Geodetic measurement of tectonic deformation in the Santa Maria Fold and Thrust Belt, California

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Abstract
We have analyzed geodetic observations to resolve tectonic deformation across the Santa Maria Fold and Thrust Belt, northwest of Santa Barbara, California. The geodetic network forms a braced quadrilateral with ∼40 km sides whose southwest corner is the Vandenberg very long baseline interferometry station. The data include (1) historical triangulation from the 1880s, 1920s and 1950s; (2) electronic distance measurement trilateration from 1971 and 1985; and (3) Global Positioning System observations from 1986 and 1987. We combine the three different types of data to estimate two‐dimensional station positions and strain rate parameters simultaneously. Using a model which constrains the relative velocity field to be linear in space and constant in time, we find significant strain rates. The maximum compressive strain is oriented N17°E±5°E, and the compressive strain rate in this direction is 0.13±0.03 μstrain/yr. Under the assumption that the unresolved rotational component of the velocity field is zero, we estimate that the integrated rate of deformation across the basin is 7±1 mm/yr oriented at N03°E±13°. This vector can be decomposed into 6±2 mm/yr of crustal shortening on the general structural trend of N30°E and 3±1 mm/yr of right‐lateral shear across this axis. On the basis of these values and earthquake focal mechanisms in the area, we infer that the deformation occurs on northwest trending folds and thrusts within the belt. These results are consistent with the rate and direction of deformation across the central California Coast Ranges inferred by balancing Pacific‐North America plate motion against San Andreas slip and Basin and Range extension. They imply that the Santa Maria Fold and Thrust Belt is the primary active element in transforming motion from the Coast Ranges to the western Transverse Ranges and the Santa Barbara Channel.