SEISMIC MONITORING NEAR YUCCA MOUNTAIN

Comprehensive seismic monitoring has been in effect near the potential high-level nuclear waste repository at Yucca Mountain since 1978. At that time a 56-station, primarily single-component, analog network with an aperture of about 125x125 km was implemented to characterize the seismic activity rates for faults and seismic source zones with implications for the seismic hazard at the site. In late 1995 a modern high-dynamic range digital network was set into operation as the analog network was phased out. The 30-station digital network was implemented within 50 km of the site with a dense distribution of stations within 20 km of the repository area. The digital network has resulted in a decrease in the detection threshold of roughly 1.0 magnitude unit near Yucca Mountain. The observed low seismicity in and around the Yucca Mountain block based on analog network monitoring has been confirmed with the more sensitive digital net; a few very small earthquakes (< 0.0) have been detected with digital network that would not have been recognized prior to 1995. The most significant earthquake in the 25 years of comprehensive monitoring has been the M 5.6 Little Skull Mountain earthquake that occurred about 20 km southeast of the site. This earthquake and the aftershock sequence have been studied in detail, and ground motion data from the sequence has contributed to numerous studies that evaluate the ground motion hazard at the site. Also, the three-component, high-dynamic-range data and waveforms from an increasing array of strong motion stations have made significant contributions to understanding local attenuation properties, site effects and expected ground motions from larger events. Activity in the Little Skull Mountain aftershock zone has continued; an M 4.4 earthquake in June of 2002 provided a detailed distribution of ground motion data in the mountain block from recently installed accelerometers. A new borehole array of accelerometers at the proposed location of surface facilities should provide critical data for the design effort. The lower detection thresholds provide valuable data for constraining the activity of geologic structures throughout the monitoring region and therefore a better understanding of the tectonic processes in southern Nevada.