James B. Scott

Shallow shear velocity and seismic microzonation of the urban Las Vegas, Nevada, basin

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 Blvd. (The Strip), and borehole and surface-wave 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 intrabasin faults found near Interstate 15 and Lake Mead Blvd. Calcite cementation of alluvium (a.k.a. caliche) along the Las Vegas Strip elevates Vs30 values to 500–600 m/sec, 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.

A Shallow Shear-Wave Velocity Transect across the Reno, Nevada, Area Basin

In October and November 2001, we performed an urban shear-wave velocity transect across 16 km of the Reno, Nevada, area basin. Using the refraction microtremor method of Louie (2001) we determined shear-wave velocity versus depth profiles at 55 locations. Shear-wave velocity averaged to 30 m depth (Vs30) is one predictor of earthquake ground-motion amplification in similar alluvium-filled basins, and it is the basis of site hazard classification under National Earthquake Hazards Reduction Program-Uniform Building Code (NEHRP-UBC) provisions. A geologic map-based NEHRP classification along nearly all of our transect line would be NEHRP-D, but our measurements of Vs30 revealed that 82% of the transect is classified NEHRP-C. Relatively stiff Tertiary sediments underlie the surface of the Reno basin, and weaker soils occur east of downtown Reno in the floodplain of the Truckee River. Although 53 of our locations were on the geologically youngest and most active fluvial units, these sites showed Vs30 values ranging from 286 m/sec (NEHRP-D) to 849 m/sec (NEHRP-B). Mapped geologic and soil units are not accurate predictors of Vs30 measurements in this urban area. A test model based on gravity results showed Quaternary-alluvium depth can be combined with transect Vs30 mea- surements to predict Vs30 across the Reno basin.

A Transect of 200 Shallow Shear-Velocity Profiles across the Los Angeles Basin

This study assesses a 60 km north-northeast–south-southwest transect along the San Gabriel River for shallow shear velocities, in San Gabriel Valley and the Los Angeles Basin of southern California. We assessed a total of 214 sites, 199 along the transect at 300-m spacing, during a one-week field campaign with the refraction microtremor (ReMi) technique. The transect9s maximum 30-m shear velocity (Vs30) occurs in coarse alluvium of San Gabriel Valley where the San Gabriel River exits the San Gabriel Mountains, at 730 m/sec, upper National Earthquake Hazards Reduction Program (nehrp) site class C. Much of the northeast section of the transect (in San Gabriel Valley) is also nehrp class C, or near the CD class boundary. The section of the transect south from Whittier Narrows to Seal Beach shows nehrp-D velocities in active alluvium. The transect9s lowest Vs30, 230 m/ sec at the Alamitos Bay estuary, is also classed as nehrp-D. An increase toward the nehrp CD class boundary occurs at the shoreline beach outside Alamitos Bay, confirmed by additional measurements on Seal Beach. Our measured Vs30 values generally show good correlation with published site-classification maps and existing borehole data sets. There is no evidence in our data for an increase in velocity predicted by Wills et al. (2000) at their CD to BC site classification boundary at the San Gabriel Mountains front, nor for any decrease at their D to DE class boundary at Alamitos Bay. Very large Vs30 variations exist in soil and geologic units sampled by our survey. The Vs30 variations we measured are smaller than Vs30 variations of 30% or more we found between closely spaced (

An Idaho-Nevada-California Refraction Experiment: Utilizing Large Mine Blasts For Long-range Profiles

Utilizing commercial mine blasts and local earthquakes, as well as a dense array of portable seismographs, we have achieved high-resolution crustal refraction profiles across northern Nevada and the central Sierra Nevada Mountains. Using a dense spacing of 411 portable seismographs and 4.5 Hz geophones, the instruments were able to record events ranging from large mine blasts, small local earthquakes (approximately magnitude 2), as well as two larger earthquakes (magnitudes 2.8 and 3.8). Our instruments sensed blast first arrivals out to a distance of approximately 400 km. We have obtained 99% data recovery and clear refractions across the Sierra Nevada and the northern Great Basin regions.