TWO POST-GLACIAL EARTHQUAKES ON THE SADDLE MOUNTAIN WEST FAULT, SOUTHEASTERN OLYMPIC PENINSULA, WASHINGTON

Two earthquakes ruptured the Saddle Mountain West fault on the southeastern Olympic Peninsula after the Cordilleran ice sheet retreated northward from the Puget Sound lowland about 17 ka. Multiple Holocene scarps define the Saddle Mountain fault zone, first recognized in the early 1970s on 1939 aerial photography and more recently illuminated by LIDAR. The scarps deform Vashon glacial till and reflect northeast-trending, east-side-up reverse faults that extend up to ~18 km based on regional aeromagnetic mapping. High-resolution magnetic surveys across the western-most scarp, acquired by others, image a linear magnetic trough that is consistent with a two-dimensional fault model that offsets the Eocene Crescent Formation by >20 m at 30 m depth. To investigate the post-glacial earthquake history, we excavated a trench across the western fault strand that exposed fault-related deformation in Vashon glacial till, stratified drift and post-glacial colluvial deposits within a 1- to 2-m high scarp. Underlying the scarp, northeast-striking faults and fractures dip steeply to the southeast and northwest. Retrodeformation of strata across graben-bounding faults in the hanging wall imply that at least two earthquakes produced the observed vertical separation of ~1 m. Additional folding or faulting not evident in trench exposures probably raised the scarp to its total height of 1.7 m. Measured cobble alignments in till suggest that folding produced rotations <20°. A cobble split by a secondary fault in the hanging wall implies a dextral component of slip that is consistent with right-lateral deflections of aeromagnetic anomalies. In contrast, slip indicators reported for exposures of the eastern fault strand indicate a sinistral component of slip. A single calibrated ¹⁴C age on detrital charcoal and independent estimates of the onset of ice retreat limit the time of the earlier earthquake to 17-8.5 ka. The most recent event occurred after 1.7 ka based on ¹⁴C ages, and probably produced surface rupture that dammed Price Lake about 1,100 years ago. The total post-glacial vertical deformation rate is ~0.1 m/kyr and vertical displacements per event range from 0.2-0.8 m. Empirical estimates of earthquake magnitude using fault length and mean displacement suggest M 6.5-7 for the most recent earthquake.