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New Constraints on the Location and Subsurface Structure of the Holocene Birch Bay Fault, Northwestern Washington, USA

The Birch Bay fault, located in the Bellingham Basin of northwestern Washington, USA, is a 25 to 60 km long, northwest-trending structure that accommodates north-directed shortening in the Cascadia forearc. The offshore portion of the fault is well constrained by marine seismic and magnetic data. Near the Birch Bay coastline, the fault has been inferred from airborne and marine magnetic anomalies that coincide with an uplifted beach platform and offset marsh and tidal sediments. The extension of the Birch Bay fault on land and to the southeast is coarsely constrained by aeromagnetic anomalies. The fault lacks clear geomorphic expression but is co-located with a broad zone of uplift and folding in Quaternary glacial and marine sediments and underlying Eocene sedimentary bedrock, and thus has been interpreted as a blind thrust.

We present high-resolution 2D seismic reflection data acquired along three 1 to 1.5 km long profiles that provide the first images of the fault in the shallow subsurface and support an updated interpretation of its location and subsurface expression. The fault is revealed in two profiles south and west of Ferndale as a northeast-dipping main thrust with several southwest-dipping back-thrusts that cut latest Pleistocene glaciomarine and Holocene Nooksack River delta sediments. Flower-style structures are imaged to within 30 m depth and spanning a width of 300 to 500m; however, seismic reflection resolution limitations do not preclude discrete faulting closer to the ground surface. The absence of the fault in a third profile near the Birch Bay shoreline supports a southward relocation of its position. We interpret a late Pleistocene through Holocene vertical slip rate of 0.2 to 0.8 mm/year, similar to other active crustal faults within the Cascadia forearc. The fault’s orientation and transpressive subsurface expression imply a significant dextral component of slip. Future work will involve ground-penetrating radar surveys to determine whether signs of fault rupture persist near the ground surface and to better characterize the attendant seismic hazard.

Session: Distributed Deformation from Surface Fault Rupture [Poster]

Type: Poster

Room: Evergreen Ballroom

Date: 4/22/2022

Presentation Time: 08:00 AM Pacific

Presenting Author: William C. Duckworth

Student Presenter: No

Additional Authors