Broadband Rayleigh-Wave Dispersion Curve and Shear-Wave Velocity Structure for Yucca Flat, Nevada

The geology near a seismic source has a major effect on seismic waves recorded at distance. This can be especially true in the case of man-made explosions, due to increased geologic heterogeneity at shallow depths and interactions with the free surface. Yucca Flat (YF), a sedimentary basin on the Nevada National Security Site, has hosted hundreds of well-recorded underground nuclear tests. As such, it should be an ideal natural laboratory for the study of shallow explosions. Unfortunately, basin-wide models of such important physical properties as compressive- and shear-wave velocity are not available with sufficient fidelity to maximize the potential of the studies. We attempt to remedy this situation by creating a new shear-wave velocity model of YF. This model was generated by inverting Rayleigh-wave phase-velocity dispersion measurements. Because no single dataset provided a dispersion curve of the necessary frequency bandwidth for shallow, intermediate, and deep basin depths simultaneously, we combined three dispersion curves with complementary bandwidths from three data sources. The datasets, in order of low frequency to high, were (1) underground nuclear tests at YF, recorded on regional seismic networks (0.14-0.4 Hz); (2) a multimode spatially averaged coherency microtremor array located on YF (0.2-20 Hz); and (3) several refraction microtremor (ReMi) linear arrays, also on YF (2.5-50 Hz). Compared to previous work, our model is characterized by slower velocities. The known geologic boundaries such as the depth of the basin and water table are prominent at reasonable locations.