New research reveals that the Cascadia subduction zone—a high-risk earthquake region extending from California to British Columbia—has experienced at least 10 major quakes in the last 7,500 years. These events left behind a distinct signature: massive underwater landslides that trigger sediment flows known as turbidites. The discovery provides a clearer picture of the region’s earthquake recurrence intervals and suggests similar records may exist in other subduction zones worldwide.
Understanding Subduction Zones and Megaquakes
Subduction zones are areas where one tectonic plate slides beneath another, creating the potential for extremely powerful earthquakes. The Cascadia zone is capable of producing quakes exceeding magnitude 9.0, similar to the 2011 Tohoku earthquake in Japan which triggered a catastrophic tsunami. Determining how often these megaquakes occur is crucial for assessing risk, but historical records are often incomplete.
The Hunt for Geological Evidence
Researchers have previously relied on geological markers—such as sudden land-level changes and turbidites—to reconstruct earthquake histories. However, turbidites can also be caused by non-seismic events like storms and landslides, making it difficult to isolate earthquake-generated deposits. The new study overcomes this challenge by examining the continental slope —the steep underwater drop-off from the North American continent—in southern Cascadia.
Deep-Sea Investigation Reveals Clear Links
Using remotely operated vehicles and sediment cores, a team led by U.S. Geological Survey research geologist Jenna Hill analyzed turbidite deposits off the coast of Crescent City, California. Radiocarbon dating confirmed a direct correlation between these sediment flows and known ancient Cascadia earthquakes.
“We are able to clarify how and where the turbidites are generated,” Hill explained, “So we know they’re coming from landslides that we know are triggered by earthquakes.”
The study found that even moderate-sized quakes can trigger these deep-sea landslides, increasing the potential for tsunamis. Additionally, evidence of seafloor shaking found alongside the turbidites strengthens the connection to seismic activity.
Implications for Global Earthquake Risk
Turbidites in coastal submarine canyons have already been used to link Cascadia earthquakes to those on the San Andreas Fault. However, the deeper continental slope deposits offer more reliable markers because they are less affected by coastal influences like tides and rainfall. The researchers believe this method can be applied to other subduction zones globally, revealing previously unknown seismic histories.
The findings underscore the importance of studying deep-sea geological records to improve earthquake hazard assessments worldwide. By understanding the frequency and intensity of past megaquakes, scientists can better prepare for future events in vulnerable regions.
