Abstract

Detection capability at NVAR (Mina, Nevada array) was assessed using a sequence of 553 earthquakes (ML < 4.4) that occurred from 07/31/1999 to 09/26/1999 near Scotty's Junction, about 60 km west of the Nevada Test Site (NTS). Based on the analysis of these data, the Pg 90% detection threshold at NVAR for events at Scotty's Junction for a normal noise environment was 44 nm/s zero-to-peak velocity as measured at NV08. That corresponds to 1.9 local magnitude units as determined by the University of Nevada at Reno (UNR). Using results from the Scotty's Junction events as well as from fifty earthquakes, 1.5 < mUNR < 4.7, which occurred on NTS between December 1998 and May 1999 and were located by the Southern Great Basin Digital Seismic Network (SGBDSN) operated by UNR, we calculated the Pg 90% detection level at NVAR as 2.5 UNR local magnitude units for events on NTS.

We determined locations using a crosscorrelation method to calculate back azimuth and horizontal phase velocity. Epicentral distance was estimated using LgPg arrival time differences. Arrival times for the Pg and Lg phases were determined using wavelet decomposition methods. Using sixteen of the Scotty's Junction events (ML > 3.2) identified as having similar locations and source mechanisms by crosscorrelation of their waveforms with a large, well located event, the sample standard deviation for estimated back azimuth was 0.98°. This standard deviation, in distance, was 2.7 km at 158 km, the distance from NVAR, element NV08, to Scotty's Junction.

For the events we located on NTS, the sample standard deviation in distance, due to errors inherent to the wavelet detector, was 1.5 km. Using only the NVAR array for epicentral location, we estimated a 90% coverage area of less than 100 km2 for events at 273 km distance in the southeastern corner of NTS.

We determined Pg group velocity from Scotty's Junction to NVAR as 5.98 km/s and Lg group velocity as 3.38 km/s. For the travel path between NTS and NVAR Pg group velocity was 6.06 km/s and Lg group velocity was 3.46 km/s.

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