A 2700-yr record of Cascadia megathrust and crustal/slab earthquakes from Upper and Lower Acorn Woman Lakes, Oregon

Abstract We infer a ~2,700-year history of Cascadia megathrust and other earthquakes from two small mountain lakes located 100 km inland of the coast near the California/Oregon border. We use the characteristics of a disturbance deposit in the historic portion of the sediment cores attributed to the 1700 CE Cascadia earthquake to identify Cascadia earthquake deposits downcore. This deposit shows evidence of a small bypass turbidite, followed by a deposit composed of light-coloured silt (indicating it is enriched in calcium-rich Slickear Creek watershed-sourced sediment), without visible mica grains (which would indicate a lake bedrock source), organic grading of the deposit tail, and a basal contact with evidence of rapid loading or the influence of sediment-water interaction during seismic shaking. Seven deposits downcore have the characteristics of this deposit. An age-depth model suggests that six of these are temporal correlatives to the largest margin-wide marine turbidite event deposits from Goldfinger et al., 2012, (events T1 through T6), whereas the two deposits with some of the characteristics are potential correlatives of smaller turbidites T5a and T5b. Other thinner deposits are temporal correlatives of T2a and T3a and other smaller deposits of uncertain origin. Lake core physical property data can be correlated to those from offshore cores containing seismogenic turbidites attributed to Cascadia earthquakes. These results suggest that small Cascadia landslide-dammed lakes with sufficient sedimentation rates (~1-2 cm/decade) and mixed clastic and organic content may be good recorders of subduction earthquakes which can be differentiated from other types of deposits.