Geodetic measurement of deformation in the offshore of southern California

Citation

Webb, Francis H. (1991) Geodetic measurement of deformation in the offshore of southern California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ztnn-4v90. https://resolver.caltech.edu/CaltechTHESIS:04142011-131740440

Abstract

Geodetic surveys using signals from satellites of the Global Positioning System (GPS) have allowed for the reoccupation of historical triangulation markers for crustal deformation studies and the recovery of a long history of geodetic data that date to the late 1800's. From June 1986 to May 1988, six GPS experiments were conducted in California that incorporated more than 50 first-order triangulation stations on the channel islands of southern California and the mainland into a precise GPS network. Interstation vectors were calculated from the GPS observables using the National Geodetic Survey's (NGS) GPS22 software. The results of these analyses provided baseline component precisions better than several pans in 10^7. The vectors from all of the GPS surveys were combined and used with historic triangulation observations to estimate shear-strain rates across the Santa Barbara Channel and Oxnard plain, and across the southern California Continental Borderland. Simultaneous reduction was used to evaluate the shear-strain rates from the combination of these data. Shear-strain rates across the Santa Barbara channel are from 0.2 to 0.4 µrad/yr with a direction of maximum contraction of from N23W to N20E, averaged over a hundred years of geodetic data. For networks that cross the Continental Borderland, the data were insufficient to reliably estimate strain rates. The direction of shortening implied by the geodetic data agrees with that inferred from the trends of late Quaternary and Holocene faults and folds in the Santa Barbara channel. When the shear-strain rates across the channel are modeled as uniaxial convergence, this indicates shortening from 13±5 mm/yr at N23W±5 across the western part of the channel to 18±5 mm/yr at N20E±5 across the eastern part. By adding the western channel velocity to the geodetic velocity of the Vandenberg VLBI site with respect to North America, a velocity path across the Pacific-North American plate boundary can be constructed to the offshore islands of 56±4 mm/yr at N38W±4, exceeding the NUVEL-1 plate-model velocity for Pacific North American plate motion in California by 7 mm/yr.

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