Seismicity and focal mechanisms for the Southern Great Basin of Nevada and California in 1990

Abstract

For the calendar year 1990, the Southern Great Basin seismic network (SGBSN) recorded about 1050 earthquakes in the SGB, as compared to 1190 in 1989. Local magnitudes, M{sub L}, ranged from 0.0 for various earthquakes to 3.2 for an earthquake on April 3, 1990 5:47:58 UTC, 37.368{degrees} North, 117.358{degrees} West, Mud Lake, Nevada quadrangle. 95% of those earthquakes have the property, M{sub L} {le} 2.4. Within a 10 km radius of the center of Yucca Mountain, the site of a potential national, high-level nuclear waste repository, one earthquake with M{sub L} = 0.6 was recorded at 40-Mile Wash. The estimated depth of focus of this earthquake is 3.8 km below sea level. Other, smaller events may have also occurred in the immediate vicinity of Yucca Mountain, but would have been below the detection threshold (M{sub L}{approx}0.0 at Yucca Mountain). Focal mechanisms are computed for seventeen earthquakes in the Nevada Test Site (NTS) and in the SGB west of the NTS for the year 1990. Solutions are mostly strike-slip, although normal slip is observed for a hypocenter at Stonewall Flat, Nevada, and reverse slip is observed for a hypocenter at Tucki Mountain, California. The average direction of the focal mechanism P-axes is North 47{degrees} East, with nearly horizontal inclination, and the average direction of the T-axes is North 42{degrees} West, with nearly horizontal inclination, consistent with a regional tectonic model of active northwest extension during the Holocene epoch.