Site Characterization of the Nevada National Security Site from Body-Wave and Ambient Noise Tomography

Characterization of the geophysical properties at scales relevant at local-regional distances from the ongoing series of source physics experiments (SPE) conducted on the Nevada National Security Site (NNSS) is key for many of the objectives of the experiments. In support of the SPE we have collected both body wave travel times from local earthquakes as well as analyzed ambient noise from local stations to produce surface wave dispersion curves for the NNSS and the surrounding region. Over 750,000 P- and S-wave travel times were collected from over 50,000 earthquakes, active sources and nuclear tests that have occurred on or near the site. These data were combined in a linearized iterative tomographic inversion procedure to solve for the optimal 3-D P- and S-wave velocity structures and hypocentral locations given the travel times and subject to model smoothness constraints. Although the P-wave velocities in many areas are well-resolved within the shallowest few kilometers of the surface due to active source and nuclear test data, the shallow S-wave velocities are poorly resolved in this depth range due to lack of sources with picked S-waves. To mitigate this shortcoming, the data set is augmented with fundamental mode dispersion curves derived from analysis of ambient noise cross-correlograms among pairs of stations on the site. These dispersion curves are then combined and inverted within different frequency bands ultimately to obtain the 3-D S-wave velocity structure within the shallowest portion of the crust under the NNSS. We will compare and contrast results between the body-wave and surface-wave-based models and link these results, preparing for the next phase, which includes joint body-wave and dispersion-curve inversion to produce a single, unified model that builds off the strengths of both types of data. We will also explore the respective resolutions of the models based on the two data sets. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.