HR: 0800h
AN: S51A-0226    [Abstracts]
TI: Progress Toward More Detailed Site-Conditions Maps for California
AU: * Wills, C J
EM: Chris.Wills@conservation.ca.gov
AF: California Geological Survey, 801 K Street ms 12-32, Sacramento, CA 95814, United States
AU: Gutierrez, C I
EM: carlos.gutierrez@conservation.ca.gov
AF: California Geological Survey, 801 K Street ms 12-32, Sacramento, CA 95814, United States
AU: Silva, M A
EM: michael.silva@conservation.ca.gov
AF: California Geological Survey, 801 K Street ms 12-32, Sacramento, CA 95814, United States
AB: We have developed a map of geologic units that can be distinguished by their shear-wave velocity. In developing this map we build upon earlier work to determine the shear-wave velocity characteristics of geologic units in California (Wills and Silva, 1998) and grouping geologic units with similar Vs into NEHRP categories (Wills and others, 2000). We have further refined those categories and prepared a map showing geologic units with distinct Vs characteristics (Wills and Clahan, 2006). We are testing potential refinements of the existing statewide map based on: 1. Improving the precision of locations the geologically-defined units and 2. Improving the definition of shear-wave velocity classes. For a test area in southern California, we have compiled the available high resolution geologic mapping. We have simplified the hundreds of individual geologic units shown on those maps to simplified units based on shear- wave velocity defined units of Wills and Clahan (2006). The result is a map showing geologic units with defined Vs30 values based on the most detailed available geologic mapping. Most of the simplified geologic units have well-defined ranges of shear-wave velocity, with the very important exception of younger alluvial deposits. These deposits may be thin – and profiles to 30 m include other, higher velocity materials, and the grain size and density may vary substantially. As a result there is a wide range of measured Vs30 values in younger alluvium. To sub-divide the young alluvium Wills and Clahan (2006) attempted to use very simple geographic criteria. We developed classes called thin alluvium, deep alluvium, fine alluvium and coarse alluvium based on geographic criteria. These categories appear to have differing shear-wave velocity characteristics, but the criteria for distinguishing them was not explicitly defined, nor were options for defining these classes explored. In order to make the mapping more consistent, and more applicable to other areas, we are exploring the use of two simplified rules for sub-dividing young alluvium. Preliminary results suggest that distance from bedrock and slope may both correlate with shear-wave velocity in young alluvium. Further work will define the relations between these factors and shear-wave velocity, and result in methods for producing improved maps of Vs30 for estimating seismic amplification in southern California and elsewhere.
DE: 7212 Earthquake ground motions and engineering seismology
SC: Seismology [S]
MN: 2007 Fall Meeting