Southern California Moment and Magnitude Scales from the SCEC/USGS Community Stress Drop Validation Study

Annemarie S. Baltay, & Rachel E. Abercrombie

Submitted July 1, 2024, SCEC Contribution #13468

We illustrate the systematic difference between moment magnitude and local magnitude, compared to seismic moment, for the SCEC/USGS Community Stress Drop Validation Study 2019 Ridgecrest data set, caused by underlying earthquake source physics. While the relationship between seismic moment and moment magnitude log10(M0) ~ 1.5* M is uniformly valid for all earthquake sizes [Hanks and Kanamori, 1979], the relationship between local magnitude ML and moment is itself magnitude dependent. For moderate events, ~3< M < ~7, M and M­L are coincident; for earthquakes smaller than ~M3, ML ~ 1.0 log M0 [Hanks and Boore, 1984]. This is a consequence of the corner frequency fc becoming larger and thus close to, or greater than, the upper frequency of observation, and implies that ML and M differ by a factor of 1.5 for these small events. While this idea is not new, we propose a new, continuous relationship between local magnitude and moment, for magnitudes 2 to 6 which extrapolates to smaller and larger magnitudes, applicable to southern California specific to the Ridgecrest region. We make use of the plethora of seismic moments as submitted by many participants of the Community Stress Drop study, compared to the Southern California Seismic Network (SCSN) catalog magnitudes. Overall, the seismic moments in the Community Study recover moment magnitude well, so we use our new ML-MO to convert ML to M, essentially recovering the SCSN operational MLr scale. This systematic difference between local and moment magnitude has implications for spectral stress drop estimates, earthquake ground motion modeling, as well as other magnitude scales and earthquake statistics.

Baltay, A. S., & Abercrombie, R. E. (2024). Southern California Moment and Magnitude Scales from the SCEC/USGS Community Stress Drop Validation Study. Bulletin of the Seismological Society of America, (submitted).

Related Projects & Working Groups
Community Stress Drop Validation