Empirical Site Response and Comparison with Measured Site Conditions at Anss Sites in the Vicinity of Reno, Nevada

We empirically evaluate spectral amplification relative to a nearby rock site at 18 Advanced National Seismic System strong-motion stations within the basin containing the urban areas of Reno and Sparks, Nevada. The near-surface site conditions have a strong effect on ground motion, which is clearly demonstrated through analysis of weak motion. The study uses multiple regional earthquake events of varying azimuth. Averages of these empirical amplifications, grouped by generalized geological formations (volcanic rock, Pliocene and early to mid-Quaternary sediments, and younger Quaternary sedimentary sites), show dependence on the geology. However, when the stations are grouped according to V-S30 values, the mean amplifications are more distinct, indicating that V-S30 is a more useful predictor of amplification for the Reno-Sparks, Nevada, urban area. Spectral amplifications computed from ground motions of the local Mogul, Nevada, 2008 earthquake swarm (ML 0.6-4.7; Anderson et al., 2009) have similar spectral shapes for 8 of the 12 stations where comparisons could be made, but the absolute amplitudes are slightly inconsistent. Four stations show unexplained peaks in the amplitude spectral ratios from the mainshock ground motions. The comparisons verify that multiple regional ground motions can be used to obtain relative site-response amplification functions for seismic-hazard applications, but the discrepancies are a reminder that directional effects contribute uncertainty to the amplification functions.