The Surface Deformation of the Western US Deformation Zone Inferred From 626 Motion Vectors.

We have determined the surface deformation of the Western United States Deformation Zone (WUSDZ) by a simultaneous inversion for strain and rotation rates and surface creep on faults of 626 velocity vectors. The inverted dataset is a combination of the SCEC 2.0 velocity field (extended with 13 sites from the SCEC 1.0 velocity field) of the Crustal Deformation Working Group and a preliminary version of the WUSC003 velocity field of Bennett et al [2001]. We have parameterized all major faults in California and the Wasatch fault zone, as provided by the USGS. The Basin and Range is characterized by a large amount of small faults. We tested an approximation of these by implementing a fault per fault zone in our parameterization. In the surface deformation model we obtain very small right-lateral fault motions of several mm/yr at the San Andreas fault, suggesting surface creep along the fault. Most other faults show little to no surface creep. The resulting strain rate field complies with geological observations and portrays dominant NNE-SSW contraction in southern California rotating to N/NNW-S/SSE contraction along the big bend of the San Andreas fault, and back to NNE-SSW contraction in the Bay area. The strain rates in the Basin and Range are almost an order of magnitude smaller than the those in California. There is clear E-W extension in the Sierra Nevada Seismic Zone and along the eastern side of the Wasatch Fault zone. The rotation rate field is dominated by significant clockwise rotation surrounding the San Andreas Fault.