Quaternary Deformation in the Seattle Fault Zone: Insights from High-Resolution Marine Geophysical Data

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems
Type: Oral
Date: 4/28/2020
Time: 02:45 PM
Room: 240
Description: 

The Seattle fault zone (SFZ) is a blind, north-vergent thrust fault system located beneath the greater Seattle area. In A.D. 900-930, a M7-7.5 earthquake ruptured a primary, south dipping thrust fault in the SFZ, with probable concurrent slip on north dipping backthrusts. Due to the regional covering of water, glacially deformed Quaternary deposits, dense vegetation, the distribution, connectivity, recurrence of faulting is still debated and current models lack information from the shallow subsurface. Here, we use new marine seismic reflection data to characterize patterns in the depth extent and stratigraphic dip orientation of near-surface Quaternary strata and link them to fault-related deformation across the SFZ.

In 2017, we collected over 325 km of collocated chirp and multichannel seismic profiles throughout Puget Sound and Lake Washington, imaging the top 300 m of sediments at 0.25 m to 1 m vertical resolution. Using this dataset, we employ a coherence-based analysis to visualize fault-related patterns including: abrupt change in bedding plane dip, truncations of continuous horizons and zones of incoherence. We apply the method to legacy seismic profiles, giving a crustal-scale context to shallow observations and re-visualizing existing interpretations of tectonic deformation. Using this approach we find: 1) evidence for north-south oriented folding of inferred Pleistocene glacial deposits south of Bainbridge Island, 2) regional offsets in the depth extent of inferred ice-contact deposits in Lake Washington from the last glacial retreat and 3) growth strata and onlap of Holocene sediments in narrow windows of layered sedimentation. These fault-related features are coincident with existing interpretations of the SFZ and provide new insight into 3D patterns of crustal shortening as expressed in shallow marine and lacustrine sediments within the Holocene.

Presenting Author: Ginevra L. Moore

Authors