A Velocity Field for the North American Plate Interior

The determination of accurate continent-scale geodetic velocity fields in plate interiors is essential for investigating the processes that drive the deformation of continents and control the associated seismicity. Accurate continental-scale velocity fields are also important as reference frames for mapping crustal motions at plate boundaries. As the precision and accuracy of space geodetic techniques improves, coherent deformation patterns are now emerging from the analysis of continent-wide GPS data sets in plate interiors. In North America, studies have shown that the primary horizontal strain signal in the plate interior is consistent with that expected from Glacial Isostatic Adjustment (Calais et al., 2006; Sella et al., 2007). Current results indicate ~NS shortening at a rate on the order of 10-9/yr in southern Quebec, the northeastern U.S., and just south and west of the Great Lakes, consistent with GIA, seismic moment release, and earthquake focal mechanisms in these areas. In contrast, no detectable surface strain is found south of about 38N, at the 0.7~mm/yr precision level (95% confidence). Sub-millimeter per year accuracy reamains however a challenge at the scale of an entire continent. We present an updated velocity field for North America that results from the combination of two independent solutions and includes about 800 sites. We discuss the implications of these results for earthquake hazard, glacial isostatic adjustment models, and for defining a Stable North America reference frame for geodetic studies in western North America.