HR: 0800h
AN: S51A-0138    [Abstracts]
TI: Structure and Stresses Near Yucca Mountain, Nevada From Tomography and Focal Mechanisms
AU: * Preston, L
EM: preston@seismo.unr.edu
AF: University of Nevada Reno, Seismology MS/174, Reno, NV 89557 United States
AU: Smith, K
EM: ken@seismo.unr.edu
AF: University of Nevada Reno, Seismology MS/174, Reno, NV 89557 United States
AU: von Seggern, D
EM: vonseg@seismo.unr.edu
AF: University of Nevada Reno, Seismology MS/174, Reno, NV 89557 United States
AB: The local and regional tectonic setting of Yucca Mountain, Nevada, the designated nuclear waste repository, is characterized in geologic, neotectonic, seismological, and geodetic investigations. Although earthquake focal mechanisms have been compiled for M$>\sim$2 events for a number of years, the details and variability of the local stress field as well as the crustal velocity structure in the region is generally poorly understood. In an effort to address this, we have compiled over 1000 focal mechanisms from earthquakes that have occurred around Yucca Mountain since the early 1980's to define the regional stress field in the vicinity of the site. From 1976 to 1992, few earthquakes were observed with magnitudes over M3. However, after the Little Skull Mountain earthquake (06/29/1992 M5.6) and associated aftershocks, several earthquakes above M4 have occurred within 65 km of Yucca Mountain. Nonetheless, the vast majority of earthquakes, especially within 10 km of Yucca Mountain proper, are of M$<$1, where first-motion focal mechanisms are difficult, if not impossible, to develop even in this highly instrumented area. Including small events is crucial to our evaluation of the local stress field in and around the mountain block near the proposed repository site. We have developed a method that determines focal mechanisms and their associated statistical error bounds for events from a user-controlled combination of available first-motions and P, SV, and SH amplitudes. In order to resolve local variations in the stress field as best as possible, we have relocated the events using the hypoDD program. In addition to determining the stress field, we have imaged the 3-D velocity structure within 100 km of the mountain to approximately15 km depth using a non-linear iterative tomographic inversion applying both controlled-source and earthquake travel times. We have collected over 150,000 P and S arrival times from local earthquakes from 1980 through the present and over 4,000 first-arrivals from underground nuclear tests and the 1993 Non-Proliferation Experiment. Controlled-source locations and origin times are precisely known. The tomographic inversion simultaneous solves for optimal earthquake locations (3-D position and origin time) as well as for smooth P and S velocity structures. We compare locations determined in the joint hypocentral inversion result with relocated hypocenters applying standard location routines. Preliminary results demonstrate the commonly observed Great Basin result of a nearly 1-D, well-behaved, structure below about 3 km depth. The upper 3 km indicate a large-scale low-velocity feature that roughly corresponds to a trough (low) in the regional gravity image which includes Yucca Mountain. We will discuss the tectonic implications of the resolved stress field and structure.
DE: 8180 Tomography
DE: 7230 Seismicity and seismotectonics
DE: 8109 Continental tectonics--extensional (0905)
DE: 8164 Stresses--crust and lithosphere
SC: Seismology [S]
MN: 2004 AGU Fall Meeting