Regional Attenuation of Southern Nevada Using Multiphase Inversion

Abstract:

Seismic event amplitude estimation plays an important role in a range of endeavors including the discrimination between earthquakes and explosions and seismic hazard estimation. Reasonable amplitude estimation requires knowledge of the attenuation experienced by seismic waves as they travel through the earth. In this study, we investigate the attenuation structure in the region of 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. Phase III is planned to be a direct earthquake-to-explosion comparison in Rock Valley at the southern end of NNSS. For the Rock Valley experiment, a chemical explosion would be placed at the hypocenter of a small shallow earthquake and recorded at a common set of receivers. A sequence of unusually shallow events along the Rock Valley Fault Zone in May of 1993 was recorded by a network of stations operated by the University of Nevada-Reno (UNR) and makes this novel experiment possible. As part of a feasibility study for this phase of the SPE, LLNL, UNR and NSTec are working to improve our understanding of the region and the propagation of energy from sources in the area to local and regional stations in the western U.S. Eight new seismic stations, including two borehole sensors, located at the original 1993 sites and additional sites, have been installed and ongoing seismicity along the fault is currently being recorded.

Examination of the local attenuation structure is an important part of our site characterization. We wish to ensure that an explosion generated at a Rock Valley site will be large enough to be recorded at regional distances, and a well-constrained attenuation study will aid us in all our SPE modeling efforts. We simultaneously invert regional phases Pn, Pg, and Lg (e.g. Pasyanos et al. 2009) to examine the crust and upper mantle structure of southern Nevada and surrounding region.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.