High Resolution Regional Attenuation for the Source Physics Experiment Using Multiphase Inversion

Abstract:

Seismic event amplitude measurement plays a critical role in the discrimination between earthquakes and explosions. An accurate 2D model of the attenuation experienced by seismic waves traveling through the earth is especially important for reasonable amplitude estimation at small event-to-station distances. In this study, we investigate the detailed attenuation structure in the region around southern Nevada as part of the Source Physics Experiment (SPE). The SPE consists of a series of chemical explosions at the Nevada National Security Site (NNSS) designed to improve our understanding of explosion physics and enable better modeling of explosion sources. Phase I of the SPE is currently being conducted in the Climax Stock Granite and Phase II will move to a contrasting dry alluvium geology. A high-resolution attenuation model will aid in the modeling efforts of these experiments. To improve our understanding of the propagation of energy from sources in the area to local and regional stations in the western U.S., we invert regional phases Pn, Pg, and Lg to examine the crust and upper mantle attenuation structure of southern Nevada and the surrounding region. We consider observed amplitudes as the frequency-domain product of a source term, a site term, a geometrical spreading term, and an attenuation (Q) term (e.g. Walter and Taylor, 2001). Initially we take a staged approach to first determine the best 1D Q values; next we calculate source terms using the 1D model, and finally we solve for the best 2D Q parameters and site terms considering all frequencies simultaneously. Our preliminary results agree generally with those from the continent-wide study by Pasyanos (2013). With additional data we are working to develop a more detailed and higher frequency model of the region as well as move toward a fully non-linear inversion.