A 2700-yr record of Cascadia megathrust and crustal/slab earthquakes from Upper and Lower Squaw 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 disturbance deposits in the historic portion of the sediment cores from the lower lake to identify a deposit from the 1700 CE Cascadia earthquake (deposit J). This deposit is composed of light-coloured silt (indicating it is enriched in 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.

Seven deposits downcore have some of the characteristics of deposit J. An age-depth model suggests that the five deposits most similar to deposit J (including deposit J) are temporal correlatives to the largest margin-wide marine turbidite event deposits from Goldfinger et al., 2012, (T1, T2, T3, T4, T5 and 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 other regional lake records and offshore cores. These results suggest that small Cascadia lakes with sufficient sedimentation rates (~1–2 cm/decade) with mixed clastic and organic sedimentation may be good recorders of earthquakes, that subduction earthquake deposits are different from those from other types of earthquake deposits and deposits from other types of disturbances, such as floods.