HR: 16:00h
AN: S14C-01    [Abstracts]
TI: Probabilistic Seismic Hazard Maps for Seattle, Washington, Based on 3D Ground-Motion Simulations
AU: * Frankel, A D
EM: afrankel@usgs.gov
AF: U.S. Geological Survey, MS 966, Box 25046, DFC, Denver, CO 80225, United States
AU: Stephenson, W J
EM: wstephens@usgs.gov
AF: U.S. Geological Survey, MS 966, Box 25046, DFC, Denver, CO 80225, United States
AU: Carver, D L
EM: carver@usgs.gov
AF: U.S. Geological Survey, MS 966, Box 25046, DFC, Denver, CO 80225, United States
AU: Williams, R A
EM: rawilliams@usgs.gov
AF: U.S. Geological Survey, MS 966, Box 25046, DFC, Denver, CO 80225, United States
AU: Odum, J K
EM: odum@usgs.gov
AF: U.S. Geological Survey, MS 966, Box 25046, DFC, Denver, CO 80225, United States
AU: Rhea, S
EM: rhea@usgs.gov
AF: U.S. Geological Survey, MS 966, Box 25046, DFC, Denver, CO 80225, United States
AB: We have produced probabilistic seismic hazard maps for Seattle using over 500 3D finite-difference simulations of ground motions from earthquakes in the Seattle fault zone, Cascadia subduction zone, South Whidbey Island fault, and background shallow and deep source areas. The maps depict 1 Hz response spectral accelerations with 2, 5, and 10% probabilities of being exceeded in 50 years. The simulations were used to generate site and source dependent amplification factors that are applied to rock-site attenuation relations. The maps incorporate essentially the same fault sources and earthquake recurrence times as the 2002 national seismic hazard maps. The simulations included basin surface waves and basin-edge focusing effects from a 3D model of the Seattle basin. The 3D velocity model was validated by modeling several earthquakes in the region, including the 2001 M6.8 Nisqually earthquake, that were recorded by our Seattle Urban Seismic Network and the Pacific Northwest Seismic Network. The simulations duplicate our observation that earthquakes from the south and southwest typically produce larger amplifications in the Seattle basin than earthquakes from other azimuths, relative to rock sites outside the basin. Finite-fault simulations were run for earthquakes along the Seattle fault zone, with magnitudes ranging from 6.6 to 7.2, so that the effects of rupture directivity were included. Nonlinear amplification factors for soft-soil sites of fill and alluvium were also applied in the maps. For the Cascadia subduction zone, 3D simulations with point sources at different locations along the zone were used to determine amplification factors across Seattle expected for great subduction-zone earthquakes. These new urban seismic hazard maps are based on determinations of hazard for 7236 sites with a spacing of 280 m. The maps show that the highest hazard locations for this frequency band (around 1 Hz) are soft-soil sites (fill and alluvium) within the Seattle basin and along the inferred trace of the frontal fault of the Seattle fault zone. Sites on more consolidated soils within the Seattle basin generally exhibit higher hazard than those on similar soils outside the basin, because of basin surface waves and focusing of S-waves by the geometry of the edges and bottom of the basin.
DE: 7203 Body waves
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7255 Surface waves and free oscillations
SC: Seismology [S]
MN: 2007 Fall Meeting