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Lacustrine Paleoseismic Proxies from Lake Whatcom, Washington, USA: Decoding Megathrust and Crustal Shaking Using 14 m Sediment Cores

Description: 

Lacustrine paleoseismic investigations in seismically active regions, including Alaska, Chile, the Alps, and the Cascades, provide valuable long-term records of megathrust and crustal earthquakes, critical for assessing seismic hazard. Here we present chirp subbottom data, shallow sediment cores (1–2 m), and two 14-m composite piston cores collected from Lake Whatcom, WA, located in the northern Cascadia subduction zone. Lake Whatcom provides a comparison with coastal lacustrine paleoseismic sites (e.g., Ozette, Crescent) as it lies ~300 km inland from the subduction interface and near multiple zones of active crustal deformation capable of generating ground motions exceeding 5.5 MMI. Lake Whatcom also presents an opportunity to examine down-core petrophysical differences between shaking-induced and hydroclimatically induced event deposits.

Lake Whatcom sediments contain mass-transport deposits (MTDs) and regionally correlated volcanic tephra (i.e., Mazama) within a stratigraphically conformable lacustrine archive. These sediments record watershed evolution from a glacially dominated regime to present-day conditions. Preliminary observations suggest that some MTDs are earthquake-triggered, forming laterally extensive event deposits with sharp basal contacts. These deposits have coarse sediments at the base that fine upward and are typically capped by fine-grained clay. In contrast, flood-induced deposits are typically fine at their base, coarsen towards peak flood discharge. As discharge decreases, sediments fine upward accordingly. To constrain depositional chronology, we sampled composite core UWO1 and obtained 54 radiocarbon ages to construct a Bayesian P-sequence age model that provides a framework for evaluating MTD timing and postglacial (~14ky) changes in sedimentation. These new observations from Lake Whatcom help distinguish megathrust from crustal earthquake shaking. Comparing Lake Whatcom’s age model with well-dated records from sites such as Lake Ozette allows us to identify proxies for regional megathrust events and distinguish them from earthquake proxies of nearby Cascadia crustal faults.

Session: Subaqueous Evidence for Earthquakes, Coseismic Landslides, Tsunamis and other Cascading Hazards [Poster]

Type: Poster

Date: 4/15/2026

Presentation Time: 08:00 AM (local time)

Presenting Author: Boe Derosier

Student Presenter: No

Invited Presentation: 

Poster Number: 158

Authors