Characterization of shallow shear wave velocity structures in southwestern Utah

Key to understanding local site conditions is the shallow shear-wave velocity structure. In the southwest corner of Utah near the rapidly growing urban areas of St. George and Cedar City, there currently exist no available data for characterizing site class units. This region has the potential for experiencing magnitude 6.5 or larger events. The University of Utah Seismograph Stations recently installed an urban strong-motion network in the region and there is also a need to characterize the shallow velocity structures at the sensor locations. In order to determine the shallow shear-wave velocity structure in and near St. George and Cedar City, we collected microtremor data using an array of four (three-component) broadband seismometers at six sites. We processed these data by (1) calculating the coherency between sensors, (2) calculating the horizontal to vertical spectral ratio (HVSR), and (3) calculating phase velocity dispersion curves. We determine the shallow S-wave velocity structure by a forward modeling approach using the Multimode spatial autocorrelation method (MMSPAC) and comparing predicted Rayleigh wave fundamental mode ellipticity curves to HVSR data. S-wave velocity models obtained at all sites seem reasonable given what is known of the geology with the exception of one site near Cedar City. The average S-wave velocity in the upper 30 meters (Vs 30) is between 360 and 760 m/s for all six sites. This is the velocity range corresponding to NEHRP site class unit C.