The Cascadia subduction zone has the potential to shake the Pacific Northwest and British Columbia with devastating earthquakes. Now a closer look at the structure of the megafualt reveals that it is divided into several main areas.
These areas can rupture individually or they can all unleash a massive tremor at once. However, the findings suggest that earthquake experiences may be different in each zone.
“It requires a lot more study, but for places like Tacoma and Seattle, it could mean the difference between alarming and catastrophic,” said study co-author Harold Tobin, a geophysicist at the University of Washington. declaration.
In many subduction zones, where oceanic crust grinds beneath lighter continental crust, small and moderate earthquakes are common. These mini-quakes provide researchers with information about what hidden parts of the tectonic plates look like and where the various faults are located.
Related: Dormant subduction zone could wake up to create new ‘Ring of Fire’ to engulf Atlantic Ocean
But Cascadia, which stretches 600 miles (1,000 kilometers) from northern Vancouver Island to Cape Mendocino, California, rarely produces small tremors, lead author Suzanne Carbotte, a marine geophysicist at Columbia University’s Lamont-Doherty Earth Observatory, told Live Science. So scientists’ best look at its structure came from research in the 1980s that used onboard instruments to send small bursts of seismic waves into the crust and then record the returning waves to create images of the subsurface.
Today’s instruments are much more sophisticated, Carbotte said, but no one has done a repeat study of the region. So in 2021, the researchers behind the new study collected new seismic data along the fault.
The data covered the 560-mile (900 km) boundary between the continental North American plate and the oceanic Juan de Fuca, Explorer, and Gorda plates, which are all subducting beneath North America at rates of 1.2 to 1.6 inches (30 to 42). millimeters) per year.
The data revealed that as oceanic crust subducts, or sinks down, it also breaks up. “Now that we have real information covering the entire area, we know that the fault surface is much more complex in its geometry than the picture we had from that very old data,” Carbotte said.
The fracture is divided into four main segments, the researchers found. One starts in Northern California and goes all the way to Cape Blanco, Oregon. Another stretches north from Cape Blanco to Alsea Bay. Here, scientists saw a lot of slip, or movement of both sides of the fault against each other. Another segment stretches north from Alsea Bay to the mouth of the Columbia River.
Finally, there is a large – and important – segment stretching from southern Washington to southern Vancouver Island. Here the plates meet at a flat, shallow angle and there is a large area of ​​contact. The size of an earthquake is directly related to the size of the area that ruptures, Carbotte says. This means that the segment from southern Washington to southern Vancouver Island is most likely to cause the largest earthquake.
The Cascadia subduction zone last produced a large earthquake in the 1700s. There are no written records of the event from that time, but drowned trees and a mysterious tsunami recorded in Japan show that an 8.7 to 9.2 magnitude earthquake shook the region on January 26 of that year. Researchers do not know whether the 1700 earthquake caused the entire fault to rupture or just one segment.
The new understanding of fault geometry should help scientists better map the hazard of the next Cascadia earthquake, Carbotte said. This includes not only potential tremors along highly populated areas such as Vancouver and Seattle, but also the risk of tsunamis along the coast.
“Segmentation means you can make more informed predictions about how the shake patterns might be different,” Carbotte said.
The findings were published Friday (June 7) in the journal Scientific advances.