3-9 Lacustrine Paleoseismology of the Darrington–Devils Mountain Fault Zone at Lake Cavanaugh, Washington: Constraints on Ground Shaking from Puget Lowland Earthquakes

Session: Lakes of the World Through Space and Time: Archives of Climate, Paleoenvironments, Ecosystems, Geohazards, and Economic Resources

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Abstract:

The near-continuous accumulation of lake sediment can preserve a proxy record of pre-instrumental earthquake shaking. Earthquakes of sufficient intensity—typically >MMI 5.5–6.0—can trigger lacustrine event deposits (e.g., turbidites and mass-wasting deposits), which are widely used to identify past shaking in seismically active areas.

Sediment cores and seismic reflection data from Lake Cavanaugh, Washington, located above the Darrington-Devils Mountain Fault Zone (DDMFZ), record a postglacial history of earthquake disturbance extending back ~14,250 years. We identify four candidate paleoseismic events: a prominent deformation and mass-wasting episode under late-glacial lake conditions, and three Holocene turbidites dated to ~6,000, ~4,000, and ~1,600 cal yr BP. Age control is based on a lake-specific sedimentation rate curve anchored by 11 radiocarbon dates and the 7,640 cal yr BP Mazama tephra.

Comparison with nearby DDMFZ paleoseismic trench studies (e.g., Personius et al., 2014), 12 km to the west of the lake, suggests the youngest event (~1,600 cal yr BP) correlates with a known surface-rupturing earthquake. However, the ~6,000 and ~4,000 cal yr BP events lack correlative records in trenches. The strong shaking inferred from the 4,000-cal yr BP turbidite suggests that some DDMFZ earthquakes may not rupture to the surface, complicating trench detection. Conversely, we do not observe the ~8,000 cal yr BP event observed in the trenches, indicating potential lateral variability in fault zone rupture dynamics. The definitive ~14.25 ka Lake Cavanaugh event is characterized by subsidence and tilting of the lake floor and a prominent mass-wasting deposit overlain by a thick turbidite. This event predates the oldest record in nearby terrestrial trenches. If all four postglacial events originated because of DDMFZ earthquakes, they imply a time-averaged recurrence interval of ~3,500 years. The three Holocene events post-dating Mazama tephra yield a mid-to-late Holocene recurrence interval of ~2,500 years.

Importantly, the absence of event layers corresponding to known regional earthquakes (e.g., 1872 CE North Cascades, 1700 CE Cascadia megathrust, 923–924 CE Seattle Fault) constrains local shaking intensities below the typical threshold typically required to generate lacustrine event deposits (Van Daele et al., 2015). These findings refine DDMFZ earthquake recurrence intervals and demonstrate the added value of lake sediment records for assessing regional shaking intensities from paleo and future western Washington earthquakes.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10870

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