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

Abstract. We infer a ∼2700-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 is composed of light-colored silt sourced from the delta and has extended organic grading of the deposit tail and a basal contact with evidence of rapid loading or coseismic settling of silt into the organic sediment below. Eight 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) (deposits T1 through T6), whereas the two deposits with some of the characteristics are potential correlatives of smaller deposits T5a and T5b. We use the characteristics of the lower of two deposits inferred to be the result of a crustal earthquake that occurred in 1873 CE to identify similar deposits downcore. As a result, temporal correlatives of T2a and T3a and smaller deposits in the marine record were identified as likely crustal fault earthquakes. These results suggest that small Cascadia landslide-dammed lakes from distances of 100 km inland of the coast with sufficient sedimentation rates (∼1–2 cm per decade) and mixed clastic and organic content may be good recorders of subduction earthquakes. Furthermore, southern Cascadia crustal earthquakes likely partially explain the more frequent earthquakes in southern Cascadia and suggest a previously unrecognized hazard in the region.