RESOLVING THE STRATIGRAPHIC RECORD OF CRUSTAL AND SUBDUCTION EARTHQUAKES IN HOLOCENE SEDIMENTS OF LAKE CRESCENT, OLYMPIC PENINSULA, WASHINGTON

Lake Crescent is a deep, steep-sided lake within the forearc of the northern Cascadia subduction margin on the northern Olympic Peninsula in western Washington. Starting about 8,000 years ago, two distinctive event deposit types began accumulating in the lake. Four meter-scale mass transport deposits resulted from large, subaerial rockslides that entered the lake, likely triggering displacement (tsunami) waves. In contrast, twenty decimeter-scale turbidites can be traced to smaller mass failures along the lake’s subaqueous slopes. CHIRP seismic reflection imaging indicates that the rockslides and thick mass transport deposits were emplaced following ruptures of the North Olympic Fault Zone beneath the lake, which likely generated localized high-frequency ground accelerations that triggered the subaerial rock mass failures. In contrast, most turbidites formed during earthquakes on the seismogenic portion of the Cascadia subduction megathrust, 70 to 190 km to the west. During these events, seismic waves originating at the megathrust will experience more attenuation, particularly the high-frequency components, so that at Lake Crescent, the soft lake sediments are likely to amplify the low-frequency accelerations. The turbidites have age ranges that overlap with the ages of Cascadia earthquakes in regional offshore and onshore paleoseismic records, including those derived from deep-sea turbidites, lakes, fjords, and salt-water marshes. Importantly, deposits correlative to two regional earthquakes that multiple researchers postulate to have occurred at the subduction interface around 500-600 and 800-900 years ago are absent to poorly developed at Lake Crescent but are represented in the stratigraphy of Ozette Lake, located 60 km to the west of Lake Crescent and closer to the subduction trench. The differences between these two lakes suggest that shaking during these subduction zone events was sufficiently attenuated in the landward direction so that Lake Crescent’s underwater slopes were minimally affected. No turbidites have formed in Lake Crescent in the last 324 years since the great subduction zone earthquake of 1700 CE, implying that shaking from crustal or intraslab earthquakes has not exceeded the necessary local threshold for turbidite emplacement.