Fault-related folds and their associated secondary faults play a critical yet often underrecognized role in accommodating strain and generating earthquakes in active fold-and-thrust belts. In the Seattle fault zone (SFZ), Washington, USA, we present new paleoseismic, geomorphic, and geophysical evidence for late Pleistocene and Holocene earthquakes on shallow, south-dipping secondary faults—the Lytle Beach and Vasa Park faults—that lie within the hanging wall of the greater SFZ and are on trend with the primary, blind Blakely Harbor fault. Our data show that these structures have ruptured independently, producing localized uplift and deformation at the surface, with the most recent event (RH2) likely occurring in the early nineteenth century. While a temporal overlap between the late Pleistocene RH1 and VP1 earthquakes raises the possibility of a ≥35 km rupture along the Blakely Harbor fault, structural and temporal evidence instead supports independent rupture on individual faults related to folding. We interpret these faults as bending-moment reverse faults that formed within a synclinal hinge zone of the main fault, reflecting mechanical and kinematic influences of the broader fault system. Combined with prior studies, our findings indicate that faulting related to folding dominates the mode of strain release within the SFZ since the late Pleistocene with more frequent earthquake recurrence (∼350 yr) over the past ∼2500 yr.
Angster et al. (Tue,) studied this question.