the azimuthal variation of residual spectra to relate the spatial variation of corner frequency to source effects such as rupture directivity. Finally, we analyze the residual spectra for any effect from target earthquake magnitude, depth and location within the complex and distributed fault system in the Ridgecrest area.

Key Words
Earthquake source complexity, Stress drop, Corner frequency

Neupane, A. S., Ruhl, C. J., & Abercrombie, R. E. (2022, 09). Source Complexity of M 3.5 – 5.5 Earthquakes in the 2019 Ridgecrest Earthquake Sequence. Poster Presentation at 2022 SCEC Annual Meeting.

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