High-Resolution Marine Seismic Imaging of the Seattle Fault Zone: Near-Surface Insights into Fault Zone Geometry, Quaternary Deformation, and Long-Term Evolution

Abstract

ABSTRACT The Seattle fault zone (SFZ) is a north-directed thrust fault system that underlies the greater Seattle metropolitan area. Evidence of past land level changes, landslides, liquefaction, and a local tsunami indicate that this 70-km-long fault system can host up to M 7–7.5 earthquakes. Both the geometry and earthquake recurrence of the SFZ are debated and surveys of the shallow subsurface have not yet been incorporated into deeper crustal-scale structural interpretations, especially where the SFZ cuts across marine portions of the Puget Lowland. Here we use a new high-resolution marine seismic reflection dataset to image fault-related deformation in Quaternary sediments and Tertiary bedrock throughout Puget Sound and Lake Washington. We use this perspective of shallow geology as a link between existing crustal-scale geophysical insights into fault geometry at depth and paleoseismological observations of faulting at the surface and propose a refined structural model for the SFZ. We interpret that our new seismic reflection data in the Rich Passage area of Puget Sound images evidence of an inactive, south-dipping strand of the SFZ, which is overprinted by Quaternary folding and slip along north-dipping backthrusts within the hanging wall of a blind, south-dipping fault located 6 km farther north. To explain these results, we propose that the SFZ is a normal sequence fault propagation fold that has stepped northward through time, and we show the plausibility of this model through trishear forward modeling. Growth strata and faulting imaged in Quaternary sediments in Lake Washington and Rich Passage are consistent with the spatial distribution of folding and backthrusting that occurred during an M 7–7.5 earthquake in A.D. 900–930, corroborating existing evidence that the SFZ has been active throughout the Quaternary.