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

Abstract. We compare disturbances from the historic portion of the sedimentary record from Lower Squaw Lake, Oregon, to the historic record of events from the region to (1) determine if the lake records Cascadia megathrust earthquakes, and (2) if sediment deposits can be differentiated by disturbance type. We use the sedimentological characteristics and geochemically inferred provenance of the deposits (labelled A–J) from the historic portion (post 1650 CE) of the record to discriminate between types of deposits. We show that earthquake-triggered deposits are complex and flood deposits are simpler but vary depending on flood characteristics. Disturbance deposit J dates close to 1700 CE (1680–1780 CE) through multiple approaches. This deposit suspected to result from the magnitude (M) 8.8–9.2 1700 CE Cascadia megathrust earthquake is composed of unusually well-sorted, normally graded, medium-grained silt derived from distal rocks in the upper watershed. The silt grades upward, increasing in organic content forming a long, organic-rich tail. Load structures of silt into the organic-rich sediment below suggest rapid deposition. In contrast, a deposit attributed to the ~M7.0 1873 CE intraplate earthquake is a normally graded, medium-grained, watershed-sourced silt overlain by an organic tail and preceded by a lake-wide deposit interpreted as a wall failure from an earthquake that caused the landslide dam to fail. These results suggest that inland lakes can be sensitive recorders of earthquakes, and that it is possible to discriminate between plate margin and other types of earthquakes, and floods.