Along-strike variations in the shallow seismic velocity structure of the Seattle fault zone: Evidence for fault segmentation beneath Puget Sound
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| Authors: | Calvert, AJ; Fisher, MA; Johnson, SY |
| Year: | 2003 |
| Journal: | J. Geophys. Res.-Solid Earth 108 Article Link (DOI) |
| Title: | Along-strike variations in the shallow seismic velocity structure of the Seattle fault zone: Evidence for fault segmentation beneath Puget Sound |
| Abstract: | Around 1100 years ago, the Seattle fault, which trends east-west beneath Puget Sound and the greater Seattle metropolitan area, experienced a M>7 earthquake. We present high-resolution images of the shallow P wave velocity variation across the fault zone. These images were obtained by tomographic inversion of the first arrivals recorded along two north-south oriented seismic reflection lines shot within Puget Sound near Seattle. Just beneath the seafloor, the fault zone includes uplifted Tertiary rocks with seismic velocities in the range of 2300 to 2600 m s(-1). These velocities contrast markedly with values of similar to1600 m s(-1) in shallow Holocene sediments. South of the Seattle fault zone volcanic rocks of the Crescent Formation, which exhibit velocities >3700 m s(-1),are identified at depths of only 900 m. Seismic velocities of around 2600 m s(-1), which represent Oligocene rocks, are found in the hanging wall of the Seattle fault beneath eastern Puget Sound. In the west, lower, 2300 m s(-1) seismic velocities occur, probably due to the presence of Miocene rocks, which are not found in the east. Along-strike velocity variations arise from the folding of Tertiary rocks and the presence of distinct fault splays, including a north striking tear fault characterized by depressed seismic velocities that was intersected by the eastern seismic line. Along-strike differences in the uplift of Tertiary rocks beneath Puget Sound are likely associated with the existence of a segment boundary of the Seattle fault system. |
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