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An 8000-Year Holocene Earthquake Record From the Northern Cascadia Forearc: Evidence for Multiple Sources at Lake Crescent, Washington

Lake Crescent is a deep, steep-sided lake within the forearc of the northern Cascadia subduction margin on the northern Olympic Peninsula in western Washington. Like other lakes worldwide, Lake Crescent’s stratigraphy points to an earthquake-recording sensitivity that depends on the climate, sediment accumulation rates, and shaking intensity. Starting about 8,000 years ago, two distinctive types of event deposits began to accumulate in the lake. Four meter-scale mass transport deposits resulted from multiple large mass failures on subaerial and subaqueous slopes triggered by ruptures along the North Olympic Fault Zone beneath the lake. On the other hand, twenty decimeter-scale turbidites correlate to much smaller mass failures on the lake’s subaqueous slopes. The turbidites have age ranges that overlap with regional offshore and onshore paleoseismic records, including those derived from deep-sea turbidites, Vancouver Island lakes and fjords, and Cascadia salt-water marshes. While these correlations suggest that most of the Lake Crescent turbidites formed during great Cascadia subduction earthquakes, we cannot rule out that some had other sources, including nearby crustal faults. Importantly, deposits correlative to two regional earthquakes that multiple researchers postulate to have occurred at the subduction interface around 500-600 and 800-900 years ago are absent to poorly developed at Lake Crescent but are represented in the stratigraphy of Ozette Lake, located 60 km to the west of Lake Crescent and closer to the subduction trench. The differences between these two lakes suggest that shaking during these subduction zone events was sufficiently attenuated in the landward direction so that Lake Crescent’s underwater slopes were minimally affected. Additionally, no turbidites formed in Lake Crescent in the 324 years since the last great subduction zone earthquake of 1700 CE, implying that shaking from crustal earthquakes has not exceeded the necessary local threshold for turbidite emplacement during this period.

Session: From Faults to Fjords: Earthquake Evidence in Terrestrial and Subaqueous Environments - I

Type: Oral

Room: K’enakatnu 6

Date: 5/1/2024

Presentation Time: 08:15 AM (local time)

Presenting Author: Elana Leithold

Student Presenter: No

Additional Authors