JOINT INVERSION OF HIGH-RESOLUTION GRAVITY AND SEISMIC DATA IN ROCK VALLEY, NV

Direct comparisons of earthquake and explosion seismic wave propagation through geologic structure are sparsely available. Differentiating between the characteristics of shallow earthquake sources and explosions allows for robust assessment of discrimination models. To compare source signals, characterization of the underlaying geologic medium including the fault structure at depth is important. Geophysical data collected at Rock Valley (RV) within the Nevada National Security Site offer insights into the structural properties needed to update the local Geologic Framework Model (GFM). Here, we present observations from recent dense gravity surveys as well as results from joint inversion or gravity and seismic data which constrain the underlying velocity structure. Dense gravity surveys were conducted by Sandia National Laboratories and Los Alamos National Laboratory, consisting of 10 high-resolution gravity lines (100-250 m station spacing) that augment regional USGS gravity measurements, which are sparse in RV. Relative gravity measurements are tied to a USGS absolute gravity reference station south of RV and are integrated with the regional survey data. Corrections are applied to the gravity dataset including instrument drift, latitude, earth tide, terrain, free air, and Bouguer corrections. Bouguer anomaly maps correlate with the exposed lithologic units, with a deepening alluvium and sedimentary deposits in central RV flanked by Tertiary volcanics and shallowing Paleozoic carbonate basement. Forward modeling gravity from the existing GFM alongside geologically reasonable permutations of the structure allow us to further constrain the model. Active source (Accelerated Weight Drop) seismic data has also been collected in the RV region and has been previously inverted for P and S velocity structure. Studies of the broader NNSS applied joint inversion methods on regional gravity and seismic data, which was found to enhance resolution of the upper 1-2 km velocity model. Our joint inversion results from RV follow a similar methodology, better constraining the shallow structure relevant for comparison of the 1993 RV earthquake sequence (<3 km) and planned chemical explosions of the Rock Valley Direct Comparison experiment. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525