Potential Lacustrine Records of Cascadia Great Earthquakes

Lacustrine sediments have been used successfully over the past few decades to develop earthquake chronologies and rupture assessments in a variety of locations and settings, from large lakes in Japan and Chile to Alpine lakes in central Europe. Although inland lakes in the Pacific Northwest have been used extensively for fire and vegetation reconstructions, they have been largely ignored with respect to their tectonic setting. Strong shaking from great earthquakes at subduction zones is known to produce significant environmental disturbance and can result in lake deposits that are distinctive and datable records of these events. Cascadia paleoseismic studies, including those at Lake Washington, Bradley Lake, and Effingham and Saanich Inlets, provide direct evidence that records of Cascadia great earthquakes are preserved in a variety of sedimentary archives. The field of marine turbidite paleoseismology has resulted in advancements which we have now begun to apply to inland lacustrine sediments using the records at Sanger and Bolan Lakes (both spring-fed, alpine cirque lakes), and Upper Squaw Lake (a stream-fed, landslide-dammed lake) located 45-100 km inland from the coast near the California/Oregon border. Inorganic terrigenous layers are visible in these sediments, and physical property data (via CT scans, magnetic susceptibility and gamma density) show characteristics that correlate between lakes, and more surprisingly, correlate great distances to seismogenic offshore turbidite deposits. The highest resolution site is Upper Squaw Lake, a 7.2 ha landslide-dammed lake which drains a 40 km2 watershed. A 10 m core spanning the past 2,000 years was extracted from this site, and is comprised of silty gyttja interbedded with inorganic turbidite deposits. Six major events are observed this core, similar to the number of events in the marine turbidite record in the same time period, with supporting age control. The characteristics of the physical property data are remarkably similar to those found at the Smith Apron offshore site approximately 160 km away. Striking correlation examples include a multiple peaked event or events at ~1500 years before present (ybp), a single peak fining upward with a small excursion near the top of the sequence at ~1000 ybp, and a large single peak for an event poorly constrained at ~500 ybp. The number of observed events is also comparable to those found at the Oregon coastal site Bradley Lake which records tsunami inundation. The great distance and multiple depositional environments over which these events correlate suggests these are earthquake-generated deposits and supports the hypothesis that gravity-driven seismogenic sediment deposits may record a crude primary signal of shaking which we call a "paleoseismogram". We hope to expand this investigation, by increasing the number of inland sites to be used in conjunction with marine and coastal records, to refine Cascadia paleo-rupture models by improving estimates of along-strike segmentation and the down-dip extent of the locked zone.