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Testing Community Velocity Models of Southern California using Gravity Modeling

Masooma Hasnain, & Simon L. Klemperer

Submitted September 11, 2022, SCEC Contribution #12220, 2022 SCEC Annual Meeting Poster #261

Southern California Earthquake Center (SCEC) has developed two distinct community models, CVM-H and CVM-S, that span Southern California. Many other models have been produced for the same region, e.g. Barak et al. (2015, G-cubed, hereafter BKL-15). Here we test which model (BKL-15, CVM-H15.1, CVM-S4.26) best predicts the observed regional gravity field. Since 3D tomographic models typically have poor resolution at shallow depths due to limited station coverage and all models have resolution that degrades below the Moho, we only model the gravity field due to wavespeed variations between 5–40 km. We also test a range of published wavespeed-to-density conversions, as the appropriate conversion is dependent on the geology. We initially focus on a simple geologic setting, the well-modeled high-contrast gravity signature (Langenheim et al. 2004, 2014) of the Peninsular Ranges Batholith (PRB). We sampled all three models at the same voxel size, 1 km vertical, 0.05° horizontal. We converted the 3D wavespeed models into 2D density cross sections with cross-strike smoothing, preserving the original voxel size. We used Fast-Grav, a freeware gravity modeling software, to calculate the gravity along each transect. We removed DC offsets from the model and used the mean rms deviation between wavespeed-predicted gravity and observed gravity as our metric for goodness of fit.

Across the Peninsula Ranges batholith, BKL-15 (rms fit ~ 7 mGal) is somewhat better than CVM-H (rms fit ~ 16 mGal), but CVM-S has a substantially higher rms fit of ~40-50 mGal. CVM-S over-predicts wavespeed variations in the crust in this part of southern California. We speculate that east-directed underthrusting of the PRB shown in BKL-15 and seemingly validated by our gravity modeling leads to the northeast-dipping San Andreas fault “propeller” at this latitude.

Hasnain, M., & Klemperer, S. L. (2022, 09). Testing Community Velocity Models of Southern California using Gravity Modeling. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)