EMPIRICAL LITHOLOGY AND SHEAR-WAVE VELOCITY RELATIONSHIPS FROM 62 SITE-SPECIFIC IMASW MEASUREMENTS: NEHRP SITE-RESPONSE CLASSIFICATION MAPS FOR ALBUQUERQUE, NEW MEXICO

Hazards resulting from strong ground motion can be understood as an interaction of three primary variables: (1) seismic source; (2) attenuation of seismic wave propagation; and, (3) effects of local surface geology. Local, regional, and national ground motion prediction equations now often require estimates of 30-m average shear-wave velocities (Vs30) to estimate ground shaking parameters (NGA, 2008). Shear-wave velocity (Vs) profiles from 62 sites across a representative range of geologic units in the Middle Rio Grande basin were collected using Interferometric Multichannel Analysis of Surface Waves (IMASW) technique. Of the various methods presently available to measure shear-wave velocity in the shallow subsurface as a function of depth (i.e., Vs30), few are economically viable to collect data on the scale required to develop statistically significant empirical relationships between lithology and Vs. Vs-based NEHRP soil classification criteria are correlated to geologic units to develop revised digital NEHRP site classification maps for Albuquerque, New Mexico. We developed a generalized geologic map from existing detailed geologic maps (Connell, 2006), by correlating geologic units that are similar in age, depositional process, consolidation, and grain size and assigned shear wave velocities to these units. A statistical analysis of Vs data yields representative Vs values to classify each geologic unit based on the NEHRP site soil classification scheme to produce a revised predictive site-response map for Albuquerque.