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Lacustrine Paleoseismic Investigation in the South Washington Cascade Range: Geophysical and Sedimentological Observations From Keechelus, Kachess, and Cle Elum Lakes

Description: 

Lacustrine sediments within Keechelus, Kachess, and Cle Elum Lakes, located within the Yakima River Basin in the western Cascade Range (WA, USA), may hold records of strong ground motion occurring within the Pacific Northwest. Multibeam bathymetry, high-resolution seismic profiles, and sediment core data recently collected from these three deepwater lakes image subaqueous landslides, buried mass transport deposits (MTDs), and tephra deposits, which aided the correlation of stratigraphic markers amongst the three lakes. Using 42 (1.0-1.8m) percussion-driven gravity cores, we analyzed the physical properties and age of sediment deposited during the last ~7.5 kyr B.P. Within each lake, we find a 50-100 m-thick sedimentary sequence that includes 10-15 m of Holocene sediments that conformably overlay thick late Pleistocene sediments. Three Mt. Helens tephra deposits are found in each lake, and Mazama ash was sampled in Lake Keechelus.

Stacked sequences of MTDs within each lake were mapped in 3D to determine each deposit’s source location (e.g., delta versus non-delta). MTDs sourced from multiple locations are inferred to be shaking–induced. While we do not observe definitive evidence for deposits generated by historical megathrust (e.g., 1700), intraslab, or crustal (e.g., 1872) events, we present strong evidence for comparatively large and synchronous megaturbidite deposition in each lake at 2468-2716 cal yr B.P. The coincident timing, the large volume of remobilized sediment, and the depositional style suggest considerable ground motion occurred in the Yakima River Valley; therefore, we favor a proximal source of this shaking event (e.g., an active structure within the Olympic-Wallowa Lineament). However, we continue to assess other potential sources for shaking, such as the Southern Whidbey Island Fault or a Cascadia megathrust event. This study highlights the value of lacustrine paleoseismic approaches in regions where surficial fault deformation has been obscured in the geologic record or where traditional paleoseismic techniques are not feasible.

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: Boe

Student Presenter: No

Invited Presentation: 

Authors