MEASUREMENTS AND PREDICTIONS OF VS30, Z1.0, AND Z2.5 IN NEVADA

Seismic shear-wave velocity (Vs) is a critical measurement for seismic design and performance assessment of major civil infrastructure and lifelines throughout Nevada. The Nevada Seismological Lab has posted a public database of Refraction Microtremor (ReMi) survey results of Vs versus depth in the Reno-area basin, and additional locations (sites.google.com/view/vs-profile-archive). Most of the Vs profiles were published previously in peer-reviewed journal articles. The database collects values of Vs30, Z1.0, and Z2.5 measured at 170 sites throughout the basin. Vs30 is the time-averaged Vs between the surface and 30 m depth. Z1.0 is the depth to the first occurrence of Vs = 1.0 km/s or greater, and Z2.5 is the depth to Vs = 2.5 km/s. The database contains many more Reno-area sites with Vs30 measurements that did not also have a Z1.0 measurement; this report does not examine sites without Z1.0 values.

All but a few sites have Vs30 between 260 m/s and 760 m/s, with a majority in NEHRP hazard class C. Sites that are geologically on bedrock have unexpectedly low Vs30, <760 m/s. Intensive surveying of bedrock sites shows extreme lateral variability of Vs30 as well as Z depths, with great variations over 20 m distances due to differential weathering of volcanic rocks. There appear to be no geological- or soil-mapping criteria able to predict Vs30 in Nevada, consistent with previous work. Neither Vs30 nor Z1.0 can distinguish basin from bedrock sites in Nevada. Basin sites may well have faster Vs30 measurements than nearby bedrock sites. Some bedrock sites have Z1.0 >0.1 km, and some basin sites have Z1.0 <0.05 km. In the Reno area, measured Z1.0 varies from 0.015 km to 0.45 km; Z2.5 varies from 0.1 km to 0.9 km. The ratio of Z2.5/Z1.0 is established with a minimum of 1.0, and with some sites exhibiting a ratio as high as 4.5. In 49 Las Vegas measurements along Las Vegas Blvd., Z1.0 varies from 0.05 to 0.68 km; Z2.5 measurements are not available. Z1.0 is typically between 0.05 km and 0.11 km. Variations are likely due to highly variable occurrence of Tertiary caliche cementation. Comparing the Vs30 and Z values to gravity-derived basin depths correlates the depths and allows development of a practical approach for roughly estimating Z1.0 and Z2.5 using ReMi or gravity data.