Depth to bedrock using gravimetry in the Reno and Carson City, Nevada, area basins

Sedimentary basins can trap earthquake surface waves and amplify the magnitude and lengthen the duration of seismic shaking at the surface. Poor existing gravity and well-data coverage of the basins below the rapidly growing Reno and Carson City urban areas of western Nevada prompted us to collect 200 new gravity measurements. By classifying all new and existing gravity locations as on seismic bedrock or in a basin, we separate the basins' gravity signature from variable background bedrock gravity fields. We find an unexpected 1.2-km maximum depth trough below the western side of Reno; basin enhancement of the seismic shaking hazard would be greatest in this area. Depths throughout most of the rest of the Truckee Meadows basin below Reno are less than 0.5 km. The Eagle Valley basin below Carson City has a 0.53-km maximum depth. Basin depth estimates in Reno are consistent with depths to bedrock in the few available records of geothermal wells and in one wildcat oil well. Depths in Carson City are consistent with depths from existing seismic reflection soundings. The well and seismic correlations allow us to refine our assumed density contrasts. The basin to bedrock density contrast in Reno and Carson City may be as low as -0.33 g/cm(3). The log of the oil well, on the deepest Reno subbasin, indicates that Quaternary deposits are not unusually thick there and suggests that the subbasin formed entirely before the middle Pliocene, Thickness of Quaternary fill, also of importance for determining seismic hazard below Reno and Carson City may only rarely exceed 200 m.