Complex VS profiles to 100 m depth from Rayleigh waves and 3-D VS model for Las Vegas Valley

Abstract from Manuscript 1, "Complex Shear-Wave-Velocity Profiles to 100 m Depth from Rayleigh Waves for Las Vegas, Nevada": Shear-wave velocity (VS) profiles were developed for 12 sites in the Las Vegas Valley, Nevada, which is situated on a deep alluvium-filled basin. The work was performed to support earthquake site response analyses. Data were acquired using the spectral analysis of Rayleigh-type surface waves (SASW) method. Sources used were an IVI Inc. "minivib" Vibroseis and an instrumented hammer. The combination of sources allowed VS profiles to be developed to 100 m and deeper without sacrificing resolution at shallow depths. The profiles were developed from the experimental dispersion data using straightforward linearized inversion and also following a method that incorporates the global search method of simulated annealing to optimize the starting model for linearized inversion. For all 12 sites, the two optimization processes resulted in nearly identical fits to the target dispersion curves. The VS of most layers from the two processes is within 20 percent. Resolution matrices for the two processes are comparable. Use of simulated annealing provided a measure of confidence in the correctness of the final VS profiles. Data from one site that is known to have a shallow, high-velocity inclusion were analyzed with the benefit of this independent information. The depth to and thickness of the high-velocity inclusion appear to be iv modeled accurately. Overall, the VS profiles obtained are consistent with expectations based on previous earthquake microzonation of the Valley; VS values are lower for finegrained sediments than for coarse-grained and cemented sediments. Average VS profiles previously developed for these two sediment-response units were updated; these profiles will enable refinement of previously developed earthquake ground-response projection envelopes.;Abstract for Manuscript 2, "Three-dimensional Shallow Shear-Wave Velocity Model for Las Vegas Valley": A three-dimensional (3-D) shear wave velocity (VS) model was developed for the heterogeneous shallow sediments (to nearly 400 m) of the Las Vegas Valley (LVV), Nevada. The model was based on more than 200 VS profiles and 1400 geologic well logs. Five sediment units including a cemented unit were defined from geologic log descriptions. A characteristic VS profile for four of the units was obtained by correlating between closely spaced pairs of VS and sediment data; a constant VS was assigned to the cemented unit. VS profiles were then assigned to each well location based on type of sediment according to the representative profiles. This assigned-velocity dataset was merged with measured VS profile data so that the measured data are honored in the model. The combined dataset results in a model with better resolution than a model developed using either of the two datasets independently. The software EarthVision was used to perform the 3-D interpolation of VS across the Valley. The model demonstrates the strong lateral variability of VS in the LVV. It also fits known patterns of sediment deposits: velocity in the central part of the Valley, where clay is the predominant sediment, is lower than velocity to the west and on the margins of the Valley, where gravel is predominant. The model may be used to predict Valley-wide earthquake ground-shaking patterns.