In this project the long-term record of large Cascadia earthquakes contained in the sedimentary fill of Lake Quinault, a deep, glacial moraine-dammed lake on the Olympic Peninsula, Washington, will be investigated. Lake Quinault is ideally situated to provide a long record of both seismicity in Cascadia and its upland drainage-basin scale impacts. A high resolution seismic reflection survey will be conducted to provide information about the thickness and overall geometry of the lake's sedimentary infill and to identify lake-margin slump deposits that may have been triggered during earthquake events over the past several thousand years. Long (ca. 12 m) piston cores will be recovered from at least five locations near the basin center, and event layers will be identified and interpreted using core descriptions, geophysical measurements, and x-radiography. The layers will be correlated to the lake-margin stratigraphy to help establish their co-seismic origin, and radiocarbon analyses of macroscopic plant debris will be used to evaluate their relationships to a well-established regional paleo-seismological history. Geochemical analyses of the sediments will be performed to test the hypothesis that following a great subduction zone earthquake, the production of sediment by deep-seated bedrock landslides will overwhelm erosion via nearer-surface mechanisms on a decadal time scale. Because of their hypothesized greater depth of origin from beneath the surface, we predict that compared to other strata, the organic matter and sediments accumulating in Lake Quinault immediately after a large earthquake and continuing for several years will be relatively depleted in isotopes including 14C, 10Be, and 26Al and will have relatively higher 26Al/10Be ratios.
The goal of this project is to improve understanding of the terrestrial upland response to great subduction or other large earthquakes on the Cascadia margin of North America. Previous studies of subsided coastal lowland soils, low-lying coastal lakes and lagoons, fjords, and deep sea turbidites have established that these events have occurred in this region at 300-500 year frequencies. Based on research in other areas of the world with large topographic relief, earthquake-triggered landslides are likely to be one of the most significant hazards to life and infrastructure faced during and in the immediate aftermath of future events. By documenting the sedimentary and geochemical record of landslides triggered by repeated earthquakes spaced over the past thousands of years, this study will offer insight into potential decadal-scale impacts of future events on sediment supply to riverine and coastal environments, with implications for both fresh and salt water ecosystems. Our results will provide important information for stakeholders in the Pacific Northwest, including the Quinault Indian Nation (QIN), who have jurisdiction over the lake, and will develop tools for assessing earthquake hazards in other regions. Our study, moreover, will help to elucidate the role that landslides and other mass wasting processes associated with large earthquakes have played in the evolution of topography above Cascadia and other subduction margins over millennial time scales.
