Earthquake Magnitudes from Dynamic Strain: Application to the 2019 Ridgecrest Earthquakes

Andrew J. Barbour, & Noha S. Farghal

Submitted August 8, 2019, SCEC Contribution #9384, 2019 SCEC Annual Meeting Poster #253

We investigate the attenuation and geometrical spreading characteristics of dynamic strain measurements of seismic waves from local earthquakes. We assemble a catalog of earthquakes along the Pacific/North-American plate boundary with moment magnitudes (Mw) greater than 3.5, and collect high-frequency (20 Hz) strain data from all available Plate Boundary Observatory borehole strainmeters within 500 km of a given event. In total we have 2962 records of dynamic strain. In this dataset we find hypocentral-distance (R) decay terms for broadband root-mean-square strains (S): log⁡ S0 = -4.03 - 0.000757R - 1.46 log⁡ R. This expression is similar to scaling terms used by the California Integrated Seismic Network to determine local magnitude from seismometer displacements. Inspecting residual strains reveals the largest source of variability in this relationship comes from lumped source properties – some combination of stress drop, apparent rupture velocity, fault geometry, and radiation pattern. Further, we assess the relationship between our strain-based earthquake magnitudes and catalog moment magnitudes directly, finding that when site effects are accounted for by linear mixed-effects regression: Mw = -4.44 + 0.992 (log⁡S - log⁡ S0), indicating that strain magnitude is directly proportional to moment magnitude. Using recordings from three strainmeters located within 50 km of the recent Ridgecrest earthquakes, we use the above relationship to determine Mw = 6.32 ± 0.28 for the July 4 event and 7.18 ± 0.18 for the July 6 mainshock.

Key Words
Ridgecrest, strainmeter, dynamic strain, ground motion, ShakeAlert

Citation
Barbour, A. J., & Farghal, N. S. (2019, 08). Earthquake Magnitudes from Dynamic Strain: Application to the 2019 Ridgecrest Earthquakes. Poster Presentation at 2019 SCEC Annual Meeting.


Related Projects & Working Groups
Ridgecrest Earthquakes