Variability and reliability of stress drops from the SCEC/USGS Community Stress Drop Validation Project

Annemarie S. Baltay, & Rachel E. Abercrombie

Submitted September 7, 2025, SCEC Contribution #14563, 2025 SCEC Annual Meeting Poster #TBD

The international USGS/SCEC Community Stress Drop Validation Study provides insight on the variability and reliability of spectral stress drop estimates which should be considered in application and interpretation of earthquake stress drop estimates. The earthquake stress drop, a measure of the release of stress on the fault during slip, is a fundamental source parameter in modeling earthquake source physics. It is a similarly fundamental parameter in ground motion modeling because of its major control on the high frequency shaking. Numerous estimates of stress drop made from earthquake and acoustic-emission spectra exist, but concern about their large uncertainties and lack of consistency has led to doubt and confusion in their reliability for interpretation. The Stress Drop Validation Study was designed to address this fundamental problem (https://doi.org/10.1785/0120240158) and enables direct comparison of many independently applied approaches. We provide guidelines based on the comparison for assessing reliability of published values and improving future estimates.
Different assumptions and methods result in different estimation of source parameters (principally corner frequency, fc, spectral stress drop, Δσ, and seismic moment, M0,) obtained from modeling calculated source spectra. In the Ridgecrest, California, earthquake sequence using regional California data, we find that no present method is reliable for M<2.5. We model the observed trends of submitted fc with both magnitude and depth to understand the physical dependencies of the spectral Δσs. Although the absolute values are highly method-dependent, we find consistent relative variation between individual events, as well as small-scale spatial variation implying there is real source variability that can be gleaned from spectral analysis. This large-scale comparison implies that absolute spectral Δσ estimates are dependent on the methods used; in particular, magnitude and depth dependent trends should be treated with caution, and studies of different regions or using different methods should not be directly compared. We welcome new members to the ongoing community study, wishing to observe, learn or more actively participate. Visit https://www.scec.org/events/2025-scec-stress-drop-workshop/ for more information.

Key Words
stress drop, community validation study

Citation
Baltay, A. S., & Abercrombie, R. E. (2025, 09). Variability and reliability of stress drops from the SCEC/USGS Community Stress Drop Validation Project. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology