HR: 0800h
AN: S21A-0234    [Abstracts]
TI: Liquefaction Scenarios in the Northern Santa Clara Valley for a Repeat of the 1868 Hayward Fault (M6.7-7.0) Earthquake
AU: * Holzer, T L
EM: tholzer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS977, Menlo Park, CA 94025, United States
AU: Noce, T E
EM: tnoce@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS977, Menlo Park, CA 94025, United States
AU: Bennett, M J
EM: mjbennett@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS977, Menlo Park, CA 94025, United States
AB: The spatial distribution of the probability of liquefaction in the northern Santa Clara Valley, California, was predicted for a repeat of an earthquake like the 1868 Hayward Fault (M6.7-7.0) earthquake. Probabilities were computed with the methodology for probabilistic liquefaction hazard mapping that was developed by Holzer and others (USGS OFR 02-296, 2006). The methodology relies on field-based plots of cumulative frequency of the liquefaction potential index (LPI) for spatially homogenous surficial geologic units. LPI, which is a scalar parameter that integrates the liquefaction potential of the entire soil column, was computed for 164 seismic cone penetration tests (SCPT) that were conducted in Holocene and Pleistocene geologic units. The plots of cumulative frequency were used to estimate the liquefaction probability distribution for each surficial geologic unit given peak ground acceleration (PGA) and earthquake magnitude. Scenario maps were produced with ArcGIS Model Builder. PGA at each node in a 50-m grid was estimated with the new attenuation relation proposed by Boore and Atkinson (2007, v. 3.04). Regional averages of V_S30 values, which were based on the SCPT, were used to account for local site amplification. The probability of liquefaction was estimated at each node using the liquefaction probability distribution appropriate for the surficial geology at the node. For a M7 earthquake and an assumed water-table depth of 1.5 m in the central part of the valley, liquefaction probabilities range from 0.1 to 0.2 along Coyote and Guadalupe Creeks, but are less than 0.05 elsewhere. For an M6.7 earthquake, probabilities remain greater than 0.1 along Coyote Creek but decrease along Guadalupe Creek to less than 0.1. For assumed water-table depths greater than 5 m, liquefaction probabilities are less than 0.05 throughout the valley. The probability of lateral spreading is less than 0.05 throughout the valley for both water table depths and both earthquakes. Liquefaction probabilities are substantially higher for an M7.8 1906-type earthquake on the San Andreas Fault, exceeding 0.3 along Coyote and Guadalupe Creeks for the 1.5-m water table depth. Predicted probabilities are highest in areas where liquefaction and lateral spreading were reported following the 1868 and 1906 earthquakes.
DE: 7212 Earthquake ground motions and engineering seismology
SC: Seismology [S]
MN: 2007 Fall Meeting