Seismicity in the vicinity of Yucca Mountain, Nevada, for the period October 1, 1999 to September 30, 2000

Starting on October 1, 1995, the monitoring of seismicity within the southern Great Basin near Yucca Mountain was performed with a new digital network. This network features three-component recording with 24-bit A/D conversion in the field. Continuous data are collected at 20 sps, and event triggered windows are collected at 100 sps. A seismic bulletin of events is made by automatically associating triggers among stations, classifying the local earthquake events, and locating the earthquakes and computing their magnitudes with conventional methods. This report covers the operational and seismic results of the fifth full year (FYOO) of the improved, digitally based, monitoring. The FYOO earthquake bulletin includes approximately 2300 events within about 50 km of Yucca Mountain. The largest event in FYOO, magnitude (ML) of 3.1, is anomalously low relative to the largest events in each of the 22 prior years of monitoring. Nearly one-half of the FYOO earthquakes are concentrated in the aftershock zone of the 1992 Little Skull Mountain (LSM) earthquake. Another primary contribution to the catalog are aftershocks of the ML 4.7 January 27, 1999, Frenchman Flat earthquake, which continued through FYOO. Hypocentral depths are largely concentrated in the range of 7-12 km, consistent with previous years. This is primarily a consequence of the depth distribution of aftershocks of the LSM earthquake; the LSM rupture surface was confined to between 6 and 12 km. The observed minimum detection thresholds for earthquakes within the network range in ML from -0.3 to 0.5 or greater, with the lower threshold achieved for earthquakes in the LSM and Yucca Mountain areas where the network is most dense. From the FYOO earthquakes, 28 new short-period first motion focal mechanisms were reliably determined. These, and the nearly 260 from the previous four years, show a consistent picture of the overall stress field in the region of the digital network. The tension axis is oriented at roughly 60° west of north, at shallow dip, and the pressure axis at roughly 30° east of north, with greater variability in the dip direction accounting for a collection of both strike-slip and dip-slip faulting within NW-SE extension. In FYOO three additional small events occurred within 10 km of the ESF, all with magnitudes \u3c 0. First-motion focal mechanisms could not be determined for any of these due to the limited number of stations that detected the events. In addition, two events were located in the southern part of the Yucca Mountain block, more than 10 km from the ESF; and a first-motion focal mechanism could be determined for one of these