The role of oblique strike-slip faulting in the tectonics of the Puget Lowlands and throughout the Pacific Northwest

Abstract:

Upper-plate seismicity in the Puget Lowland, away from and oblique to known fault structures, challenges our understanding of active tectonics and seismic hazards of the region. The challenge arises in part from the thick glacial sediments and extensive water bodies that obscure active faults. Multichannel seismic reflection data in waterways, combined with aeromagnetic and gravity data, can effectively map fault structures throughout the Puget Lowland. Seismic reflection data indicate a NE-SW zone of recent high-angle faulting and shallow sediment deformation crossing the Seattle Uplift and the Seattle Basin, distinct from previously interpreted E-W fault systems including the main Seattle Fault. These NE-SW faults correlate with a zone of active seismicity, cut or deform sediments at the seafloor, and trend across the central Puget Lowland at an oblique angle to major regional structures. Aeromagnetic data show that the trend continues southeastward across the Seattle Uplift and connects deformation of shallow sediment in the Puget Sound with deformation of shallow sediment in Hood Canal. Two additional zones of faulting have NW-SE trend and cut through the Seattle Basin and Kingston Arch. Though strike-slip motion is a new interpretation for deformation along the southern edge of the Seattle Basin, it is not anomalous in the Puget Lowland; it is observed to the north along the southern Whidbey Island Fault (Sherrod et al., 2008) and Darrington-Devils Mountain Fault (Personius et al., 2014) and to the south along the Mount St. Helens seismic zone. Strike-slip motion accompanying regional compression and thrust-faulting occurs elsewhere in the Pacific Northwest including in south-central Washington (Blakely et al., 2014). Oblique strike-slip faults may contribute more significantly to deformation and seismicity within the Puget Lowland than previously recognized. Future work will evaluate the impact of deformation along these structures in tectonic models and seismic hazard assessments.