A comprehensive analysis of geodetic slip-rate estimates and uncertainties in California

Developing a comprehensive model of tectonic continental deformation requires assessing (1) fault-slip rates, (2) off-fault deformation rates, and (3) realistic uncertainties. Fault-slip rates can be estimated by modeling fault systems, based on space geodetic measurements of active surface ground displacement such as Global Navigation Satellite Systems (GNSS) and Interferometric Synthetic Aperture Radar (InSAR). Geodetic slip-rate estimates may vary widely due to measurement and epistemic (model) uncertainties, presenting a challenge for both estimating slip rates and accurately characterizing uncertainties: models may vary in the number of faults represented and the precise location of those faults. Since 2003, 33 published geodetic deformation models have produced slip-rate estimates within California. Variability among these models represents variability among valid model choices and may be considered a proxy for model uncertainties in geodetic slip-rate estimates. To enable rigorous comparison between geodetic slip-rate estimates, I combine models on a georeferenced grid and find an average standard deviation on slip rate of approximately 1.5 mm/yr over 542 grid cells (average area of 1304 km (super 2) /cell). Furthermore, the average strike-slip and tensile-slip rates over all 33 studies, in each grid cell, may then be projected onto Unified California Earthquake Rupture Forecast (UCERF) v.3.1 faults for a single summary model of geodetic slip rates. Slip rates that do not project perfectly onto UCERF3.1 faults form a summary model of off-modeled-fault (OMF) deformation. Most of this OMF deformation occurs in grid cells that intersect UCERF3.1 faults, suggesting that off-fault deformation may be, in part, a product of epistemic uncertainty in geodetic slip-rate estimates and may be physically accommodated on, or very near, UCERF faults.