Washington, Vancouver Island epicentres of future Cascadian earthquakes: Study

The Cascadia megathrust, a formidable fault system capable of producing catastrophic magnitude 9 earthquakes, is poised to unleash its most severe impacts offshore of Washington state and Vancouver Island, according to recent data.

Cascadia megathrust is a major geological feature located off the Pacific Northwest coast of North America. It is a convergent plate boundary, or subduction zone, where the Juan de Fuca Plate is being forced, or subducted, beneath the North American Plate. This tectonic interaction has a potential for large earthquakes and tsunamis.

This revelation from the study emerged from the most extensive survey of the fault to date, published in the June 7 issue of Science Advances.

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Unlike previous assumptions that portrayed the megathrust as a single, continuous fracture, the new data revealed that it comprises at least four distinct segments. Researchers identified the segment stretching from southern Vancouver Island through Washington state as the most perilous.

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Seismologist Edwin Nissen of the University of Victoria in British Columbia, who was associated with the study, said, “The Cascadia megathrust is a huge risk to people living in the Pacific Northwest," reported Science News.

He noted that while the segment near Vancouver Island’s southwest coastline might get the largest earthquakes, southern segments along Oregon’s coast could experience smaller but more often quakes.

The Cascadia megathrust spans roughly 1000 kilometres from British Columbia to northern California.

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Cascadia has been rocked by 19 quakes in the past 10,000 years. These quakes exceeded magnitude 9. The most recent, in 1700, triggered a tsunami that reached Japan. Even as there is a major threat, the fault has been less understood compared to similar faults in Japan and New Zealand.

Survey findings

The seismic survey along a 900-kilometrestretch of the zone was done by using underwater air guns. With the help of the underwater air guns, sound waves were sent into the seafloor. They mapped the fault and rock layers beneath. This method provided unprecedented insight into the fault’s structure.

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The data revealed that the Juan de Fuca plate, as it moves under the North American plate, breaks into segments due to the uneven distribution of rigid rocks above. One particularly concerning finding is that the segment extending from southern Vancouver Island is relatively smooth and shallow-dipping which makes it easier for quakes to propagate towards the coast.

(With inputs from agencies)