TECTONIC IMPLICATIONS OF A DENSE CONTINUOUS GPS VELOCITY FIELD AT YUCCA MOUNTAIN, NEVADA

A dense, continuous GPS network was established in the Yucca Mountain area in 1999 to provide the most reliable measurements possible of geodetic strain patterns across the Nation's only proposed permanent repository for high-level radioactive waste.  The network lies astride a boundary between the geodetically stable central Great Basin and the active western Great Basin, which at the latitude of Yucca Mountain is undergoing distributed right-lateral (engineering) shear at a rate of ~60 nstrain/yr.  Monitoring from 1999 to 2003 (3.75 yrs) yields a velocity field characterized by nearly homogenous N20°W right-lateral shear of 20 ± 2 nstrain/yr, or a net velocity contrast of ~1.2 mm/yr, across a 60 km aperture that includes the proposed repository site.  Comparison of time series of continuous results with earlier campaign surveys indicating ~50 nstrain/yr of west-northwest extension from 1991 to 1997 suggest that the more rapid rates were in part transient motions associated with the 1992 M_s 5.4 Little Skull Mountain earthquake.  Post-seismic motions do not appear to affect the 1999 to 2003 velocity field in either campaign or continuous data.  Both the magnitude of the velocity contrast across the area and the linearity of the gradient are difficult to explain by elastic bending of the crust associated with the Death Valley fault zone, a major right-lateral strike-slip fault about 50 km west of the repository site.  The results therefore suggest significant right-lateral strain accumulation, with displacement rates in the 1 mm/yr range, may be associated with structures in the Yucca Mountain area.  The absence of structures in the area with equivalent Late Quaternary displacement rates underscores the problem of reconciling discrepancies between geologic and geodetic estimates of deformation rates.