Seismicity in the vicinity of Yucca Mountain, Nevada, for the period October 1, 2001 to September 30, 2002

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 seventh year (FY02) of the digital network monitoring. The FY02 earthquake bulletin includes over 5000 events within about 65 km of Yucca Mountain. This is roughly twice the average number of events in the previous six years and is due to the occurrence of a significant M = 4.4 earthquake on 06/14/2002 within the aftershock zone of the 1992 M = 5.6 Little Skull Mountain earthquake. This event was followed by nearly 3000 aftershocks through September 30,2002. Hypocentral depths of the recent aftershocks are largely concentrated in the range of 8-12 km, consistent with previous years. Earthquakes outside the LSM aftershock zone are largely in the 4-12 km range. The observed minimum detection thresholds for earthquakes within the network range in ML from -0.5 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. The M 4.4 earthquake at Little Skull Mountain on 06/14/2002 was the major event in the network during FY02. During FY02 only the M 4.4 Little Skull Mountain earthquake provided usable strong-motion recordings. The maximum acceleration of 0.084 g was observed at the station LSC nearly directly above the hypocenter, and maximum accelerations in a range of .004 to .032 g were observed near Yucca Mountain, with top end of the range observed at station WHBS on the pad of the proposed Waste Handling Building. Recorded accelerations were roughly in agreement with those predicted in the PSHA (Probabilistic Seismic Hazard Analysis) study for Yucca Mountain. From the FY02 earthquakes, 46 new short-period, first-motion focal mechanisms were reliably determined. These, and the roughly 300 from the previous six years, show a consistent picture of the overall stress field in the region of the digital network. The mean tensional 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 range of both strike-slip and dip-slip faulting within the general NW-SE extension. In FY02 five additional small earthquakes occurred within 10 km of the ESF, all with magnitudes \u3c 0.0. In addition, a small earthquake was located in the southern part of the Yucca Mountain block, more than 10 km from the ESF, and another in Crater Flat. In the Death Valley region, two earthquakes measured M \u3e 3 within the park boundaries. Little of the observed seismicity in the Death Valley region can be related to the two large faults there: Furnace Creek and Death Valley