Shafaqna Science: New data indicate that the most destructive Cascadian earthquakes are likely to occur offshore from Washington state and Vancouver Island. The Cascadia megathrust, a massive fault capable of producing devastating magnitude 9 earthquakes like the 2011 Tohoku quake, has been challenging to study. Recent research has shown that instead of being a single continuous fracture, the fault consists of at least four segments, with the most hazardous segment stretching from southern Vancouver Island through Washington state, according to a June 7 report in Science Advances.
Seismologist Edwin Nissen from the University of Victoria, not involved in the study, notes that while this section of the fault is most likely to experience the largest earthquakes, segments farther south along Oregon’s coast might see smaller, more frequent quakes.
The Cascadia megathrust, a subduction zone where the Juan de Fuca plate slides beneath the North American plate, follows the west coast of North America from British Columbia to northern California. This fault has caused 19 quakes greater than magnitude 9 in the past 10,000 years, including the 1700 quake that triggered tsunamis reaching Japan.
Despite its significant threat, the Cascadia fault has been poorly understood compared to other subduction zones, which typically have frequent small earthquakes providing valuable data. The recent survey, conducted by marine geophysicist Suzanne Carbotte and her team aboard the Marcus G. Langseth research vessel, used sound waves to map the fault’s structure over a 900-kilometer stretch, revealing its segmented nature.
The segmentation of the fault, driven by the irregular distribution of rigid rocks in the North American plate, can influence earthquake size. Segmented faults may limit the extent of an earthquake’s spread, potentially making quakes smaller.
However, some quakes could still affect multiple segments, as evidenced by the 1700 earthquake. The survey also found that the segment extending from southern Vancouver Island is smooth and inclined at a very shallow angle, which could allow earthquakes to propagate further east and impact major cities like Victoria, Seattle, and Vancouver more than previously thought.
The new data will help assess and forecast earthquake risks, with the next update to the Pacific Northwest’s hazard model expected in 2029. The survey also identified smaller, nearshore faults that could contribute to tsunami waves, potentially impacting the coast much sooner if they slip alongside a megathrust quake.