Exploring the Use of Historic Earthquake Information to Differentiate Between Deposit Triggers for the High-resolution Stratigraphy from Squaw Lakes, Oregon, USA

Abstract:

The unique setting and high-resolution stratigraphy at Squaw Lakes, Oregon provides an opportunity to apply lake paleoseismology to southern Cascadia forearc lakes. These lakes were formed when a landslide dammed Squaw Creek located ~100 km from the Oregon coast at the Oregon/California border separating the drainages at the confluence of Squaw and Slickear Creeks. The upper lake contains evidence of disturbance events much too frequent to be the result of earthquakes alone. A link to historic events provides information that may be used to differentiate between deposit triggers and improve the interpretation of the prehistoric portion of the sedimentary record.

Regional newspapers published historic accounts of earthquakes experienced by the local people, the most notable of which is the November 23 (or 22nd), 1873 Crescent City, CA earthquake. Although the 1906 San Francisco earthquake was also felt in this region, reports indicate that shaking was much stronger near Jacksonville, Oregon (only 25 miles to the north of Squaw Lakes) as a result of the 1873 earthquake. The depth range that most likely contains sediment deposited within a few years of 1873 can be determined using a new high-resolution age model for the Upper Squaw Lake sediment core (Gavin et al., in prep). This depth range in the core contains a thick deposit that is similar in structure to deposits deeper in the core that have been proposed to correlate with the marine record of Cascadia great earthquakes. These disturbance event deposits are thicker, graded deposits, where grading is dominated by the percentage of organic content as compared to those interpreted to be a result of watershed disturbances. Recently acquired radiocarbon ages for the Lower Squaw Lake core suggests the thicker Upper Squaw Lake deposits correlate to those recorded in the lower-resolution sedimentary record at Lower Squaw Lake. The character of the likely contemporaneous deposits from the lower lake show grading more typical of seismogenic turbidites. These observations provide additional evidence that can be used to identify seismogenic deposits in these cores.