Document Type

Technical Report

Publication Date

9-8-2003

Abstract

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 sixth full year (FY01) of the improved, digitally based, monitoring. The FY01 earthquake bulletin includes nearly 2000 events within about 65 km of Yucca Mountain. The two largest events in FYO1, both having magnitude (ML) of 3.4, are relatively small compared to the largest events in each of the past 8 years of monitoring by the Nevada Seismological Laboratory. Nearly one-half of the FYO 1 earthquakes are concentrated in the aftershock zone of the 1992 Little Skull Mountain (LSM) earthquake. Hypocentral depths of the aftershocks are largely concentrated in the range of 8-12 km, consistent with previous years. Earthquakes not in 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 -0.3 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 FY01 earthquakes, 37 new short-period first-motion focal mechanisms were reliably determined. These, and the nearly 300 from the previous five 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 NW-SE extension. In FY01 three additional small earthquakes occurred within 10 km of the ESF, all with magnitudes < 0.6. A first-motion focal mechanism was determined for one of them — its tensional axis was nearly identical to the mean of the entire set in the last six years. In addition, five small earthquakes were located in the southern part of the Yucca Mountain block, more than 10 km from the ESF. In the Death Valley region, an M 4.3 earthquake occurred in the zone of the Eureka Valley earthquake aftershocks, and five other earthquakes measured M > 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. During FY01 only two local earthquakes provided usable strong-motion recordings. Accelerations were all below l%g for these recordings.

Keywords

Earthquakes; Nevada – Yucca Mountain; Seismic networks; Seismological stations

Disciplines

Earth Sciences | Geophysics and Seismology

Language

English

Comments

Prepared by the Nevada Seismological Laboratory, Reno, Nevada for the U.S. DOE/UCCSN Cooperative Agreement Number
DE-FC08-98NV12081
Task 12: Southern Great Basin Seismic Network Operations
Document ID: TR-02-003, Rev. 0
Signatures have been redacted for privacy and security measures.

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