Southern Great Basin seismological data report for 1981 and preliminary data analysis

Earthquake data for the calendar year 1981 are reported for earthquakes occurring within and adjacent to the southern Great Basin seismograph network. Locations, magnitudes, and selected focal mechanisms for these events and events from prior years of network operations are presented and discussed in relation to the geologic framework of the region. These data are being collected to aid in the evaluation of the seismic hazard to a potential repository site at Yucca Mountain in the southwestern Nevada Test Site. The regional stress field orientation, as inferred from focal mechanisms, is characterized by a northwest-directed least compressive stress and a northeast-directed greatest compressive stress. We infer from this stress orientation that faults of north to northeast trend are most susceptible to slip. Faults of this orientation exist within the Yucca Muntain block, but they probably have not moved significantly in the last 500,000 years. Yucca Mountain lies within a fairly large area of relatively low level seismicity extending west to the Funeral Mountains, south to the Black Mountains and Nopah Range, and southeast to the Spring Mountains. One M 1.7 earthquake has been located in the Yucca Mountain block in about I year of intense monitoring. At present somewhat conflicting geologic, seismologic, and stress evidence hinder definitive conclusions about the seismic hazard at the proposed repository site.