HR: 1340h
AN: S53B-1270    [Abstracts]
TI: Site Response in the San Joaquin/Sacramento River Delta
AU: * Fletcher, J B
EM: jfletcher@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, ca 94025, United States
AU: Boatwright, J
EM: boat@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, ca 94025, United States
AB: The Sacramento/San Joaquin River Delta lies on the western edge of the Great Valley and contains a system of levees that are thought to be prone to catastrophic failure from a major earthquake in the San Francisco Bay area or on faults along the western border of the Great Valley. To assess this risk we deployed digital recorders and broadband sensors in late 2006 and 2007 at 3 levee sites in the Delta (each site had a top and base sensor) and at one reference site to the west. Cone penetrometer data show that at the base, the soils have low S-wave velocities of 170 to 240 m/s. Upper soil layers are typically peats and aeolian sands. During the nine months of deployment, we recorded 3 local events (45km<r<60km) that occurred on or near to the Hayward Fault. The events range in depth from 5 to 16 km with magnitudes of 3.7 to 4.2. Site response is estimated by ratios of S wave spectra at each site to spectra from a reference site to the west of the Delta (Byron Hot Springs , BYR) or Black Diamond Mine (BDM, part of the Berkeley Digital Seismic Network). Spectra are normalized for distance. Each spectrum is smoothed with an algorithm that tries to preserve peaks; a running mean filter is also applied to the spectra from the reference site to reduce the possibility of holes in the reference spectrum appearing as a resonance in the ratio. Our primary observation is that many of the spectral ratios show large resonances, typically at 1-3 Hz and may represent a substantial risk to the levees. Sites at the tops of levees typically have stronger resonances in the 1-3 Hz range compared to base sites. The character of these ratios, however, differs substantially for each event. For example, the top site at Bethel Isl. has peaks in the site response with amplitudes between 6 and 15 (2-3Hz) for an earthquake located near Berkeley using either reference site, but is only apparent in the ratios using BDM for the other two events. This is because BYR has more amplitude in the 2- 3 Hz band then BDM typically does. The event with a depth of 16.2 km appears to have the smallest resonances when using BYR as a reference. Long-period surface waves arrive as much as 20s late compared to the high- frequency S waves on records from Delta sites producing very long codas on these records. Because of a strong resonance at the top site compared to the base site at Bethel Isl. we investigated whether the resonance could be explained by a topographic effect. Polarizations at top sites are not consistent with the focal mechanism, nor are they simply related to the strike of the levees suggesting that the source of such a marked change in the character of the ground motion on the levees is differences in subsurface geology.
DE: 7212 Earthquake ground motions and engineering seismology
SC: Seismology [S]
MN: 2007 Fall Meeting