The Cascadia Subduction Zone has the potential to shake the Pacific Northwest and British Columbia with devastating earthquakes. Now, if we look closer than ever at the structure of the megafast, it appears that it is segmented into several major regions.
These regions can disintegrate individually, or they can all cause a massive shock all at once. But the findings indicate that the experience of an earthquake can be different in each zone.
“It requires a lot more research, but for places like Tacoma and Seattle it could mean the difference between alarming and catastrophic,” co-author Harold Tobin, a geophysicist at the University of Washington, said in a study. rack.
In many subduction zones, where oceanic crust rubs beneath lighter continental crust, small and medium-sized earthquakes are common. These miniquakes give researchers information about what the hidden parts of tectonic plates look like and where different faults are located.
Related: The dormant subduction zone could awaken and form a new ‘Ring of Fire’ that swallows the Atlantic Ocean
But Cascadia, which stretches 1,000 kilometers from North Vancouver Island to Cape Mendocino, California, rarely produces small earthquakes, says lead author Suzanne Carbottea marine geophysicist at the Lamont-Doherty Earth Observatory at Columbia University told LiveScience. So the best view scientists had of its structure came from 1980s research that used onboard instruments to send small bursts of seismic waves into the Earth’s crust and then record the returning waves to create images of the subsurface to make.
Today’s instruments are much more advanced, Carbotte said, but no one had done a repeated study of the region. So in 2021, the researchers behind the new study collected new seismic data along the fault line.
The data covered 560 miles (900 km) of the boundary between the continental North American plate and the oceanic Juan de Fuca, Explorer, and Gorda plates, all moving at a rate of 1.2 to 1.6 inches (30 to 42 inches). ) diving under North America. millimeters) per year.
The data showed that when the oceanic crust sinks, or plunges, it also breaks up. “Now that we have factual information about the entire region, we know that the fault surface is much more complex in geometry than the picture we had from that very old data,” Carbotte said.
The fault is split into four major segments, the researchers found. One starts in northern California and extends to Cape Blanco, Oregon. The next stretches north from Cape Blanco to Alsea Bay. Here the researchers saw a lot of slip or movement of the two sides of the fault relative to each other. The next segment extends north from Alsea Bay to the mouth of the Columbia River.
Finally, there is a large – and important – segment that extends from southern Washington to southern Vancouver Island. Here the plates meet at a flat, shallow angle and there is a lot of contact surface. The size of an earthquake directly corresponds to the size of the area that ruptures, Carbotte says. This means that the southern segment from Washington to southern Vancouver Island is most likely to produce a major earthquake.
The Cascadia Subduction Zone last produced a major earthquake in 1700. There are no written accounts of the event from that time, but drowned trees and a mysterious tsunami in Japan reveal that on January 26 that year, an earthquake with a magnitude of 8.7 to 9.2 shook the region. Researchers don’t know whether the 1700 earthquake was caused by the rupturing of the entire fault line or just one segment.
The new understanding of the fault’s geometry should help scientists better map the hazards of the next Cascadia earthquake, Carbotte said. This includes not only potential quakes along densely populated regions such as Vancouver and Seattle, but also the risk of tsunamis along the coast.
“The segmentation means you can make more informed predictions about how the shaking patterns might vary,” Carbotte said.
The findings were published Friday (June 7) in the journal Scientific progress.