SHALLOW SHEAR VELOCITY AND SEISMIC MICROZONATION OF THE LAS VEGAS URBAN BASIN

In July 2003 we performed a seismic microzonation study of the Las Vegas basin along a 15 kilometer transect. Using 120 Ref-Tek RT-125 “Texans” on loan from PASSCAL, we completed this transect in two days. We used 4.5 Hz geophones to collect Rayleigh wave data for dispersion analysis. Only passive microtremor seismic sources are needed for this type of analysis; freeway and commuter traffic are used in this case. Using a spacing of 20 meters, several 800 meter sections were compiled to create a profile of 30-meter shear velocity approximately parallel to Interstate 15 from Cheyenne Avenue at the north to Tropicana Avenue at the south. We used three refraction lines, with a sledgehammer source, to augment the microtremor dispersion data with P velocities. The study is a first step in the characterization of the effects of ground shaking due to a seismic source in the surrounding region. Similar studies have been completed in the Reno area basin and in the Los Angeles Basin. The prior studies suggest that shallow shear velocity does not correlate well with geologic map units. The range of velocities within one map unit is greater than the average difference between units. This survey suggests a medium-scale study with a limited budget can be completed in a short time.