INVESTIGATING THE EARTHQUAKE RUPTURE HISTORY OF THE NORTHERN CASCADIA SUBDUCTION ZONE USING COASTAL LACUSTRINE DIATOMS, LAKE OZETTE, WASHINGTON, USA

Tidal wetland stratigraphy along the Cascadia subduction zone (CSZ) preserves geologic evidence of coseismic land-level changes and tsunami inundation from past great (M_w >8) earthquakes, while offshore turbidites provide a proxy for shaking during past megathrust ruptures. Together, onshore and offshore records document up to 19 great earthquakes at the CSZ over the last 10,000 years. Most onshore evidence is concentrated in the central and southern CSZ where records extend thousands of years longer than the northern CSZ. The limited onshore record of past earthquakes and tsunamis in the northern CSZ leave questions about the spatial and temporal variability of past events, however, a possible way to address onshore data gaps in earthquake records is to target signatures preserved in coastal lakes. Earthquake magnitude thresholds for generating depositional signatures in lakes are typically lower (MMI 5.5) than other environments, suggesting that lakes may contain a more complete record of regional shaking history.

Extensive coring within Lake Ozette, located on the Olympic Peninsula in Washington State, has revealed more than 25 Holocene subaqueous mass transport deposits (MTDs) inferred to have been emplaced during strong shaking along the megathrust. The last 11 MTDs can be dated and correlated to other geologic records of great earthquakes along the CSZ. Microfossils, such as diatoms, preserved within lake sediments may provide an independent test of MTD origin (i.e., seismic versus hydroclimatic). Here, we explore the diatoms preserved across potential MTD contacts and a known river flood deposit from Lake Ozette to test for deposit origin. Our initial results across MTDs show a distinct difference in diatom species within the deposit compared to surrounding lake sediments. We anticipate that diatoms will record a distinct, but different, signature during river floods as compared to earthquake-triggered deposits, enhancing our ability to use diatoms to distinguish the origin of deposits in the fossil record. The implications of this research are to understand the recurrence interval of great earthquakes along the CSZ, improve spatial correlation of seismic events, and enhance the earthquake record for northern Cascadia by developing a more reliable method for identifying past earthquake occurrences.