A 2700 Year Record of Earthquakes from Snake Monster Lake, Oregon, USA

Finding evidence of megathrust earthquakes in small Cascadia lakes would provide the opportunity to better determine the source, and therefore hazard potential, of southern Cascadia earthquakes. An analysis of the sedimentary record from Snake Monster Lake, a landslide dammed lake in southern Oregon, identified a disturbance deposit dated to 1680-1760 CE. This deposit has a sharp basal contact with evidence of loading, is composed of watershed-sourced silt (suspected to result from liquefaction of the lake's delta), and has a long (5-20 cm) organic tail. To determine if this deposit was formed in response to the 1700 CE Cascadia megathrust earthquake, deposits with similar characteristics were identified downcore and the timing of these deposits was compared to the timing of published record of offshore seismoturbidites and onshore paleoseismic records.

Deposits with characteristics most similar to the deposit attributed to the 1700 CE Cascadia earthquake are temporal equivalents to offshore deposits T1-T6, where T2 and T4 equivalents in the lake record are thinner than the other events, similar to correlatives in the offshore record. A younger deposit (deposit H, dated to 1820-1880 CE; attributed to a slab earthquake which occurred in 1873 CE) also has similar characteristics. Deposit D, dated to 1870-1940 CE (possibly the result of the 1906 San Andreas earthquake), is also composed of watershed-sourced silt. A deposit, possibly equivalent to T2a (550-660 BP), is composed of schist, similar to deposit I (attributed to the failure of the landslide dam attributed to the 1873 CE earthquake), suggesting a different source. The record may also include equivalents to events T5b and T5c, prominent in the offshore record, but are of uncertain attribution sedimentologically. These results suggest that Snake Monster Lake contains evidence of both Cascadia and other seismic (and possibly aseismic) events, and suggest that southern Cascadia lakes may provide useful information that can be used to determine the seismic sources influencing the offshore record.