New data on ‘the Big One’ offers a clearer picture of the inevitable earthquake catastrophe

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On January 26, 1700, a tsunami struck Japan. We know this from written documents describing coastal flooding. But what’s most striking about these documents is what they don’t describe. Tsunamis are the result of water displacement by earthquakes, but the 1700 tsunami did not correlate with any earthquake felt in Japan; there was no warning quake. Instead, this “orphan tsunami” was the result of a massive earthquake all the way on the other side of the Pacific Ocean, off the coast of present-day Oregon and Washington. The same event is traceable in the oral traditions of the indigenous people of the Pacific Northwest, which include stories of earth shaking and flooding. And also in the annual rings of long-drowned ghost forestswho died suddenly before the beginning of the growing season of 1700.

The earthquake had an estimated magnitude of 9, a catastrophic event for the Pacific Northwest, according to stories that tell of the disappearance of entire settlements and tribes. largest known earthquakes in human history. Modern geological analyzes have now made it clear that the 1700 earthquake was one of the… more than 40 significant vibrations that has rocked the region over the past 10,000 years, and one of about two dozen earthquakes of magnitude 9 or greater from the same period. Each was the result of the Cascadia subduction zone, an approximately 600-mile-long fault line that runs from southern British Columbia to northern California. Along the fault line, the small oceanic tectonic plate Juan de Fuca is sliding down and pushing up towards the much larger North American plate, building friction and pressure in anticipation of an inevitable eruption.

For more than a decadeseismologists have warned about this another earthquake is coming. On average over time, large earthquakes (about a magnitude 8 or larger) in the region have occurred about every 230 years, while even larger ones (magnitude 9 and higher) have occurred about every 500 years. It’s been over 324 years since the last one. And while the sudden shifts and ruptures that cause earthquakes don’t come like clockwork, they do come eventually.

A new study offers a deeper understanding of two colliding tectonic plates than ever before. The research, published on June 7 in the magazine Scientific progress, contains images of the lower plate terrain and confirms the segmentation of the entire subduction zone into smaller pieces. The data and analyzes will be instrumental in improving predictions of future earthquakes and tsunamis, and will help prepare the Pacific Northwest for its seismic fate.

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It provides an “unprecedentedly detailed view of the Cascadia [fault]which we know has the potential to cause large earthquakes,” Erin Wirtha research geophysicist at the US Geological Survey in Seattle who was not involved in the new work, says Popular science. “These results will provide crucial input for future earthquake and tsunami models,” she says.

A schematic cross-section of the Cascadia Subduction Zone shows the ocean floor plate (light gray) moving beneath the North American continental plate, along with other features. CREDIT: U.S. Geological Survey

In 2021, members of the research team spent 41 days at sea aboard an advanced research vessel, with an array of thousands of underwater recording devices trailing for miles behind the ship. They shot powerful sound pulses to and through the seabed and collected the echoes with their bevy of sensitive hydrophones. Based on the strength and timing of those return signals, the scientists were able to examine the topography and characteristics of much of the fault, explains Harold Tobin, study co-author and professor of geophysics at the University of Washington. “It works in an analogous way to radar,” he notes.

But while the essence of the methods may have been simple, analyzing the vast amount of information they collected was not. “It’s supercomputer-level work to do all the processing. That’s one of the reasons why the paper is [only] is coming out now,” Tobin adds. He expects this study to be the first of many as he, his colleagues and other scientists dig further into the data set.

This article “is really just the beginning,” agrees Chris Goldfinger, professor of marine geology at Oregon State University. Goldfinger was not involved in the new research, but is one of the world’s leading experts on the Cascadia subduction zone. “They have done a tremendous amount of work, but the dataset has much more depth. New things will come out of it for decades to come,” he says.

Based on this first dive into the data pool, Tobin and his co-authors have already identified some key results. First, their analysis confirms the already existing theory that smaller, horizontal faults cut through both the upper and lower plates, dividing the subduction zone into segments. (The exact number of segments depends on how the dividing lines are defined, but it is somewhere between three and five.) An earthquake can involve any number of these segments, and the more segments slide at once, the larger the area affected . The magnitude of the quake and the extent of the catastrophe are probable.

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carbotte suduction copy2
Subseafloor map of the Cascadia subduction zone, showing the depth of the fault between the eastward-moving Juan de Fuca locality and the North American plate. Yellow/orange indicates shallow depths; green, deeper; blue/purple the deepest. Diagonal black lines approximate the dividing lines between different segments of the zone. The wavy red line to the right indicates the seaward edge of stiff continental rocks that apparently cause the zone to break up into these segments. CREDIT: Modified from Carbotte et al., Science Advances, 2024

Another key finding: The segment (or segments, depending on how you slice it) of the Juan de Fuca plate off the coast of Washington is very smooth and flat, says Tobin. “It is counterintuitive that a fault that is smoother at the boundary between two plates has the potential for larger earthquakes than a fault that is rougher,” he adds. Smooth means more contact between the plates, more friction over a larger area, and fewer geological features that hinder the spread of a slip event. If that segment disappears from Washington, it will be particularly dramatic. “We can conclude that this represents the majority of the danger from a future earthquake,” he says.

The study also provides finer-scale information about the subduction zone, including fault depth across the area. In many regions along the fault, the new data show that the fault is shallower (i.e. closer to the seafloor surface) than previously thought. “That could potentially have consequences for ground shaking and tsunami hazard,” says Wirth. “The closer you are to a fault or the closer the fault is to the surface, the stronger the ground shaking could be and the greater the tsunami generation could be.” However, she emphasizes that more analysis and quantification is needed to better understand the exact effects of this – and of each observation in the study. That’s where the models will appear.

“There is no guarantee as to what exactly the next event will do,” Tobin says. The big (or a really big one) “could happen tomorrow, but it could also be literally centuries in the future… The more you know, the better guide you can build.”

Wirth and her USGS colleagues are already starting to add this new information to their simulations. Ultimately, the results of those computer tests will be used by hazard researchers to make them more accurate shuffle cardsby engineers to test the design and responses of buildings, and by local and state governments to determine building codes.

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Although unprecedented in scale and depth and instrumental for improving future models, even the new silent data has its limits. Images of the part of the fault closest to the coastline are missing, Writh says, which is “quite important” when it comes to predicting the effects of earthquakes. And the boat’s path leaves about 30 miles between each data collection leg along the study zone, Goldfinger says. Based on previous work, it is possible to fill those 50 km gaps with some information, but nothing as detailed as the new data. “It’s a very good data set,” he says, but “the 50 km line spacing is quite large, and there’s a lot going on in between that we still can’t see.”

Still, we don’t really need more data to know that the Pacific Northwest is still woefully unprepared for the possibility of a major earthquake. “The scientific consensus is so strong that the story will not fundamentally change,” says Goldfinger. “We are in this situation where we have discovered a ticking time bomb [beneath] vulnerable cities.” Some grassroots efforts to do that renovate schools and build tsunami evacuation towers have been successful in recent years, but Oregon and Washington lag significantly behind other earthquake-prone places like Japan and even neighboring California. People tend to think that collapsing buildings are limited to Turkey or Haiti – places where they are [no or poorly enforced] building codes,” says Goldfinger. “But the Pacific Northwest is in the same boat as Turkey and Haiti because we didn’t have building codes that were correct until 1994.”

There are ideally collapsible, unreinforced masonry buildings all over Seattle, Portland, Tacoma and the other major cities in the earthquake zone. There are shoals along the coast within the tsunami flood zone no available evacuation route. Even the standard advice on how best to protect yourself in an earthquake probably not applicable to many of the places and people at risk in the Cascadia zone, Goldfinger says. “We have a very fragile infrastructure and a coming major earthquake. What should we tell people about that?”

Correction June 7, 2024 7:17 PM: The attribution of the quote about the smooth Juan de Fuca sign has been updated. Details about Turkey’s building regulations have been clarified.

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