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High-Resolution, Marine Geophysical Insights Into Quaternary Deformation and Long-Term Structural Evolution of the Seattle Fault Zone

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems I

Type: Oral

Date: 4/21/2021

Presentation Time: 09:45 AM Pacific

Description: 

The Seattle fault zone (SFZ) is a distributed, N-vergent thrust fault system that underlies the Seattle metropolitan area. Evidence of Holocene land-level changes, landslides, liquefaction, and a local tsunami indicate that this 70-km-long fault system is active and can host M7-7.5 earthquakes. Here we use >325 km of co-located multichannel seismic reflection imagery and chirp profiles to image the upper ~300 m of the subsurface at <1 m resolution across active faults in Puget Sound and Lake Washington. We use near-surface images of bedrock unconformities and overlying Quaternary geology to connect deeper crustal-scale structural interpretations from legacy seismic data with geomorphic and paleoseismic evidence of onshore Holocene fault slip to develop an improved structural model of the SFZ.

In Rich Passage, south of Bainbridge Island, we image the near-surface expression of the Orchard Point fault (OPF), one of many primary S-dipping strands of the SFZ mapped from regional-scale geophysics. Here the Oligocene Blakeley Harbor formation dips 45-80º to the north, likely in the forelimb of a fault propagation fold on the OPF and Blakely Harbor fault, another nearby S-dipping strand of the SFZ. Onlapping Quaternary glacial units and Holocene deformation patterns indicate that the OPF is inactive, and that the Rich Passage is now overprinted by backthrusting and folding in the hanging wall of a blind, S-dipping fault strand farther north. These shallow subsurface interpretations, integrated with existing bedrock geology, crustal-scale geophysics, and kinematic modeling of the 900-930 AD Restoration Point earthquake, suggest that the SFZ is a fault propagation fold, with an active deformation front that has stepped forward (north) over time. Trishear forward modeling of the proposed multi-phase, normal-sequence fault propagation fold geometry indicates that the SFZ could be accommodating ~20% of the 4-6 mm/yr north-south shortening throughout the Puget Lowland over the last 13 million years.

Presenting Author: Ginevra L. Moore

Student Presenter: Yes

Authors