In October and November 2001, we performed an urban shear-wave velocity transect across 16 km of the Reno, Nevada, area basin. Using the refraction microtremor method of Louie (2001) we determined shear-wave velocity versus depth profiles at 55 locations. Shear-wave velocity averaged to 30 m depth (Vs30) is one predictor of earthquake ground-motion amplification in similar alluvium-filled basins, and it is the basis of site hazard classification under National Earthquake Hazards Reduction Program-Uniform Building Code (NEHRP-UBC) provisions. A geologic map-based NEHRP classification along nearly all of our transect line would be NEHRP-D, but our measurements of Vs30 revealed that 82% of the transect is classified NEHRP-C. Relatively stiff Tertiary sediments underlie the surface of the Reno basin, and weaker soils occur east of downtown Reno in the floodplain of the Truckee River. Although 53 of our locations were on the geologically youngest and most active fluvial units, these sites showed Vs30 values ranging from 286 m/sec (NEHRP-D) to 849 m/sec (NEHRP-B). Mapped geologic and soil units are not accurate predictors of Vs30 measurements in this urban area. A test model based on gravity results showed Quaternary-alluvium depth can be combined with transect Vs30 measurements to predict Vs30 across the Reno basin.

No abstract