Connectivity of Holocene Fault Network Between the Southern Olympic Mountains and the Puget Lowland

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

The extent and connectivity of crustal faults in the forearc of the Cascadia subduction zone is poorly understood due to limited paleoseismic and lidar data and surface processes such as recent glaciation, dense vegetation, and rapid erosion that obscure the geomorphic record. Documenting the extent and rupture histories of forearc faults and understanding how earthquake-related stress changes affect nearby structures, however, are crucial to characterizing seismic hazards in the region. Fault studies in western Washington have helped identify and map a Holocene-active, sub-vertical fault network that trends ENE from north of Grays Harbor (Canyon River fault-CRF) and along the southeast flank of the Olympic Mountains (Frigid Creek fault-FCF and Saddle Mountain fault zone-SMFZ), and trends eastward into the Puget Lowland along the south-dipping Seattle fault zone (SFZ). Geomorphic mapping indicates the CRF may be up to 60 km-long and recent trench results on the western CRF indicate it has displaced Holocene fluvial deposits during one or two left-lateral oblique earthquakes. New OxCal age modeling of these radiocarbon data constrains the most recent earthquake on the western CRF to ~4500 cal. yr BP. We integrate this mid-Holocene earthquake with timing information for younger (mid- to late-Holocene), left-lateral earthquakes farther east within this fault network (central CRF, FCF, SMFZ) and the youngest and easternmost earthquake on the adjacent north-vergent SFZ. Per-event vertical deformation on these faults ranges from ~2–8 m, suggesting significant modification to the static stress field. We use Coulomb stress-transfer modeling to evaluate how different ruptures may interact, particularly where paleoseismic data are insufficient to discriminate between or correlate events with large, overlapping uncertainties. These results further support a model that the Canyon River, Frigid Creek, Saddle Mountain, and Seattle faults comprise a kinematically-linked, ~150 km-long fault network that actively accommodates N-S shortening above the Cascadia subduction zone.

Presenting Author: Scott E. K. Bennett

Authors