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The data from 28 continuous BARGEN GPS stations in the Yucca Mountain region were processed for January 2000 to June 2004. The data have been processed independently using both the GIPSY and GAMIT software packages for quality assurance purposes, yielding an RMS of velocity differences between the solutions of 0.06 mm/yr for the east and 0.10 mm/yr for the north. The velocity solution for the local Yucca Mountain network has a relatively smooth signal showing NW-trending rightlateral shear. The magnitude of the velocity contrast across the local network, from east to west, is 0.95 ± 0.04 mm/yr. The GPS results were inverted, using an elastic dislocation model, to estimate fault parameters for the San Andreas (SA), Owens Valley (OV), Panamint Valley-Hunter Mountain (PV-HM) and Death Valley-Furnace Creek (DV-FC) fault systems. Estimated slip rates are 30.4 ± 0.2 mm/yr, 2.6 ± 0.7 mm/yr, 4.5 ± 1.3 mm/yr, and 5.0 ± 0.6 mm/yr, respectively. Estimated locking depths are 12.0 ± 1.0 km, 8.2 ± 7.8 km, 11.5 ± 6.6 km, and 33.0 ± 2.4 km, respectively. The estimated locking depth of ~33 km for the DV-FC fault system is unreasonably deep and caused by the inversion fit to the relatively steep velocity gradient across Yucca Mountain. With a hypothetical local model fault at Yucca Mountain included in the inversion, estimated slip rates are 30.8 ± 0.2 mm/yr, 3.2 ± 0.6 mm/yr, 4.7 ± 1.0 mm/yr, 2.8 ± 0.4 mm/yr, and 0.8 ± 0.2 mm/yr for the SA, OV, PV-HM, DV-FC and local model faults respectively. This results in a total Eastern California Shear Zone (ECSZ) slip rate of 11.5 ± 1.2 mm/yr, which agrees with the GPS estimate of 11.5 ± 0.1 mm/yr. Estimated locking depths are 11.9 ± 1.0 km, 6.6 ± 4.9 km, 7.6 ± 4.6 km, 11.8 ± 3.4 km, and 12.5 ± 3.2 km for the SA, OV, PV-HM, DV-FC and local model fault systems, respectively. Although the RMS of velocity differences between model and GPS results is not significantly improved by including a local model fault, this puts the DV-FC locking depth at a more physically reasonable level of ~12 km. The addition of a local model fault (which could represent deformation across any number of the mapped faults) is, therefore, one way to explain the GPS results. The models do not account for factors such as varying fault orientation, finite fault length, postseismic deformation, earthquake cycle effects, or oblique slip, and the use of improved fault models is likely to change the estimated fault parameters. However, the inversion results do suggest that it is difficult to explain the GPS results for the local network using only models of the ECSZ faults. Estimated total right-lateral shear strain rate for the local Yucca Mountain network, based on the fault parameters estimated for the ECSZ faults, is 11.3 ± 1.4 ns/yr. This compares with a GPS estimate for all stations in the local network of 18.1 ± 0.7 ns/yr, oriented N16 ± 1oW. The GPS estimate of shear strain rate assumes uniform strain across the network, which is not the case in reality. Estimated model shear strain for stations within the local network at and to the west of Yucca Mountain is 13.6 ± 1.6 ns/yr. This agrees to within one standard deviation with the GPS estimate for the western cluster of stations, which is 15.7 ± 1.1 ns/yr, oriented N20 ± 2oW. The estimate of model shear strain rate for stations in the local network to the east of Yucca Mountain is 8.9 ± 1.1 ns/yr. This is significantly lower than the GPS estimate of 25.1 ± 1.3 ns/yr, oriented N8 ± 2oW. This suggests that it is possible to explain the GPS-measured strain rate for the western cluster of stations using dislocation models of the ECSZ faults (with no local fault necessary), but it is not possible to explain the GPS estimate of strain rate for the eastern cluster using these models. This highlights the need to investigate further the possibility of leftlateral deformation across the NE-trending Rock Valley fault zone, particularly since the GPS estimate of strain rate is ambiguous, and could represent left-lateral strike-slip faulting on a N82 ± 2oE trending fault system.


Earthquake zones; Geodesy; Global Positioning System; Nevada – Yucca Mountain; Shear zones (Geology)


Earth Sciences | Geophysics and Seismology | Tectonics and Structure




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