Lake Crescent, a 180-m-deep, glacially carved lake located on the Olympic Peninsula in western Washington, USA, overlies the Lake Creek-Boundary Creek fault zone, a system of structures with at least 56 km of late Pleistocene to Holocene surface rupture. Investigation of the lake's sediment, including a reflection seismic survey and analysis of piston cores, reveals evidence that the fault beneath the lake has ruptured four times in the past ~7200 years, producing unusually thick deposits termed megaturbidites. The earthquakes triggered rockslides that entered the lake and caused displacement waves (lake tsunamis) and seiches, most recently ca. 3.1 ka. Seismic reflection results from beneath the depth of core penetration reveal at least two older post-glacial ruptures that are likely to have similarly affected the lake. The stratigraphy of Lake Crescent provides insight into the behavior of a fault system that partially accommodates regional clockwise rotation and contraction of the northern Cascadia forearc through oblique dextral shear, and highlights the potential for disruption to critical infrastructure, transportation corridors, and industry on the North Olympic Peninsula during future surface-rupturing earthquakes. Our results illustrate the potential synergism between lacustrine paleoseismology and fault-scarp trench investigations. More precise dating of strong earthquake shaking afforded by continuous accumulation of lake sediment improves earthquake histories based on trenched fault scarp exposures, which are commonly poorly dated.
Bibliographical noteFunding Information:
Our work in Lake Crescent was supported by the U.S. Geological Survey (USGS) National Earthquake Hazards Reduction Program (Grant G15AP00091). Research in Olympic National Park, Washington, USA, was facilitated by Drs. Steven Fradkin and Jerry Freilich under U.S. Department of the Interior Scientific Research and Collecting Permit OLYM-00366. We are indebted to the talented personnel of LacCore, the National Lacustrine Core Facility at the University of Minnesota, Minneapolis, Minnesota, USA, including Anders Noren, Dr. Mark Shapley, Ryan O'Grady, and Christina Brady Shannon, without whom we could not have accomplished our research. Field sample collection was facilitated by the educators and staff of NatureBridge at Olympic National Park. The crew of the USGS R/V Parke Snavely were instrumental in the collection of high-resolution bathymetry data from the lake. Thorough and constructive reviews by Drs. Alan Nelson and Maarten Van Daele significantly improved the manuscript.
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