Continuous GPS Observations of Postseismic Deformation Following the 16 October 1999 Hector Mine, California, Earthquake (M_w 7.1)

Abstract

Rapid field deployment of a new type of continuously operating Global Positioning System (GPS) network and data from Southern California Integrated GPS Network (SCIGN) stations that had recently begun operating in the area allow unique observations of the postseismic deformation associated with the 1999 Hector Mine earthquake. Innovative solutions in fieldcraft, devised for the 11 new GPS stations, provide high-quality observations with 1-year time histories on stable monuments at remote sites. We report on our results from processing the postseismic GPS data available from these sites, as well as 8 other SCIGN stations within 80 km of the event (a total of 19 sites). From these data, we analyze the temporal character and spatial pattern of the postseismic transients. Data from some sites display statistically significant time variation in their velocities. Although this is less certain, the spatial pattern of change in the postseismic velocity field also appears to have changed. The pattern now is similar to the pre-Landers (pre-1992) secular field, but laterally shifted and locally at twice the rate. We speculate that a 30 km × 50 km portion of crust (near Twentynine Palms), which was moving at nearly the North American plate rate (to within 3.5 mm/yr of that rate) prior to the 1992 Landers sequence, now is moving along with the crust to the west of it, as though it has been entrained in flow along with the Pacific Plate as a result of the Landers and Hector Mine earthquake sequence. The inboard axis of right-lateral shear deformation (at lower crustal to upper mantle depth) may have jumped 30 km farther into the continental crust at this fault junction that comprises the southern end of the eastern California shear zone.

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