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 megathrust earthquakes. We use the sedimentological characteristics and X-ray fluorescence (XRF) provenance of disturbance deposits (labeled A–J) from the historical portion (∼ 1650 CE and younger) of the record to discriminate between deposit types. We show that earthquake-triggered deposits can be differentiated from flood deposits, and Cascadia megathrust earthquake 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 M8.8–9.2 1700 CE Cascadia megathrust earthquake. Event deposits H and I are interpreted to be the result of the ∼ M7.0 1873 CE Brookings earthquake, which is explained here to be a crustal earthquake immediately followed by a subduction earthquake. These results demonstrate the usefulness of lake sediments to infer earthquake hazards in Cascadia.