Strain partitioning along the western margin of North America

Abstract

This paper describes an elastic block model for the interseismic horizontal crustal velocity field occurring in that part of the United States located west of longitude 100° W and between latitudes 31°N and 49°N. We developed the model by simultaneously inverting 6873 GPS-derived velocity vectors and 166 geological fault slip rates for the angular velocities (i.e. the Euler poles relative to the North America plate) of 46 elastic blocks, horizontal strain rate tensors for 38 of these blocks, and the spatially variable elastic coupling coefficients on faults that bound adjacent blocks.While the model covers all of the western United States located between Canada and Mexico, this paper focuses on the region residing south of Cape Mendocino where plate boundary deformation is accommodated predominantly by slip on the San Andreas fault system. Block strain rates (which account for deformation associated with distributed faults within blocks) are systematically higher in blocks located in the western part of the model and adjacent to the plate boundary. Strain rate magnitudes range from over 10−7/yr for some blocks adjacent to the San Andreas fault system to values of about 10−9/yr for blocks located in eastern Nevada and western Utah. Blocks adjacent to the San Andreas fault system are characterized by strain rate tensors that correspond to uniaxial contraction perpendicular to the local strike of the San Andreas. The highest rates of fault normal contraction are associated with the northern end of the fault (north of San Francisco) and in the southern end (south of Los Angeles). The central San Andreas (including the creeping segment of the fault) is characterized by strain rate tensors more consistent with dextral shear. Thus the northern and southern ends of the fault are consistent with a transpressional strain partitioning model with strike slip occurring on the San Andreas fault system and distributed shortening occurring within the blocks adjacent to this fault system. There is no evidence of strain partitioning in the central San Andreas.