Lacustrine Paleoseismic Records of Large Cascadia Earthquakes from Lake Ozette, Washington

Abstract:

Lakes often contain the high-fidelity sedimentary proxies needed to reconstruct the timing of punctuated environmental disturbances, such as 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 known active upper-plate faults capable of producing large magnitude earthquakes. The USGS led a three-week field campaign in 2019 aimed at acquiring high-resolution bathymetry, sub-bottom profiles, and sediment cores. Combined with new subaerial LiDAR data, topo-bathymetry data of the entire lake basin is resolved to ~2 m resolution. A dense grid of subbottom Chirp and Boomer profiles provide three-dimensional constraints on the lacustrine stratigraphic framework 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 small deltas is blocky, rugged, and appears to be covered in MTDs. Here Chirp profiles image a succession of up to twenty-five stacked high-amplitude layers that onlap surrounding slopes. Vibracores confirm the five youngest of these are discrete, graded sand beds. Core sites within distal locations (relative to fluvial inputs) contain far less sand, and appear to represent condensed sections of laminated silt and mud spanning the last 4–5 ka. For example, one site (VC08) is isolated from fluvial sources, but located downslope of nested headwall scarps; it contains up to ten discrete, 2-3 cm thick, organic-rich MTDs. Radiocarbon dating suggests that the MTDs in VC08 were deposited during the last ~3.5 ka. Our preliminary interpretation is that shaking and failure of surficial lake sediment generated the MTDs during large megathrust ruptures, and that Lake Ozette contains a complete Holocene record of shaking along this portion of the Cascadia subduction zone.