Oblique fault systems crossing the Seattle Basin: Geophysical evidence for additional shallow fault systems in the central Puget Lowland

Upper plate seismicity in the Puget Lowland is more broadly distributed than mapped fault systems and presents a conundrum for understanding the active tectonics of the region. Although many previous studies have mapped faulting in the Puget Lowland from subsurface geophysical data, many of these efforts have focused specifically on mapping the structure of the Seattle Fault Zone and the South Whidbey Island Fault. The thick glacial sediments and extensive water bodies may conceal additional active faults away from these major structures. We map fault networks in Quaternary sediments broadly throughout the central Puget Lowland using existing marine multichannel seismic reflection data sets with widely distributed profiles to extend the results of previous work. We identify a NE‐SW zone of recent high‐angle faulting and shallow sediment deformation crossing the Seattle Uplift and the Seattle Basin that segments the Seattle Fault Zone and is distinct from previously mapped fault systems. Faults in this zone cut or deform sediments at the seafloor, and the zone trends across the central Puget Lowland at an oblique angle to major regional structures. Two additional zones of faulting trend NW‐SE and cut through the Seattle Basin and the Kingston Arch, respectively. Aeromagnetic lineations extend the NE‐SW trend of deformation across the Seattle Uplift and connect deformation of shallow sediment in the Puget Sound with deformation of shallow sediment in the Hood Canal. These oblique fault structures may partially control the wide distribution of seismicity within the central Puget Lowland and should be considered in seismic hazard assessments.