Comparison of EGF methods for Ridgecrest Sequence: Can EGF be used to help resolve ambiguity in isolating source spectra?

Rachel E. Abercrombie, Xiaowei Chen, Yihe Huang, & Shanna Chu

Under Review November 18, 2024, SCEC Contribution #14128

The spectral stress drop is a popular parameter to quantify and characterize an earthquake source and its expected seismic radiation simply, enabling investigation of earthquake spatial and temporal variability for larger numbers of events. Also, spectral measurements are one of the few possible for earthquake characterization and hazard prediction in regions of low seismicity. However, spectral stress drop estimates are uncertain, especially as recorded earthquakes may be too complex to characterize ideally with a single parameter. Empirical Green’s function (EGF) approaches to isolate the earthquake source are widely regarded as one of the best for individual analysis of well-recorded earthquakes. However, analysis decisions related to the selection of stations, EGFs, time-windows, frequency bandwidth, and source models can cause discrepancies in resulting estimates of the source spectrum, source time function, and source parameters. We present results following one well-developed EGF approach, and compare it with those from three other independent methods applied to earthquakes in the 2019 Ridgecrest, California, earthquake sequence selected for the SCEC/USGS Community Stress Drop Validation Study. The common data set consists of two weeks of earthquakes from the 2019 Ridgecrest earthquake sequence, including nearly 13,000 events of M1 and greater, recorded on stations within 100 km. We obtain estimates of corner frequency and spectral stress drop for 75 earthquakes (M2.2-4.6) and find varying degrees of similarity among studies. We investigate four events in detail (M2.7-4.1) and find that we obtain consistent results when the sources are relatively simple. Multiple EGFs produce good ratios and source time functions at stations with a good azimuthal distribution. This suggests that there is a role for such approaches to resolve the inherent ambiguity in larger scale inversions between source scaling and attenuation and site effects.

Citation
Abercrombie, R. E., Chen, X., Huang, Y., & Chu, S. (2024). Comparison of EGF methods for Ridgecrest Sequence: Can EGF be used to help resolve ambiguity in isolating source spectra?. Bulletin of the Seismological Society of America, (under review).

Related Projects & Working Groups
Stress Drop Community Validation Study, Seismology