SHEAR WAVE VELOCITY CHARACTERIZATION TO ADDRESS IMPACTS OF SHALLOW SEDIMENTS ON GROUND SHAKING FOR LAS VEGAS

In the event of a major earthquake near Las Vegas, weak-ground-motion data have shown that the intensity and spectral content of ground shaking will be variable across the Las Vegas Basin. The Basin, which covers approximately 1600 square kilometers in surface area, is home to about 2 million people. A preliminary microzonation, based on predominant sediment type in the upper 30 m and validated using weak ground motion measurements, has identified two zones. One zone encompasses the central to eastern portion of the Basin where fine-grained sediments predominate, and the other encompasses the western portion of the Basin and around the Basin margins where gravels predominate. Because shear wave velocity is a key parameter in defining the response of a sediment column to dynamic input, the microzonation effort is being advanced by expanding the velocity map of the Basin, in terms of both coverage and detail. Emphasis is on characterizing velocities and their variation using surface waves. Through use of a “minivib” vibroseis and passive-source methods, dozens of detailed, one-dimensional profiles are being resolved, in some cases to depths of 100 m or more. The database is supplemented with 160 simpler shear wave velocity profiles that were collected for development purposes and filed in public records. When coupled with deep shear-wave velocity data collected using single-station group-wave velocity measurements, the data will facilitate generation of a three-dimensional shear-wave velocity map of the Basin. Intelligent interpolation of velocity data will account for sediment type, the presence of faults that cut the sediments, and possibly alluvial-fan source materials.

In addition to the shear wave velocity of the shallow sediments, other key factors influencing ground-surface shaking in the Basin are multi-dimensional basin-edge interference effects, near-fault effects and the dynamic response of the Basin's deeper sediments. Supplementary to the velocity maps, analyses are planned to investigate the impacts of these variables on sediment response. Amplification factors developed through this process can be applied, along with the characteristics of the earthquake-producing faults, to build seismic hazard maps for use in urban planning.