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Earthquakes can trigger gravity-driven debris flows, known as mass wasting events. When these occur underwater, debris surges downslope in a sediment-rich slurry and settles on the seafloor in deposits known as turbidites. These turbidites can contain clues about the frequency and intensity of earthquakes along fault boundaries, preserving a history of seismic activity in their layered sediments.

7 images of the Cascadia subduction zone
Hazards of the Cascadia Subduction Zone: tsunamis, landslides, site amplification, turbidites, land-level change, and episodic tremor and slip.

For decades, researchers studying seafloor turbidites along the Cascadia Subduction Zone (CSZ) faced considerable uncertainty regarding the age and spatial correlation of these deposits, often due to lack of data. Reliably distinguishing earthquake-derived turbidities (seismoturbidites) from storm-derived turbidites was another uncertainty. 

A new study from USGS researchers at the Pacific Coastal and Marine Science Center offers the most comprehensive spatial database to date of submarine landslide scarps—the steep cliff faces often caused by mass wasting events—across the CSZ. 

“The offshore record of earthquakes comes from these turbidites,” said Research Geologist Jenna Hill, lead author of the study, “Our work helps to understand the context of how those deposits are formed, so that we can better interpret the earthquake record along the entire Cascadia margin.” 

Spanning 1,000 kilometers (620 miles), the CSZ is formed by the Pacific tectonic plate sliding beneath the North American plate, giving rise to volcanic and tectonic activity throughout the U.S. Pacific Northwest and western British Columbia.

Using high-resolution multibeam bathymetry, the team mapped and identified more than 8,700 underwater landslide scarps, finding that mass wasting is most common along the steep lower slope of the CSZ. Most of the landslides caused by intense seismic activity occurred closest to the deformation front, the leading edge of the subducting Pacific Plate. This suggests that the most intense earthquakes along the CSZ may occur far offshore in deep water, along the margin’s steep lower slope. 

"Finding that the CSZ’s lower slope is a major source of landslides is key—it changes our thinking about how turbidites are generated during big earthquakes,” said Research Geophysicist Danny Brothers, a co-author of the study. “Our crew is currently aboard the research vessel Bold Horizon pursuing the next critical piece of the story: Collecting piston cores in key locations along the CSZ to ground truth and eventually radiocarbon-date these landslide deposits." 

Subduction zones are the source of some of the largest earthquakes in recorded history. Understanding how these tectonic margins behave—how often and how powerfully they move, or rupture—is critical to assessing the potential hazards they pose to communities and infrastructure. 

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