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Ozette Lake is located above the locked portion of the northern Cascadia megathrust and is relatively isolated from other active faults. The lake's sediments have preserved a detailed record of seismic events, including underwater landslide events known as mass transport deposits and related turbidite layers, which were likely triggered by powerful earthquakes.

The USGS has long been involved with deep-sea fault mapping, which typically uses a combination of sonar and sediment sampling to study faults offshore, often far beneath the seafloor. Applying similar techniques to lake sediments is a relatively new and growing field. 

"Compared to sediments deposited in marine environments, lakes are usually less disturbed, and in some cases almost pristine, with sediment layers going back thousands of years," said Danny Brothers, Research Geophysicist and lead author of the study. "For decades researchers have studied lake sediments in a paleoclimatic context, analyzing pollen content and other environmental proxies, and then carbon-dating plant macro-fossils to better understand past climates. Lakes are very sensitive to environmental perturbations, and so we’ve started looking at lake sediments to find evidence of prehistoric shaking."

Using a combination of high-resolution bathymetry, seismic reflection profiles, and sediment core analysis, the study’s researchers were able to reconstruct Ozette Lake's post-glacial stratigraphy and identify 30 to 34 distinct event deposits. These deposits, spanning the last 14,000 years, provide a rare glimpse into the frequency and magnitude of past earthquakes in the region.

Radiometric dating of the sediments allowed the team to develop a detailed timeline for the last 5,500 years, revealing that the most recent 12 event layers correspond to earthquakes occurring approximately every 365 to 405 years. Among these, at least 10 events were likely caused by megathrust earthquakes, with a recurrence interval of 440 to 560 years.

Lakes such as Ozette are thus critical to understanding the seismic history of the northern Cascadia subduction zone. Combining offshore fault studies with onshore lake-based studies not only contributes to our knowledge of past Cascadia earthquakes but also have important implications for assessing future seismic risks in the Pacific Northwest. 

Read the study, Post-glacial stratigraphy and late Holocene record of great Cascadia earthquakes in Ozette Lake, Washington, USA, in Geosphere.

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