SEISMIC REFLECTION IMAGERY AIDS LATE QUATERNARY EARTHQUAKE HISTORY RECONSTRUCTIONS AT LAKE CRESCENT, WASHINGTON, USA

Lake Crescent is a 180-m-deep, glacially carved lake located in Olympic National Park, Washington, within the forearc of the Cascadia subduction zone (CSZ). The lake’s sediments are a high-resolution recorder of local paleoseismic activity, including from the CSZ and regional upper-crustal faults. Previous work at Lake Crescent has revealed a series of at least four meter-scale mass transport (MT) deposits in sub-bottom sediments at the lake during the last ~7,200 years, the two most recent of which are dubbed MTA and MTB. In addition to the MT deposits, numerous stacked centimeter-to-decimeter-scale turbidites are also preserved. The MT deposits, which include thick megaturbidite units, are attributed to a series of ruptures along the North Olympic Fault Zone that consists of three faults that in part transit beneath Lake Crescent, including the Lake Creek-Boundary Creek, Sadie Creek, and Barnes Creek faults. This study uses seismic reflection data in combination with existing geochronological and sedimentological data to characterize the upper two MT deposits and further analyze the post-MTA record (< ~3,100 YBP) of mass wasting (MW) deposits and their correlation to turbidite layers.

Newly acquired CHIRP seismic data allow us to better resolve presumed debris flow and overlying turbidite deposits in MTA and MTB and to identify correlative mass wasting deposits along the basin margins. Isopach modeling indicates maximum layer thicknesses of 3.9 and 7.6 m for the two layers, respectively, in the lake’s northern basin and 2.9 and 5.8 m in the southern basin. Cumulative wet volumes for MTA and MTB are estimated at 8.3 x 10⁶ and 9.9 x 10⁶ m³. The seismic dataset also reveals five groups of slope-marginal MW deposits above MTA in the southern basin, composed of at least 20 individual deposits. These MW deposits transition basinward into turbidites deposited in the past 3100 years, indicating that many turbidites were sourced from more than one slope failure location. Previous research indicates that multiple co-eval mass wasting deposits along a lake’s margin are a reliable indicator of an earthquake-induced trigger (Schnellman et al., 2002). These findings highlight the utility of lake deposits for assessing the seismic hazards to the Olympic Peninsula posed by the North Olympic Fault Zone and the Cascadia Subduction Zone.