HR: 1320h
AN: S43A-0980    [Abstracts]
TI: Three Urban Transects of Shallow Shear-Velocity Using the Refraction Microtremor Method
AU: * Scott, J B
EM: jscott@seismo.unr.edu
AF: Nevada Seismological Laboratory and Dept. of Geological Sciences, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, Reno, NV 89557 United States
AU: Clark, M
EM: mclark@seismo.unr.edu
AF: Nevada Seismological Laboratory and Dept. of Geological Sciences, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, Reno, NV 89557 United States
AU: Lopez, C T
EM: ctlopez@unr.nevada.edu
AF: Nevada Seismological Laboratory and Dept. of Geological Sciences, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, Reno, NV 89557 United States
AU: Pancha, A
EM: pancha@seismo.unr.edu
AF: Nevada Seismological Laboratory and Dept. of Geological Sciences, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, Reno, NV 89557 United States
AU: Rasmussen, T
EM: mighty_mouse_76@yahoo.com
AF: Nevada Seismological Laboratory and Dept. of Geological Sciences, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, Reno, NV 89557 United States
AU: Smith, S B
EM: sbsmith@unr.nevada.edu
AF: Nevada Seismological Laboratory and Dept. of Geological Sciences, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, Reno, NV 89557 United States
AU: Thelen, W A
EM: wethelen@seismo.unr.edu
AF: Nevada Seismological Laboratory and Dept. of Geological Sciences, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, Reno, NV 89557 United States
AU: Louie, J N
EM: louie@seismo.unr.edu
AF: Nevada Seismological Laboratory and Dept. of Geological Sciences, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, Reno, NV 89557 United States
AB: Surveys of shallow shear velocity in the Reno, Los Angeles and Las Vegas urban areas give us information assisting in the mitigation of earthquake hazards, and in response to a damaging earthquake. The three transects (16, 60, and 15 km long respectively) were completed quickly and economically using the refraction microtremor method, providing characterizations of over 300 separate sites. Shear-wave velocity averaged over 30 m depth (Vs30) is unexpectedly smooth along all three transects, exhibiting realistic fractal dimensions of 1.5 to 1.8. For the Reno transect, which follows the Truckee River, most of the urban area shows NEHRP class C velocities. Very little of the profile shows velocities in the NEHRP class D range. Neither geological nor agricultural soil map units can serve to predict NEHRP hazard class or shallow velocities in the Reno area, aside from distinguishing between volcanic rock and sediments. Measured along the San Gabriel River, Vs30 values through Azusa and the Santa Fe Dam area are significantly higher than the NEHRP C/D range (270-555 m/s) expected from existing hazard mapping. Vs30 values in the southern 30 km of the Los Angeles transect were well within the previously expected NEHRP class D range (180-360 m/s). Measurements at Alamitos Bay were slightly above the expected NEHRP D/E range (90-270 m/s). In Las Vegas, the lowest velocities in the transect, at the NEHRP C/D boundary, are near intra-basin faults found near I-15 and Lake Mead Blvd. Velocities then rise smoothly to the middle of the NEHRP-C range near Sahara Blvd. to the south. Caliche cementation of alluvium in parts of Las Vegas elevates Vs30 values to 500-600 m/s. Vs100 values for the three transects, averaging to the 100 m depth to which most of our measurements are valid, show trends mimicking the Vs30 trends. Across all three cities our measurements correlate poorly against available USDA soil maps, geologic mapping, and hazard mapping. Surface maps do not predict the conditions of any individual site with accuracy sufficient for engineering application. A detailed stratigraphic model derived from deep water-well logs in Las Vegas predicts Vs30 better than maps, but only in well-measured areas.
UR: http://www.seismo.unr.edu/hazsurv
DE: 7223 Seismic hazard assessment and prediction
DE: 7294 Instruments and techniques
DE: 7205 Continental crust (1242)
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting