Using SCEC/USGS Community Stress Drop Validation Study seismic moments to examine Southern California Moment and Magnitude Scales and implications for b-value modeling

Annemarie S. Baltay, Rachel E. Abercrombie, Justin L. Rubinstein, & Nicholas J. van der Elst

Published September 8, 2024, SCEC Contribution #14030, 2024 SCEC Annual Meeting Poster #065

We illustrate the systematic difference between moment magnitude (M or Mw) and local magnitude (ML) caused by underlying earthquake source physics, using seismic moment (M0) measurements submitted to the Statewide California Earthquake Center/United States Geological Survey Community Stress Drop Validation Study using the 2019 Ridgecrest Earthquake sequence. Moment magnitude is defined to be proportional to 2/3 log10 (M0) for all earthquake sizes (Hanks and Kanamori, 1979), with constants for Mw chosen so that Mw and ML are approximately coincident in the range ~3< M < ~6. However, for earthquakes smaller than ~M3, ML ~ 1.0 log10 M0 (Hanks and Boore, 1984) and thus the relationship between Mw and ML is itself magnitude dependent. This is a consequence of the corner frequency fc becoming larger than the upper observed frequency and implies that ML and Mw differ by a factor of 1.5 for these small events. While this idea is not new, consistent estimates of seismic moment for earthquakes too small for regional moment tensor modeling (M<3.5) are relatively rare. Furthermore, catalog “preferred” magnitude types are mixed, and not always in a systematic way, so it is imperative to know and understand these differences implied by Mw vs ML when utilizing magnitudes as basic inputs. The plethora of seismic moments submitted by participants of the Community Stress Drop study enables us to propose a new, continuous relationship between ML and M0, for magnitudes 2 to 6 earthquakes in the Ridgecrest region of southern California. We fit the relationship between the averaged submitted moments and the ML from the Southern California Seismic Network (SCSN) with an empirical relationship, pinning the slopes at the small and large magnitude asymptotes to the theoretical slopes, so that our new relationship extrapolates correctly. Overall, the seismic moments in the Community Study recover moment magnitude well, so we use our new ML - M0 relationship to convert ML to Mw, essentially recovering the SCSN operational revised local magnitude MLr scale. The confirmed slope difference of 50% at small magnitudes has implications for spectral stress drop estimates and earthquake ground motion modeling, as well as other magnitude scales and earthquake occurrence statistics. We specifically examine how systematic differences in Mw and ML from the same data set affect corresponding b-value estimates, which can make a huge difference in trying to forward predict earthquake rates.

Key Words
seismic moment, magnitude, catalog magnitude, stress drop, b-value

Citation
Baltay, A. S., Abercrombie, R. E., Rubinstein, J. L., & van der Elst, N. J. (2024, 09). Using SCEC/USGS Community Stress Drop Validation Study seismic moments to examine Southern California Moment and Magnitude Scales and implications for b-value modeling. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology