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Investigating the Earthquake Rupture History of the Northern Cascadia Subduction Zone Using Lacustrine Diatoms, Lake Ozette, Washington, Usa

Description: 

Offshore turbidites and onshore tidal wetland stratigraphy have provided exceptional geologic records of great (M_w >8) earthquakes along the Cascadia subduction zone (CSZ). Together, these geologic records document up to 19 great earthquakes at the CSZ over the last 10,000 years, however, 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 events along the entire megathrust boundary. Coastal lakes located in the northern CSZ may provide a more complete picture of past megathrust earthquakes since earthquake magnitude thresholds for generating depositional signatures in lakes are typically lower (MMI ~5.5) than other environments (e.g., coastal marshes).
A combination of extensive coring and chirp profiles collected from Lake Ozette, located on the coastal rim of the Olympic Peninsula in northern Washington state, has revealed more than 25 Holocene subaqueous mass transport deposits (MTDs), however, questions about the source of the MTDs still remain. Diatoms, a type of siliceous microalgae, preserved within lacustrine sediments may act as bioindicators for MTD origin (i.e., seismic versus hydroclimate). Here, we explore the diatom signatures within multiple MTD and river-inflow flood deposits to address questions about source mechanism. Preliminary results suggest diatoms record distinctly different signatures during earthquakes and high river inflow events, enhancing our ability to use diatoms to distinguish the source of deposits in the fossil record. The broad-scale 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.

Session: Constraining Seismic Hazard in the Cascadia Subduction Zone

Type: Oral

Date: 4/20/2023

Presentation Time: 02:15 PM (local time)

Presenting Author: Jessica DePaolis

Student Presenter: Yes

Invited Presentation: 

Authors