Shallow Offshore Deformation in the Seattle Fault Zone: Insights From High Resolution Seismic Reflection Imagery

Date: 4/24/2019

Time: 11:30 AM

Room: Puget Sound

The Seattle fault zone (SFZ) is an east-west striking, north-vergent thrust fault system that is located 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 the SFZ, resulting in surface folding that produced up to 8 meters of land level changes, a local tsunami, and probable concurrent surface rupture on north dipping back-thrusts in the SFZ hanging wall. Over the last few decades, kilometer-scale geophysical datasets, lidar topography, and paleoseismology studies have been used to constrain the onshore SFZ geometry, and to develop several regional structural models to explain the connectivity and kinematics of the SFZ at depth. However, current models lack information from offshore studies of the shallow subsurface.

In 2017, a high-resolution seismic reflection dataset of the SFZ was acquired in the Puget Sound and Lake Washington, consisting of multichannel reflection profiles from boomer and sparker sources with collocated chirp imagery. Here, we present interpretations of this new multi-resolution seismic dataset that resolve lithologic discontinuities in the upper 300 meters of the subsurface. We integrate our offshore seismic interpretations with a geomorphic analysis of both new and existing bathymetric datasets, along with published observations of shoreline elevation changes associated with the A.D. 900-930 event. Through these efforts we find: 1) evidence that onshore scarps continue in offshore seismic sections, 2) shallow deformation features that coincide with the near-surface locations of previously identified crustal-scale structures, and 3) expressions of folding and vertical displacement of inferred Quaternary and Oligocene deposits, providing a strategy to assess the amplitude and relative timing of deformation on these structures.

Presenting Author: Ginevra L. Moore

Authors