Magnitude, depth and methodological variations of spectral stress drop within the SCEC/USGS Community Stress Drop Validation Study Using the 2019 Ridgecrest Earthquake Sequence

Rachel Abercrombie, & Annemarie Baltay

Submitted March 14, 2025, SCEC Contribution #14175

We present the first ensemble analysis of the 56 different sets of results submitted to the ongoing community stress drop validation study using the 2019 Ridgecrest, California, earthquake sequence. Different assumptions and methods result in different estimation of the source contribution to recorded seismograms, and hence to the source parameters obtained from modeling these calculated source spectra. We investigate the range of relationships observed between estimated stress drops and both earthquake magnitude (M) and depth, to understand the nature of the differences and constraints. We focus on the dependencies and variations of corner frequency (fc), which is the primary parameter estimated by nearly all of the submitted methods. For earthquakes smaller than M2.5 there is negligible correlation between the values obtained by different studies. For larger magnitude events, correlation between fc measurements within even a small M range is always higher than stress drop, because the fc measurements simply reflect the underlying decrease in fc with M. We model the observed trends of submitted corner frequency with both magnitude and depth. Most methods report an increase in spectral stress drop with M, although a magnitude-invariant spectral stress drop is within the confidence limits. The depth dependence is smaller and closely linked to whether a study allows attenuation to vary with source depth; we find that a combination of depth-dependent attenuation correction, and depth-dependent shear-wave velocity can compensate for reported depth trends. We model the submitted values to remove differing M and depth variation producing an “adjusted” set of spectral stress drops to investigate the relative inter-event variability. We find consistent relative variation between individual events, and also lower spectral stress drop in the northwest of the aftershock sequence, and higher on the cross fault, and in the region of main fault intersection.

Citation
Abercrombie, R., & Baltay, A. (2025). Magnitude, depth and methodological variations of spectral stress drop within the SCEC/USGS Community Stress Drop Validation Study Using the 2019 Ridgecrest Earthquake Sequence. Bulletin of the Seismological Society of America, (submitted).

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