New data on the 'big quake' provides a clearer view of the inevitable earthquake catastrophe

On January 26, 1700, a tsunami struck Japan. We know this from written records describing coastal inundation. However, what is most notable about these records is what they do not describe. Tsunamis are the result of water displaced by earthquakes, but the 1700 tsunami was not associated with any earthquake felt in Japan – there was no warning earthquake. Instead, this “orphan tsunami” was the result of a massive earthquake all the way across the Pacific Ocean, off the coast of present-day Oregon and Washington. The same event can be traced in the oral traditions of indigenous peoples of the Pacific Northwest which contain accounts of earth shaking and floods. And also in the tree rings of long-submerged ghost forests, which died suddenly before the beginning of the growing season in the year 1700.

The earthquake measured 9.0 magnitude, a catastrophic event for the Pacific Northwest according to stories about the disappearance of entire settlements and tribes, and among the largest known earthquakes in human history. Through modern geological analysis, it is now well established that the 1700 earthquake was one of more than 40 large earthquakes that shook the region over the past 10,000 years, and one of about two dozen earthquakes of magnitude 9 or greater from the same time period. . Both were the result of the Cascadia Subduction Zone, a fault line about 600 miles long that runs from southern British Columbia to northern California. Along the fault, the small Juan de Fuca oceanic tectonic plate slides down and pushes the larger North American plate upward, building up friction and pressure waiting for the inevitable eruption.

For more than a decade, seismologists have warned that another earthquake is on its way. On average, over time, large earthquakes (magnitude 8 or greater) occur in the region approximately every 230 years, while larger earthquakes (magnitude 9 and greater) occur approximately every 500 years. It has been more than 324 years since the last time. Although the sudden shifts and ruptures that cause earthquakes do not come like clockwork, they do come eventually.

A new study provides a deeper glimpse into the colliding tectonic plates than ever before. The research, published June 7 in the journal Science Advances, includes images of the bottom plate terrain and confirms the fragmentation of the entire subduction zone into smaller parts. The data and analyzes will be useful for improving predictions of future earthquakes and tsunamis, and helping prepare the Pacific Northwest for its seismic fate.

It provides an unprecedentedly detailed view of Cascadia [fault]“We know it has the potential to host large earthquakes,” Erin Wirth, a research geophysicist at the USGS in Seattle, who was not involved in the new work, tells Popular Science. “These results will serve as critical 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: USGS

For 41 days in 2021, members of the study team were at sea on a sophisticated research vessel, with an array of thousands of underwater recording devices tracking for miles behind the ship. They fired powerful pulses of sound toward and across the seafloor, collecting the echoes using an array of sensitive hydrophones. Based on the strength and timing of these return signals, scientists were able to explore the topography and properties of much of the fault, explains Harold Tobin, co-author of the study and a professor of geophysics at the University of Washington. “It works in a similar way to radar,” he points out.

But although the essence of the methods may be simple, the analysis of the huge amount of information collected was not. “It's supercomputer-level work to do all the processing. That's one of the reasons the paper exists [only] “He's coming out now,” Tobin adds. He expects this study to be the first of many as he, his colleagues and other scientists delve deeper into the data set.

Chris Goldfinger, a professor of marine geology at Oregon State University, agrees that this paper “really just scratches the surface.” Goldfinger was not involved in the new study, but he is one of the world's leading experts on the Cascadia subduction zone. “They've done a tremendous amount of work, but the data set has much greater depth. New things will come out of it for decades to come,” he says.

In this initial dive into the database, Tobin and his colleagues have already identified some important findings. First, their analysis confirms the pre-existing theory that smaller horizontal faults penetrate the upper and lower plates, dividing the subduction zone into parts. (The exact number of parts depends on how the divisions are defined, but it ranges between three and five.) In the event of an earthquake, any number of these parts may be involved, and the more parts slide simultaneously, the larger the affected area. The size of the earthquake and the scale of the disaster it is likely to be.

Undersea map of the Cascadia subduction zone, showing the depth of the rift between the eastward-moving Juan de Fuca site and the North American plate. Yellow/orange indicates shallow depths; Green, deeper. Deeper blues/purples. Diagonal black lines approximate divisions between different parts of the region. The wavy red line to the right indicates the offshore margin of solid continental rock that apparently causes the region to intrude into these parts. Credit: Modified from Carbotte et al., Science Advances, 2024

Another key discovery: Tobin says the portion (or portions, depending on how you slice it) of the Juan de Fuca plate off the coast of Washington is very smooth and flat. He adds: “Contrary to what is expected, a smoother fault at the boundary between two plates has the potential to cause larger earthquakes than a rougher fault.” Smooth means more contact between plates, more friction over a larger area, and fewer geological features that impede the propagation of a slip event. When this part of Washington goes, it will be especially dramatic. “We can conclude that it represents the bulk of the risk of future seismic rupture,” he says.

The study also provides precise information about the subduction zone, including the depth of the fault throughout the region. In many areas along the rift, new data show that the rift is shallower (i.e. closer to the surface of the seafloor) than previously thought. “This could have an impact on ground shaking and tsunami risk,” Wirth says. “The closer you are to the fault or the closer the fault is to the surface, the stronger the ground shaking and potentially also the greater the tsunami generation.” However, she stresses that more analysis and quantification is needed to better understand the precise effects of this, and for each observation in the study. This is where models will come in.

“There's no guarantee exactly what the next event will do,” Tobin says. The big event (or really big event) “could happen tomorrow – but it could also happen centuries in the future… The more you know, the more evidence you can build.”

Wirth and her colleagues at the USGS have already begun adding this new information to their simulations. Ultimately, the results of these computer tests will be used by risk researchers to make more accurate vibration maps, by engineers to test building design and responses, and by local and state governments to determine building codes.

Although unprecedented in its scale, depth, and effectiveness in improving future models, even new hard data has limitations. Reith says he's missing images from the section of the fault closer to the coast, which is “very important” when it comes to predicting the effects of an earthquake. Goldfinger says the boat route leaves about 50 kilometers between each data collection route along the study area. With previous work, it's possible to fill in those 50-kilometre gaps with some information, but nothing nearly as good as new data. “It's a very good data set, but the distance between the 50-kilometre lines is very large, and there are a lot of things happening in between that we still can't see,” he says.

However, we don't really need more data to know that the Pacific Northwest remains woefully unprepared for the possibility of a major earthquake. “The scientific consensus is strong enough that the story doesn't change radically,” Goldfinger says. “We are in this situation where we have discovered a time bomb [beneath] Fragile cities.” Some grassroots efforts to upgrade schools and build tsunami evacuation towers have been successful in recent years, but Oregon and Washington still lag far behind other earthquake-prone places like Japan and even neighboring California. People tend to think that “collapsing buildings are limited to “On Turkey or Haiti, which are the places where this happened.” [no or poorly enforced] “Building codes,” Goldfinger says. “But the Pacific Northwest is in the same boat as Turkey and Haiti because we didn't have adequate building codes until 1994.”

There are collapsible, unreinforced masonry buildings throughout Seattle, Portland, Tacoma, and other major cities in the earthquake zone. There are schools along the coast within the tsunami flood zone and there is no evacuation route available. Even standard advice about how to best protect yourself in the event of an earthquake likely won't apply to many of the places and people at risk in the Cascadia region, Goldfinger says. “We have a very fragile infrastructure and a big earthquake is coming. What are we supposed to tell people about that?”

Correction June 7, 2024 7:17 PM: Updated citation attribution about Juan de Fuca's smooth painting. Details regarding building regulations in Türkiye are explained.

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