Cascadia Subduction Zone, One of Earth’s Major Hazards, Comes into Sharper Focus – State of the Planet

There is a 600-mile stretch along the coast of southern British Columbia, Washington, Oregon, and northern California where the Pacific Ocean floor dives slowly eastward under North America. This region, called the Cascadia Subduction Zone, contains a megathrust fault, a place where tectonic plates are moving against each other in a very dangerous manner. Plates can periodically lock and create stress over wide areas—eventually releasing as they bend against each other. Effect: The world’s largest earthquakes shake both the ocean floor and the land, creating tsunamis 100 feet high or more. Such a mistake by Japan caused the 2011 Fukushima nuclear disaster. Similar zones exist elsewhere in Alaska, Chile and New Zealand. In Cascadia, major earthquakes are believed to occur approximately every 500 years, give or take a couple hundred. The last one happened in 1700.

Scientists have long worked to understand the underground structures and dynamics of the Cascadia subduction zone in order to define the places most susceptible to earthquakes, how large they might be, and what warning signs they might produce. There is no such thing as predicting an earthquake; Instead, scientists try to predict the probabilities of multiple scenarios, hoping to help officials design building codes and warning systems to minimize damage when something does happen.

A newly published study It promises to take this initiative a step further. A research vessel towing an array of state-of-the-art geophysical instruments across nearly the entire zone has produced the first detailed survey of many complex structures beneath the sea. These include the geometry of the subducting oceanic plate and overlying sediments, and the makeup of the overlying North American plate. The study was recently published in the journal Science Advances.

„The models currently in use by public agencies are based on a limited set of old, low-quality data from the 1980s,” he said. Suzanne GarbottMarine geophysicist at Columbia University Lamont-Doherty Earth Observatory, who led the study. „The megathrust has a more complex geometry than previously thought. This study provides a new framework for earthquake and tsunami hazard assessment.

The data was collected by Lamont’s research vessel during a 41-day cruise in 2021, funded by the US National Science Foundation. Marcus G. Longseth. Researchers on board penetrated the ocean floor with powerful sound pulses and read the echoes, which were then converted into images, similar to how doctors create scans of the inside of the human body.

A key discovery: The megathrust fault zone is not only a continuous system but is divided into at least four segments, each partially insulated against the movements of the others. Scientists have long debated whether past events, including the 1700 earthquake, ruptured the entire zone or just part of it—a key question because the longer the rupture, the larger the earthquake.

The data show that the segments are separated by buried features, including major faults, where opposite sides slide against each other perpendicular to the shore. This helps to translate the movement of one segment to the next. „We can’t say for sure that only single parts will rupture, or for sure that the whole thing will go at once,” said Harold Tobin, a geophysicist at the University of Washington and co-author of the study. „But it adds to the evidence that there are discrete ruptures.”

The images also suggest the reasons for the separation: the hard edge of the North American continental plate is made up of many types of rock, formed at different times over millions of years, some thicker than others. This type of continental crust causes the incoming, highly flexible oceanic plates to bend and twist to accommodate differences in pressure above. In some places, the sections go down at relatively steep angles, in others they are shallow.

The researchers zeroed in on one segment in particular, which runs from southern Vancouver Island along Washington state, more or less ending at the Oregon border. The subterranean topography of other segments is relatively rough, with oceanic features, faults that rub against the upper plate, and subducted seafloors – which erode the upper plate and limit how far any earthquake can propagate within the region, thus limiting the size of the earthquake. In contrast, the Vancouver-Washington segment is much smoother. This means that its entire length will be deformed simultaneously, making it a very dangerous part.

And in this section, the coastline dips under the continental crust at a shallower angle compared to other areas. In other segments, the interface between most earthquake-prone plates is offshore, but here the study found that the shallow subduction angle extends directly under Washington’s Olympic Peninsula. This can magnify any tremors in the ground. „It requires more study, but for places like Tacoma and Seattle, it could mean the difference between dangerous and catastrophic,” Tobin said.

With funding from the US Geological Survey, a consortium of state and federal agencies and academic institutions is already analyzing the data.

As for the tsunami risk, it’s „still a work in progress,” said Kellin Wang, a research scientist at the Geological Survey of Canada who was not involved in the study. Wang’s team uses the data to model features of the seafloor on Vancouver Island that can generate tsunamis. (Normally, a tsunami occurs when the deep ocean floor moves up or down during an earthquake, sending waves to the surface that concentrate their energy and gather height when they reach shallow coastal waters.) Wang said his results will be passed on to another group. Models the tsunamis themselves, and then to another group that studies the hazards on land.

Practical assessments that could affect building codes or other aspects of preparedness could be released as early as next year, the researchers say. „There’s a lot more complexity here than previously assumed,” Garbott said.