Two Post-Glacial Earthquakes on the Saddle Mountain West Fault, Southeastern Olympic Peninsula, Washington

Abstract

Abstract Two earthquakes ruptured the western strand of the Saddle Mountain fault after the Cordilleran ice sheet began its retreat from the Puget Sound Lowland about 17 ka. The fault system includes multiple northeast-trending thrust faults on the southeastern Olympic Peninsula delineated on light detection and ranging topography by prominent, east-side-up scarps that deform Vashon till and extend over 6–7 km. The Cargill Creek Trench on the Saddle Mountain West fault exposed faulted drift and postglacial colluvium below a 1–2 m high scarp. Steeply dipping northeast-striking faults form a 10 m wide zone beneath the scarp. Structural relations in the trench indicate that two earthquakes produced at least 1 m of vertical separation. Additional folding or faulting beyond the trench raised the scarp to a total height of 1.7 m. Aligned cobbles dipping approximately 40° to the north attest to earthquake-related tilting. A cobble firmly lodged in till and split by a secondary fault indicates that hanging-wall deformation involved a dextral component of slip. However, slip on the master fault may have involved a left-lateral component based on slip indicators at adjacent trench sites in the region. A single charcoal age and estimates of the onset of ice retreat limit the time of the oldest earthquake to 17–7.7 ka. The most recent event occurred after 1.7 ka, and may be the same earthquake that dammed Price Lake about 1100 yr ago known from related studies. Empirical relations suggest that the most recent earthquake was about M 7±0.2 and produced an estimated total slip of 1.3–1.7 m. The total postglacial slip rate ranges from 0.1 to 0.3 m/k yr. These results support the hypothesis that the Saddle Mountain fault is part of a larger fault system that accommodates transpressive strain as the Puget Lowland crust migrates around the eastern flank of the Olympic Mountains.