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1 Shallow Shear Velocity and Seismic Microzonation of the Urban Las Vegas , Nevada Basin
| Content Provider | Semantic Scholar |
|---|---|
| Author | Scott, James B. Rasmussen, Tiana Luke, Barbara Taylor, Wanda J. Wagoner, Jeffrey L. Smith, Shane B. Louie, John N. |
| Copyright Year | 2006 |
| Abstract | Las Vegas Valley has a rapidly growing population exceeding 1.5 million, subject to significant seismic risk. Surveys of shallow shear velocity performed in the Las Vegas urban area included a 13 km-long transect parallel to Las Vegas Boulevard (“The Strip”), and borehole and surfacewave measurements of 30 additional sites. The transect was completed quickly and economically using the refraction microtremor method, providing shear velocity versus depth profiles at 49 locations. The lowest velocities in the transect, NEHRP D class, are near intra-basin faults found near I-15 and Lake Mead Boulevard. Calcite cementation of alluvium (a.k.a. caliche) along the Las Vegas Strip elevates Vs30 values to 500-600 m/s, NEHRP C class. Our transect measurements correlate poorly against geologic map units, which do not predict the conditions of any individual site with accuracy sufficient for engineering application. Some USDA soil map units do correlate, and Vs30 predictions based on measurements of soil units match transect measurements in the transect area. Extending soil-map predictions away from the area of dense measurement coverage generally failed to predict new measurements. Further, for several test sites the predictions were not conservative, in that the soil model predicted higher Vs30 than was later measured (predicting lesser potential ground motion). Subsurface information is needed to build a Vs30 model extending predictions throughout Las Vegas Valley. A detailed stratigraphic model built by correlating >1100 deep well logs in Las Vegas predicts Vs30 better than surface maps, but again only in parts of the Valley well-measured for velocity. The stratigraphic model yields good predictions of our transect Vs30 measurements. It is less accurate, although at least conservative, when extended to sites away from the transect. Introduction Las Vegas, Nevada is a rapidly growing population center that occupies a basin with known Quaternaryage faults. In addition, recent seismicity has been recorded in the broader region. Therefore, an up-todate assessment of the potential seismic hazard is needed. Vs30 is one predictor of earthquake ground motion amplification and potential hazard in similar alluvium-filled basins in California (Field et al., 2000). Under NEHRP-UBC provisions (BSSC, 2000) sites are categorized for shaking hazard using Vs30. Due to the costs of borehole and penetrometer measurement methods (e.g., ASTM standards D4428 and D5778), site classification in the Las Vegas area was previously based largely on geological maps with sparse downhole measurement support. Wills et al. (2000) prepared a site-conditions map for nearby areas of California on such a basis. Extending their classification from the California border into Las Vegas Valley, the Holocene alluvium covering the fans and valley floor would result in a predicted NEHRP class of D (Vs30 = 180-360m/s) for the entire Valley. Such extrapolation is unwarranted, given the remoteness of the borehole measurements used by Wills et al. (2000). A shallow shear-velocity measurement campaign in Las Vegas Valley is needed to provide accurate microzonation mapping. The purpose of this study was to determine the NEHRP site hazard classifications along the transect by providing a large number of measurements (49) of shear-wave velocity averaged to 30 m depth (Vs30). For two days in July of 2003, we conducted a refraction microtremor experiment across 13 km of the Las Vegas, Nevada basin (fig. 1). The transect follows the I-15 freeway south from Cheyenne Blvd. and parallels “The Strip” to Tropicana Blvd. We attempted to extend the area of applicability of our refraction microtremor results with a basin depth model (Langenheim, et al., 2001), valley-wide stratigraphy data derived from >1100 deep well logs, thirty additional shallow shear-wave velocity measurements using crosshole, refraction microtremor, and SASW methods (often comparing multiple techniques at the same site), and geologic and soil map-based surficial data. This information was combined to produce trial valley-wide Vs30 models. Refraction Microtremor Method Most of our shear-wave velocity measurements of the Las Vegas basin were based on the refraction |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://www.seismo.unr.edu/hazsurv/2005044_Scott-pp.pdf |
| Alternate Webpage(s) | http://pubs.usgs.gov/sir/2005/5116/pdf/sir2005-5116_B.pdf |
| Alternate Webpage(s) | http://crack.seismo.unr.edu/hazsurv/2005044_Scott-pp.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
