AGU 1998:
HIGH RESOLUTION ARRIVAL-TIME TOMOGRAPHY IN THE PUGET SOUND REGION, WASHINGTON USING DATA FROM THE 1998 SHIPS EXPERIMENT

N P Symons, R S Crosson, K C Creager, G C Thomas, A Qamar, B D Ruppel
(University of Washington, Geophysics, Box 351560, Seattle, WA, 98195)
T S Yelin, R D Norris, K L Meagher (U.S. Geological Survey, Seattle, WA, 98195)
T M Brocher, M A Fisher (U.S. Geological Survey, Menlo Pk., CA, 94025)

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P-wave tomographic imaging in the Puget Sound region of western Washington using data from the Pacific Northwest Seismograph Network (PNSN) is augmented with preliminary data from the 1998 Seismic Hazards in the Puget Sound (SHIPS) experiment. During SHIPS, about 30,000 air gun shots were fired at approximately 100 m intervals in the waterways of western Washington and British Columbia. We stacked waveforms recorded at about 50 PNSN stations to obtain approximately 1000 travel-times of first arriving compressional waves from a fairly uniform distribution of shots and receivers. Maximum offsets were about 200 km. The tomographic imaging incorporates 3-D ray tracing using finite-difference travel-time calculations and simultaneous relocation of earthquakes. The addition of the SHIPS data to the existing earthquake dataset produces an increase in resolution in the upper 20 km of our model. The new model includes the following features: (1) Clear evidence of low velocity basins under the cities of Seattle, Tacoma, and Everett. The best resolved of these, the Seattle basin, is about 10 km deep. The upper 4 km of the Seattle basin has P-wave velocities of ~ 2.5 km/sec, consistent with sonic well logs in unconsolidated sediment 15 km north. (2) A clear image of the low velocity core rocks of the Olympic Mountains and the contact of the core rocks with the underlying subducting slab of the Juan de Fuca plate at a depth of ~ 20 km. (3) The contact between the Crescent Formation and the Olympic core rocks. This contact coincides with the eastern edge of extensive seismicity in the Puget Lowland. The geometry of the contact suggests that the western edge of the Crescent is warped upward overlying the accretionary prism. East of the contact, the Crescent Formation appears to form the basement of the Puget Lowland. The Crescent Formation appears to extend eastward a distance of at least 40 km from the eastern edge of the Puget Sound.

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