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Lacustrine Paleoseismic Evidence From Two Large Lakes in Cascadia: Preliminary Comparisons of Post-Glacial Sediment Records From Ozette and Whatcom Lakes, Washington

Description: 

We present a suite of geophysical and geological evidence for earthquake-triggered mass transport deposits (MTDs) and related turbidite deposition in two of Washington’s largest lakes: Ozette and Whatcom. Ozette Lake is ideally situated along the outer coast and above the locked portion of the northern Cascadia megathrust, but is relatively isolated from active crustal faults and intra-slab earthquakes. Whatcom Lake is farther inboard of the subduction zone where shaking is expected to be less severe during megathrust ruptures, but is more proximal to active crustal faults. High-resolution bathymetry data, sub-bottom Chirp profiles, and sediment cores (spanning the last ~14 kyr B.P.) are used to characterize the post-glacial stratigraphic framework of both lakes. We examine paleoseismic proxies within the lacustrine sediments, present an initial comparison of the two datasets, and explore implications for earthquake timing and recurrence between these two sites in western Washington. Both lakes contain stacked sequences of MTDs and Mazama ash marker-beds; the physical characteristics of the event deposits vary considerably depending on proximity to primary depocenters, steep slopes, and subaqueous deltas. In Ozette, more than 150 radiocarbon ages bracket the timing of 34 event deposits emplaced since ~14 kyr B.P; these are easily differentiated from watershed processes and interpreted to have been generated by intense megathrust shaking, yielding a recurrence interval of ~420 years. Stratigraphic analyses and radiocarbon dating from Whatcom reveal a more complicated depositional history. Thick (1–5 m) MTDs and thinner (<10 cm) turbidite event beds appear to have been generated from different sources of shaking, and possibly from non-seismic watershed processes. Some of the thick MTDs failed synchronously along submerged slopes during two distinct intervals of the Holocene, whereas more than 20 turbidites are observed within the same section. Shaking from nearby earthquakes (crustal faults?) may have triggered the MTDs, but determining a source for the turbidites is a focus of ongoing and future analyses.

Session: From Faults to Fjords: Earthquake Evidence in Terrestrial and Subaqueous Environments [Poster Session]

Type: Poster

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Daniel

Student Presenter: No

Invited Presentation: 

Authors