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Late Pleistocene and Protohistoric Earthquakes on Forelimb Thrusts Within the Seattle Fault Zone: Implications for Independent Hanging Wall Deformation Rates

The Seattle fault zone (SFZ) preserves a long-term deformation record of north-directed reverse faulting, although the majority of paleoseismic studies show evidence for multiple Holocene ruptures on ancillary bedding plane backthrusts within the hanging wall fold of the main fault. These paleoseismic studies elucidate the complex nature of hanging wall deformation within the SFZ and motivate our approach for evaluating fault-related folds for seismic hazard. In this study, we focus on two recently identified <2 km-long fault scarps located ~30 km apart along the SFZ that, unlike the previously studied south-facing scarps of the north-dipping backthrusts, are north-facing and spatially correlative with the main blind fault traces of the SFZ. Through geomorphic mapping, paleoseismic trenching, Bayesian analyses of ¹⁴C ages, and dendrochronology, we show that these north-facing scarps are underlain by south-dipping thrusts that yield evidence of three earthquakes. The oldest earthquake identified within the trenches across each scarp temporally overlap during the late Pleistocene but are likely separate events, while the youngest is constrained to the late Holocene on the western-most scarp and is possibly Historic. To understand the structural significance of these faults relative to the main blind fault(s) and backthrusts, we combine ground-based magnetic transects, oriented perpendicular to the fault scarps on Bainbridge Island, with measured bedrock sections. Our analysis reveals that the high-resolution magnetic signals reflect the lithologic variability in the underlying Tertiary bedrock. The main fault location is apparent from broad changes in the magnetic data, but the fault scarps lack a distinct and coherent signal between the two transects, suggesting that the north-facing fault scarp is also a fold-associated fault. Our results further emphasize the independent rupture of the fold-associated faults, which also display shorter recurrence intervals than the main fault(s) of the SFZ. This study highlights the cryptic nature of assessing thrust and reverse fault systems for seismic hazard analysis

Session: Cryptic Faults: Advances in Characterizing Low Strain Rate and Environmentally Obscured Faults - II

Type: Oral

Room: Tubughnenq’ 3

Date: 5/1/2024

Presentation Time: 10:30 AM (local time)

Presenting Author: Stephen Angster

Student Presenter: No

Additional Authors