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Shallow Deformation in the Central Seattle Fault Zone, Washington State, From Land-Based High-Resolution Seismic-Reflection Imaging

Description: 

The detailed structure of the Seattle fault zone is laterally complex along its ~70 km length, including through the Seattle, Washington, urban area. Much of what is known about the fault zone has come from lidar imaging, paleo-seismological investigations, gravity and aeromagnetic modeling, and multi-scale marine seismic reflection imaging. Only a limited number of land-based high-resolution seismic imaging profiles crossing the fault zone have been acquired. We analyze 24 km of land-based P-wave seismic reflection profiles that fill in critical gaps in the Seattle fault zone in West Seattle, central Seattle, and Mercer Island. These data were acquired with either mini-vibroseis or accelerated weight-drop sources with nominal 5 m geophone spacings. The seismic profiles image tectonic deformation in the upper 1 km, including upwarped Tertiary rock and deformed Quaternary strata. The base of the Quaternary strata is a prominent reflector throughout the three sets of profiles. The West Seattle seismic profiles show a weaker impedance contrast at the base of Quaternary strata but provide evidence in the upper 500 m for the location of the northern fault strand currently implemented in the National Seismic Hazard Model. Deformational patterns interpreted on several profiles in the central region and Mercer Island are consistent with south-directed backthrusting with apparent dislocation on bedding planes. In the central Seattle area, an interpreted backthrust trends at approximately 45° NW compared to the nearest main thrust that trends generally east-to-west. The seismic profiles on Mercer Island provide evidence for the westward extension of both the Newcastle Hills and Newport anticlines to at least Mercer Island. The variability in thickness of the Quaternary deposits across the Seattle fault zone has implications for ground motion focusing and resonance effects.

Session: Crustal Deformation and Seismic Hazard in Western Canada, Cascadia and Alaska [Poster]

Type: Poster

Date: 4/20/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: William J. Stephenson

Student Presenter: No

Invited Presentation: 

Authors