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Lacustrine Paleoseismic Records of Cascadia Megathrust Earthquakes From Lake Ozette, Washington

Lakes often contain the high-fidelity sedimentary proxies needed to reconstruct the timing of punctuated shaking from large earthquakes and other hazards typical along active continental margins. Here we present evidence for earthquake-triggered mass transport deposits (MTDs) in Lake Ozette, Washington, a ~100 m deep coastal lake located along the outer coast of the Olympic Peninsula. Lake Ozette is likely situated above the locked portion of the northern Cascadia megathrust and is relatively isolated from active upper-plate faults. The USGS led two field campaigns in 2019 and 2021 to acquire high-resolution bathymetry, sub-bottom profiles and sediment cores to develop three-dimensional constraints on Ozette’s sedimentation history for at least the last 10 ka. Several sub-basins separated by bathymetric sills characterize the basin physiography; the eastern sub-basins are proximal to fluvial catchments and small subaqueous deltas; the western sub-basin is isolated from any significant fluvial sediment sources. The basin floor near the deltas is blocky, rugged and appears to be covered in MTDs. Chirp profiles image a succession of up to twenty-nine stacked high-amplitude layers that onlap surrounding slopes. Sediment cores sampled to a depth of 14 meters, bottoming out in coarse grained glacial outwash deposits and confirm these high-amplitude layers are discrete, normally graded sand beds (turbidities). Core sites within distal basins contain massively bedded, organic-rich silt/mud layers; each corresponds to one of the sand layers in the proximal basins. These deposits appear to have been triggered by subaqueous slope failures during megathrust ruptures. More than 50 radiocarbon dates of terrestrial plant fragments are used to constrain the ages of the youngest 11 turbidites (last ~5.5 kyr BP), yielding a recurrence interval of ~500 years. Lake Ozette may contain the most complete record (~29 events) of subaqueous MTDs generated by large megathrust ruptures along this portion of the Cascadia subduction zone.

Session: Shakes in Lakes: Frontiers in Lacustrine Paleoseismology

Type: Oral

Room: Regency A-C

Date: 4/21/2022

Presentation Time: 03:00 PM Pacific

Presenting Author: Daniel S. Brothers

Student Presenter: No

Additional Authors