New insights into Seattle fault zone geometry: incorporating shallow offshore deformation using high resolution seismic reflection imagery

The Seattle fault zone (SFZ) is an east-west striking, north-vergent thrust fault system that lies directly beneath the greater Seattle area. In A.D. 900-930, a M 7.0-7.5 earthquake ruptured a blind, south dipping thrust fault in this zone, resulting in up to 8 meters of land level changes, a local tsunami, and probable concurrent surface rupture on north dipping back-thrusts. Over the last few decades, aerial imagery, geophysical datasets, and paleoseismology studies have been used to constrain the geometry of the SFZ on land, as well as to develop several regional structural models to explain the distribution and behavior of identified compressional deformation features. Some of these models are kinematically incompatible, and uncertainty exists about whether slip on north-dipping back-thrusts only occurs concurrently with coseismic events on the primary fault zone, or if these secondary structures can rupture independently.

A detailed assessment of the location and structure of the SFZ both on and offshore is essential for understanding the associated seismic hazard. In 2017, a new high-resolution multichannel seismic reflection dataset of the SFZ was acquired throughout the Puget Sound and Lake Washington, consisting of reflection profiles from boomer and sparker sources and collocated chirp imagery. Here, we present processed and interpreted images from this new multi-resolution seismic dataset, resolving deformation features associated with crustal faulting in the top 50 meters of the subsurface. Specifically, through detailed processing and interpretation efforts we find: 1) evidence for along strike continuity of mapped scarps on land in offshore seismic sections, 2) shallow deformation features that coincide with crustal scale structures identified in legacy, deep-penetration seismic data, and 3) expressions of uplift in interpreted geologic units, providing a strategy to assess the relative time frame of deformation on these features. Together with a suite of existing marine and terrestrial data sets, we use these findings to develop an updated model of the SFZ architecture, which should help better characterize the seismic hazard in the urban Puget Lowland.