HR: 17:00h
AN: S42H-05    [PDF]
TI: A Transect of 200 Shallow Shear-Velocity Profiles Across the Los Angeles Basin
AU: * Thelen, W A
EM: wethelen@seismo.unr.edu
AF: Nevada Seismological Lab, Mackay School of Mines, University of Nevada 174, Reno, NV 89557 United States
AU: Clark, M
EM: mclark@seismo.unr.edu
AF: Nevada Seismological Lab, Mackay School of Mines, University of Nevada 174, Reno, NV 89557 United States
AU: Lopez, C
EM: ctlopez@unr.edu
AF: Dept. of Geological Sciences, Mackay School of Mines, University of Nevada 172, Reno, NV 89557 United States
AU: Loughner, C
EM: loughner@seismo.unr.edu
AF: Nevada Seismological Lab, Mackay School of Mines, University of Nevada 174, Reno, NV 89557 United States
AU: Park, H
EM: hpark@unr.edu
AF: Dept. of Geological Sciences, Mackay School of Mines, University of Nevada 172, Reno, NV 89557 United States
AU: Scott, J B
EM: jscott@seismo.unr.edu
AF: Nevada Seismological Lab, Mackay School of Mines, University of Nevada 174, Reno, NV 89557 United States
AU: Smith, S B
EM: sbsmith@unr.edu
AF: Dept. of Geological Sciences, Mackay School of Mines, University of Nevada 172, Reno, NV 89557 United States
AU: Greschke, B
EM: passcal@passcal.nmt.edu
AF: IRIS-PASSCAL Instrument Center, New Mexico Tech, 100 East Road, Socorro, NM 87801 United States
AU: Louie, J N
EM: louie@seismo.unr.edu
AF: Nevada Seismological Lab, Mackay School of Mines, University of Nevada 174, Reno, NV 89557 United States
AB: In July 2003 we evaluated a 60 km transect for shallow shear wave velocities along the San Gabriel River, in the San Gabriel Valley and Los Angeles Basin of southern California. The purpose of the experiment was to evaluate spatial variability of earthquake shaking hazard, and any velocity correlations to mapped geological and soil units. We recorded urban microtremor on 300-m linear arrays, analyzing Rayleigh-wave dispersion curves to forward-model 200 shallow shear-velocity profiles to depths exceeding 100 m. Each 300-m array had 15 seismometers at 20-m intervals. We used IRIS/PASSCAL "Texan" single-channel recorders with 4.5-Hz vertical geophones. Shear velocities, vertically averaged to 30-m depths (V30), provided NEHRP soil classifications of ground shaking potential. We also performed P-wave refraction analyses at several points along the transect. Arrays in a highway cut on the gneissic bedrock of the San Gabriel Mts. yielded V30 between 520 and 640 m/s, NEHRP hazard class C. Measurements approach NEHRP-B ($>$760 m/s) only at the locations of the coarsest surficial alluvium in Azusa, below the mouth of San Gabriel Canyon, and near Santa Fe Dam. The maximum V30 along the transect was 730 m/s. Our results suggest much of the line north of Whittier Narrows is NEHRP class C (350-760 m/s), or on the C/D boundary. We find NEHRP-D values for V30 (150-350 m/s) from Whittier Narrows south to Seal Beach. The lowest velocities from the transect, 230 m/s, are still well above the NEHRP-E range. The lowest velocities occur near the south end of the line at Alamitos Bay. At Seal Beach, in the littoral sands, velocities rise slightly toward the NEHRP-C/D boundary. We interpret a lesser shaking hazard in the northern San Gabriel Valley and at Alamitos Bay than do assessments based on existing geologic maps. In San Gabriel Mts. bedrock on the other hand, rated NEHRP-B/C on maps, we interpret a somewhat elevated hazard, finding a NEHRP-C velocity of 640 m/s at most. Variations in V30 we measured may well be explained by the presence of a steep hydraulic gradient and a large average clast size near the San Gabriel Mts., with the gradient decreasing and the sediments fining downstream to Alamitos Bay. Corroborating measurements near Alamitos Bay, Long Beach, and Azusa, at locations removed from the active river channel, proved to match results from the transect.
UR: http://www.seismo.unr.edu/hazsurv
DE: 0935 Seismic methods (3025)
DE: 5194 Instruments and techniques
DE: 6334 Regional planning
DE: 7212 Earthquake ground motions and engineering
DE: 7223 Seismic hazard assessment and prediction
SC: Seismology [S]
MN: 2003 Fall Meeting