A HOLOCENE EARTHQUAKE RECORD FROM LAKE CRESCENT, OLYMPIC PENINSULA, WASHINGTON

The sedimentary fill of Lake Crescent on the Olympic Peninsula in western Washington preserves a record of Holocene earthquakes of varied source and shaking intensity. Rupture of the Lake Creek-Boundary Creek Fault, an upper crustal structure with oblique-slip motion that runs directly beneath the lake, has had particularly dramatic effects on the stratigraphy. Four ruptures of this fault in the past 7200 years have deformed the lacustrine strata, and have triggered large rockslides on the surrounding steep, forested slopes that have entered the lake. The ensuing lake tsunamis/seiches deposited megaturbidites that are traceable over the 20 km² lake basin and exceed a meter in thickness over most of the deep lake floor. Strata above the youngest of these megaturbidites, deposited around 3100 cal yr BP, are virtually undeformed, but contain thinner turbidites that record remobilization of sediment from the lake’s subaqueous slopes with return frequencies of roughly 200-300 years. The distribution of these turbidites across separate sub-basins of the lake, as well as their volume and composition, support the interpretation that their deposition was triggered by seismic shaking.

Many of the turbidites in Lake Crescent have ages that overlap with those of co-seismically subsided soils, coastal tsunami deposits, and deep-sea turbidites previously documented in northern Cascadia, suggesting that they were triggered by rupture of the subduction interface. The sources for earthquakes that formed the remaining turbidites are presently unknown, but include possible ruptures along segments of the Lake Creek-Boundary Creek fault zone distal to the lake, or along one or more of a number of other steeply dipping, crustal faults mapped in the region. The paleoseismic archives in Lake Crescent and in several other lakes on the northern Olympic Peninsula provide an important means for determining the pre-instrumental histories of these crustal faults, which accommodate north-south contraction and clockwise rotation of the western Washington portion of the Cascadia forearc, and which are hypothesized to interact with the Cascadia megathrust and influence its behavior.