LATE QUATERNARY EARTHQUAKES ON THE CANYON RIVER FAULT, OLYMPIC MOUNTAINS, WASHINGTON: LINKING ACTIVE FAULTS IN THE PUGET LOWLAND TO OFFSHORE CASCADIA (Invited Presentation)

Active faulting and crustal deformation above the Cascadia subduction zone is driven by oblique plate convergence and clockwise rotation of the overriding North American plate. Along the south flank of the Olympic Mountains, we used field techniques and lidar analysis to map a ~60 km-long ENE-trending zone of scarps and topographic lineaments along the Canyon River fault. In the Wynoochee River valley, we mapped south-side-up scarps that cut at least four levels of latest Pleistocene to Holocene fluvial terraces, and excavated two scarp-perpendicular trenches (Mosquito and Zebra) to investigate the causes and timing of scarp formation. The 5.5 m-deep Mosquito trench crossed a 2–3 m-high scarp that cuts a terrace underlain by 9 ± 2 ka fluvial sand and gravel. Gravel beds are monoclinally folded across the ~20 m-wide scarp, but brittle faults or the underlying strath were not observed. The 3.5 m-deep Zebra trench crossed a 6–7 m-high scarp in older (~9–15 ka) terrace deposits ~250 m east of Mosquito trench and exposed a steep north-dipping fault zone that forms a negative flower structure. Fault plane striae (slickenlines) have shallow orientations (average rake of 032°), similar to slickenlines in a trench 12 km to the east (Walsh and Logan, 2007), providing evidence that the Canyon River fault accommodates south-side-up, left-lateral slip. More than half of the scarp height at the Zebra trench is the product of non-fault scarp-forming processes north of the fault zone, including fluvial downcutting and a possibly coseismic landslide. We interpret three late Quaternary surface-rupturing earthquakes from trench colluvial-wedge stratigraphy, upward fault terminations, and tree roots buried a meter below the modern root zone. Pending radiocarbon and luminescence ages for trench units will constrain the ages of these prehistoric earthquakes. The Canyon River fault is part of a kinematic link between Holocene faults in the Puget Lowland (e.g., Seattle, Saddle Mountain, Frigid Creek faults) and faults and folds north and east of Grays Harbor, possibly including latest Pleistocene structures that continue offshore. Our estimates of paleoearthquake recurrence, rupture length and magnitude will aid seismic hazard analyses and improve our understanding of the onshore-offshore structural linkages in Cascadia’s upper plate.