EARTHQUAKES, MASS WASTING, AND FISH FROM NORTHERN CASCADIA: POST-GLACIAL RUPTURE OF THE LAKE CREEK–BOUNDARY CREEK FAULT REVEALED BY CHIRP SEISMIC INVESTIGATIONS AT LAKE CRESCENT, WASHINGTON

The Lake Creek – Boundary Creek fault (LCBCF) accommodates both northward convergence and clockwise rotation of the Cascadia forearc via oblique left-lateral slip along the northern flank of the Olympic Mountains. Past studies found two probable mid-to-late Holocene rupture events near Port Angeles. We document a record of five probable post-glacial (≤14 ka) earthquakes sourced on the LCBCF in sediments of Lake Crescent, 25 km west of Port Angeles.

In 2013 we collected 75 km of Chirp seismic imagery from the lake. Using seismic stratigraphy to depths of 20 m beneath the lake bottom, the gross stratigraphic architecture is controlled by extensive, periodic, and thick (1-2 m) seismically transparent units interpreted as subaqueous mass wasting deposits (MWDs). Several meters of undeformed, finely layered sediments drape the uppermost MWD, constraining the timing of its emplacement. Near to the trace of the LCBCF, a 15-m thick seismic stratigraphic section reveals evidence for five distinct localized deformation events beneath the youngest MWD unit. The total amount of stratal deformation decreases up-section across individual MWD marker beds, suggesting repeated late Pleistocene – Holocene surface rupture and associated subaqueous slope failures.

Several seismic transects crossed subaqueous portions of two large rock avalanche deposits with 10’as of m of relief across their surfaces. Depressions on the surface of these features have collected ~ 2 m of undeformed lacustrine sediments since emplacement, and MWD units are absent, indicating that they may have been triggered by the last earthquake on the LCBCF. One avalanche deposit blocked the former outlet stream to the Elwha River, raising lake level by 25 m until a new outlet formed via the Lyre River. A rooted subfossil tree at 18 m water depth that yielded a 14-C age of 4,350 ± 50 cal ybp might date the last earthquake and rock avalanches along the Lake Crescent section of the LCBCF. In addition, earthquake-triggered rockslides are important mechanisms for the isolation and genetic diversity of aquatic species. Ancestral anadromous populations of rainbow (now known as the Beardslee Trout) and coastal cutthroat trout were isolated in Lake Crescent by the lake-damming rock avalanche and have become genetically distinct from non-isolated regional populations.