Evidence of Widespread and Historical Earthquake-triggered Deformation, Slope Failure, and Associated Deposits Within Lake Washington and Lake Sammamish
Description:
The greater Seattle, Washington region is exposed to complex seismic hazards from Cascadia megathrust, deep intraslab, and shallow crustal earthquake sources. To better constrain the timing, distribution, and effects of strong ground motion from these sources, the U.S. Geological Survey is conducting a multiyear paleoseismic investigation of Lake Washington and Lake Sammamish. These lakes lie within zones of strong to very strong predicted ground motion during a Cascadia megathrust event and are intersected or bounded by several active crustal faults, including the Seattle fault zone. Continuous postglacial sedimentary records that are sensitive to ground-motion-induced deformation are well preserved in both lacustrine basins.
Two multi-week field campaigns in 2021 and 2024 acquired chirp subbottom profiles and sediment cores from both lakes, along with high-resolution bathymetry in Lake Sammamish. Bathymetric data document multiple slope failures along basin margins, while chirp profiles image shallow faulting and deformation, basin-wide unconformities, and buried mass-transport deposits (MTDs). In Lake Washington, radiocarbon ages from sediment cores link the timing of MTDs—including prominent turbidite deposits—to the 923–924 CE Seattle fault rupture. Lake Sammamish preserves a correlative basin-wide unconformity, deformation, and turbidites, although evidence for extensive slope failure is limited.
In Lake Washington, thin sand deposits along basin margins are spatially correlated with chirp sub-bottom amplitude anomalies. 137Cesium activity profiles from the upper 50 cm of sediment show a pronounced peak attributed to maximum fallout in 1963. We interpret a shallow MTD correlated to low 137Cs values, and just above the 137Cs peak, as the lacustrine response to the 1965 M6.7 Puget Sound intraslab earthquake (local MMI ~6.5). This age estimate is compared to independent trace-metal chronology. Planned USGS coring in 2026 will further refine regional earthquake chronologies and assess spatial variability in shaking recorded in these lake basins.
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: Jared Kluesner
Student Presenter: No
Invited Presentation:
Poster Number: 163
Authors
Jared Kluesner Presenting Author Corresponding Author jkluesner@usgs.gov U.S. Geological Survey |
Jenna Hill jhill@usgs.gov U.S. Geological Survey |
Daniel Brothers dbrothers@usgs.gov U.S. Geological Survey |
Brian Sherrod bsherrod@usgs.gov U.S. Geological Survey |
Renee Takesue rtakesue@usgs.gov Pacific Coastal and Marine Science Center |
James Conrad jconrad@usgs.gov Pacific Coastal and Marine Science Center |
Peter Dartnell pdartnell@usgs.gov U.S. Geological Survey |
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Evidence of Widespread and Historical Earthquake-triggered Deformation, Slope Failure, and Associated Deposits Within Lake Washington and Lake Sammamish
Category
Subaqueous Evidence for Earthquakes, Coseismic Landslides, Tsunamis and other Cascading Hazards