UNIQUE DEPOSIT CHARACTERISTICS FROM LOWER SQUAW LAKE, OREGON DATED TO 1680-1730 AD SUGGEST A LINK TO SUSTAINED GROUND MOTIONS FROM A CASCADIA EARTHQUAKE

The sedimentary records from small lakes have the potential to provide evidence of the spatial expression of ground motions from a Cascadia earthquake inland of the coast where most people live. However, the ability to accurately identify seismogenic deposits in Cascadia lakes remains untested. Earthquakes are known to disturb lake sediment which resettle to form clastic-rich, upward-fining graded deposits, however extreme erosional events are also capable of creating similar deposits. The goal of this research is to determine if ground motions from a Cascadia earthquake can be uniquely identified in the sedimentary record from Lower Squaw Lakes, Oregon, ~100 km east of the coast near the California/Oregon border. This location is ideal for this project because it is one of two lakes that were formed when a landslide dammed two streams at their confluence, creating an upper and lower lake. Upper Squaw Lake contains a published record of erosional events and continuously overflows into the lower lake near the outflow. Our strategy is to compare the historic chronology of known extreme events to the sedimentary record of disturbances contained in well-dated cores from Lower Squaw Lake that we assume contains a disturbance from the 1700AD Cascadia earthquake. Here we show that this deposit is unique in the historic portion of the record, with evidence of sustained, not necessarily strong, ground motions. Deposit characteristics suggest that the sediment is sourced either from liquefaction or settling of the rapidly built cobble/boulder delta, requiring a lower minimum ground motion to trigger as compared to sediment disturbed from the lake margin. This finding was unexpected, and will allow us to create an accurate earthquake chronology from the sedimentary record at this location, and provides a different way of evaluating clastic layers from less-sensitive settings and greater distances from the coast than previously thought possible.