Towards an Integrated Model of the Interseismic Velocity Field Along the Western Margin of North America

Abstract

This paper describes a block model of tectonic deformation for the western part of the contiguous United States (i.e. west of longitude 100°W and between latitudes 31°N and 49°N). NOAA’s National Geodetic Survey (NGS) uses velocities predicted by this model as the basis of the horizontal velocity grids incorporated in Horizontal Time-Dependent Positioning (HTDP) software. We model the deformation of this area using 46 rotating blocks. Along with the poles of rotation, we also solve for 38 independent horizontal strain rate tensors and the elastic coupling coefficients on faults that bound adjacent blocks. For the release of HTDP 3.1, we updated estimates of model parameters by using 6,287 GPS-derived velocity vectors, that include vectors from the 2009 Plate Boundary Observatory (PBO) solution and the NGS Multiyear CORS (MYCORS) solution, and 330 geological measurements of fault slip rates and/or fault orientation. In general, the fault slip rates and the interseismic coupling coefficients are consistent with the results of previous studies; however, because of the comprehensive nature of this model, we are able to quantitatively map deformation rates over the entire plate boundary zone within the contiguous United States. Slip rates on the faults range from over 30 mm/year for the Cascadia subduction zone and parts of the San Andreas system to near zero for faults adjacent to stable North America. Block rotations play a significant role in accommodating deformation in the Pacific Northwest but make a much smaller contribution south of Cape Mendocino. Because of the variable gradient of the velocity field, HTDP3.1 incorporates a hierarchy of 5 grids with a spacing ranging from 4 nodes per degree to 100 nodes per degree.