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Hidden beneath the Pacific Ocean off the west coast of the US is the Cascadia Subduction Zone, a fault system capable of producing earthquakes of magnitude 8 or higher that can be felt hundreds of miles away, and recent studies have pinpointed the most dangerous parts of the 700-mile-long fault system.
The findings will help scientists assess earthquake and tsunami risks in the region, including Washington state, a particularly vulnerable state.
“This is a subduction zone that has not been well studied with the tools currently available,” Suzanne Calbot, a geophysicist and Bruce Heesen Lamont Research Professor at Columbia University, told Business Insider.
Using cutting-edge technology capable of probing and imaging deep beneath the ocean floor, Calbot and his team conducted the first comprehensive study of Cascadia’s complex subsurface structure, and published their findings today in the peer-reviewed journal Science Advances.
The researchers found that Cascadia is divided into at least four segments, something that had been suggested in previous studies but never confirmed, Calbot said.
“Prior to our study, the images showed a smooth surface with no obvious relationship to this section,” Calbot says, “but that smooth surface was based on very sparse data — and in some places, there is no data at all.”
The new diagram more accurately illustrates the complexity of Cascadia volcanoes and the risk they pose to the US West Coast.
How the Cascadia subduction zone causes earthquakes
The Cascades is essentially the boundary between two crustal plates: the giant North American continent and the smaller Juan de Fuca plate.
The Juan de Fuca Plate is gradually sliding (or subducting) eastward beneath the North American Plate, forming a megathrust fault (a place where plates move dangerously against one another).
Calbot explained that while the stresses that move the Juan de Fuca Plate beneath North America are continuous, the plate movement is not. Sometimes, the plates get stuck.
Once frozen like this, the plate can only absorb stress for a limited time before eventually rupturing and causing an earthquake, she said.
Scientists believe this happened about 300 years ago when the belt ruptured offshore, causing the resulting earthquake to send a giant tsunami crashing along the coast of Japan.
The Cascades haven’t had a major earthquake since 1700, but it’s only a matter of time.
Scientists cannot predict earthquakes, but by understanding the complex structure of faults deep underground, they can better understand the risk.
Calbot and her team have made great strides on that front.
Focus on risk
Calbot and his team found a lot of variation in the megafault’s structure, which likely means different places along the fault are at different hazards, said Janet Watt, a research geophysicist at the U.S. Geological Survey’s Santa Cruz campus, who was not involved in the study.
“It’s not a one-size-fits-all answer, but it helps us understand the complexities,” Watt told BI of Carbot’s findings.
Additionally, understanding that Cascadia is divided into multiple segments is key to assessing earthquake hazard, Watt says, because this segmentation could cause the megathrust to rupture in pieces rather than all at once. Shorter ruptures could trigger smaller earthquakes, which could affect the size of future earthquakes.
Moreover, the unique characteristics of each segment mean that each poses a different level of risk.Another key finding from Calbot’s study is that one of Cascadia’s segments is probably more likely to produce a major earthquake than the other.
This particularly dangerous section essentially runs along the Washington coast, from the northern Oregon border to southern British Columbia, and is flatter and smoother than other sections, which is why it has the potential to produce the largest earthquakes, Calbot told BI in an email.
What’s more, this segment is likely to spread farther into the U.S. than the others, which is bad news for Washington state: If this segment ruptures, coastal areas of Washington state would likely feel the most intense shaking, although the quake would extend far beyond state lines, Carbot wrote.
Knowing that could help the state prepare for the worst. “I think this is an example of research that can be actionable in the future in terms of increasing resilience along our coasts, and it’s exciting to see where the science takes us,” Watt said.
Calbot’s work comes in the context of many other ongoing studies aimed at providing a clearer picture of Cascadia.
“This is one particular study in a larger community effort that is currently underway. [understand] “Understanding the system, communicating what it means for coastal and inland communities, and how we can actually put the science into action,” Watt said.