Abstract

We present the first detailed geodetic image of the entire western United States south of lat 42°N, merging both campaign and continuous Global Positioning System (GPS) and very long baseline interferometry (VLBI) data sets in a combined solution for station velocities having a single, uniform reference frame. The results are consistent with a number of features previously observed through local geodetic studies and very sparse space geodetic studies, including a dominant pattern of right-lateral shear associated with the San Andreas fault, rates of the westernmost sites (along the California coast) of 46–48 mm/yr relative to a North America reference frame, and some 11–13 mm/yr of deformation accommodated east of the Sierra Nevada in the Basin and Range province north of lat 36°N. South of 36°N, the solution also shows that the southernmost San Andreas fault system accommodates effectively all interplate motion and that the southern Basin and Range is not deforming significantly. At lat 37°N, the eastern California shear zone appears to exhibit simple shear oriented between ∼N20°W and ∼N40°W relative to North America, with a fairly well defined transition zone from localized shear to diffuse spreading in the Basin and Range. Enigmatically, this transition involves a significant component of contraction normal to the overall shear-zone trend; sites in the Great Basin move southwestward at up to ∼5 mm/yr toward sites within the eastern California shear zone. To the north, in contrast, there appears to be a relatively smooth transition from east-west spreading within the eastern Great Basin to northwest-southeast shear across the westernmost Basin and Range.

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