An Ensemble of Block Models Applied to Southern California

Monica A. Diaz, & Eileen L. Evans

Submitted September 7, 2025, SCEC Contribution #14272, 2025 SCEC Annual Meeting Poster #TBD

Pacific-North American plate boundary deformation in Southern California is distributed across a geometrically complex fault system. Understanding the strain distribution across faults in such a complex network requires robust estimates of fault slip rates. Slip rates may be estimated with a block model, in which the crust is divided into microplates bounded by faults and constrained by geodetic observations such as Global Navigation Satellite Systems (GNSS). In a dense fault system such as that in Southern California, the block model solution requires regularization; we regularize the block model solution by applying damped least squares to simultaneously minimize the misfit and L2 norm of the solution. Block model results are typically based on a single fault system model and regularization parameter, limiting interpretation where the fault geometry is unknown. Here, we apply a block closure algorithm to generate a suite of block model geometries initialized with mapped southern California faults and consider a range of regularization parameters for each geometry. We develop an ensemble of block model solutions of Southern California by estimating slip rates within 18 unique block geometries with 20 different lambda values for a total of 360 block model solutions. We then subselect the model solutions to keep only those with a mean residual velocity (MVR) misfit of 1.8<MVR<2.0 mm/yr, for a final ensemble of 89 models. This approach reveals that the majority of geologic slip rates agree with at least some geodetic slip rates within the ensemble. This suggests that many discrepancies between geologic and geodetic slip rates may be due to epistemic uncertainties in modeling assumptions such as fault geometry and model regularization.

Key Words
Block Models, Tectonic, Slip rates, GNSS

Citation
Diaz, M. A., & Evans, E. L. (2025, 09). An Ensemble of Block Models Applied to Southern California. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy