Sedimentary record of historical seismicity in a small, southern Oregon lake

Abstract We compare event deposits from the historical portion of the sedimentary record from Lower Acorn Woman Lake, Oregon, to historical records of regional events to determine if the lake records Cascadia earthquakes. We use the sedimentological characteristics and x-Ray Fluorescence (XRF) provenance of disturbance deposits (labelled A-J) from the historical portion (post 1650 CE) of the record to discriminate between deposit types. We show that earthquake-triggered deposits can be differentiated from flood deposits, and Cascadia earthquakes deposits can be differentiated from other types of earthquake deposits. Event deposit J dates close to 1700 CE (1680-1780 CE) through multiple approaches, suggesting it was the result of shaking from the magnitude (M) 8.8-9.2 1700 CE Cascadia megathrust earthquake. Event deposits H and I are a complex sequence deposited in response to the ~M7.0 1873 CE Brookings earthquake. This earthquake has been previously interpreted to be an intraplate earthquake, is reinterpreted here as the result of an earthquake on a nearby crustal fault. Furthermore, the methods used have uncovered a previously unrecognized crustal earthquake deposit in the tail of deposit J, suggesting a stress relationship between Cascadia earthquakes and crustal earthquakes. These results not only demonstrate the usefulness of these methods to identify cryptic earthquake deposits and discriminate between subduction earthquake and other type of earthquake deposits in sediments from small, Cascadia lakes, but also suggest previously unknown relationships between subduction and crustal earthquakes in Cascadia.