Postglacial Mw 7.0–7.5 Earthquakes on the North Olympic Fault Zone, Washington

Abstract

ABSTRACTHolocene crustal faulting in the northern Olympic Peninsula of Washington State manifests in a zone of west-northwest-striking crustal faults herein named the North Olympic fault zone, which extends for ∼80 km along strike and includes the Lake Creek–Boundary Creek fault to the east and the Sadie Creek fault and newly discovered scarps to the west. This study focuses on the Sadie Creek fault, which extends for >14 km west-northwest from Lake Crescent. Airborne light detection and ranging (lidar) imagery reveals the trace of the Sadie Creek fault and offset postglacial landforms showing a history of Holocene surface-rupturing earthquakes dominated by dextral displacement along a steeply dipping fault zone. Paleoseismic trenches at two sites on the Sadie Creek fault reveal till and outwash overlain by progressively buried forest and wetland soils developed on scarp-derived colluvial wedges. Trench exposures of complex faulting with subhorizontal slickenlines indicate dextral displacement with lesser dip slip. Correlation of broadly constrained time intervals for earthquakes at the Sadie Creek sites and those to the east along the Lake Creek–Boundary Creek fault is consistent with rupture of much of the length of the North Olympic fault zone three to four times: at about 11, 7, 3, and 1 ka, with a shorter rupture at about 8.5 ka. Dated ruptures from trenches only partially coincide with coseismic landslides and megaturbidites in Lake Crescent, indicating that some earthquakes did not trigger megaturbidites, and some turbidites were unrelated to local fault rupture. Landform mapping suggests single-event dextral displacement of 4±1 m on the Sadie Creek fault. Inferred maximum rupture length and single-event slip imply earthquake magnitudes Mw 7.0–7.5. Dextral slip rates of 1.3–2.3 mm/yr and the ∼11,000 yr slip history suggest that the North Olympic fault zone is a prominent contributor to permanent strain in the northern Cascadia fore-arc.