Cascadia Subduction Zone, one of Earth's top hazards, comes into sharper focus (2024)

Cascadia Subduction Zone, one of Earth's top hazards, comes into sharper focus (1)

Off the coasts of southern British Columbia, Washington, Oregon and northern California lies a 600 mile-long strip where the Pacific Ocean floor is slowly diving eastward under North America. This area, called the Cascadia Subduction Zone, hosts a megathrust fault, a place where tectonic plates move against each other in a highly dangerous way.

The plates can periodically lock up and build stress over wide areas―eventually to be released when they finally lurch against each other. The result: the world's greatest earthquakes, shaking both seabed and land, and generating tsunamis 100 feet high or more.

Such a fault off Japan caused the 2011 f*ckushima nuclear disaster. Similar zones exist off Alaska, Chile and New Zealand, among other places. At Cascadia, big quakes are believed to come roughly every 500 years, give or take a couple hundred. The last occurred in 1700.

Scientists have long been working to understand the Cascadia Subduction Zone's subterranean structures and mechanics, in order to delineate places most susceptible to quakes, how big they might be and what warning signs they might produce. There is no such thing as predicting an earthquake; rather, scientists try to forecast probabilities of multiple scenarios, hoping to help authorities design building codes and warning systems to minimize the damage when something happens.

A new study promises to greatly advance this effort. A research vessel towing an array of the latest geophysical instruments along almost the entire zone has produced the first comprehensive survey of the many complex structures beneath the seafloor. These include the geometry of the down-going ocean plate and overlying sediments, and the makeup of the overriding North American plate. The study was published in the journal Science Advances.

"The models currently in use by public agencies were based on a limited set of old, low-quality 1980s-era data," said Suzanne Carbotte, a marine geophysicist at Columbia University's Lamont-Doherty Earth Observatory, who led the research. "The megathrust has a much more complex geometry than previously assumed. The study provides a new framework for earthquake and tsunami hazard assessment."

Cascadia Subduction Zone, one of Earth's top hazards, comes into sharper focus (2)

The data was gathered during a 41-day cruise in 2021 by Lamont's research vessel, the Marcus G. Langseth. Researchers aboard the ship penetrated the seafloor with powerful sound pulses and read the echoes, which were then converted into images, somewhat similar to how physicians create interior scans of the human body.

One key finding: the megathrust fault zone is not just one continuous structure, but is divided into at least four segments, each potentially somewhat insulated against movements of the others. Scientists have long debated whether past events, including the 1700 quake, ruptured the entire zone or just part of it—a key question, because the longer the rupture, the bigger the quake.

The data show that the segments are divided by buried features including big faults, where opposing sides slide against each other perpendicular to the shore. This might help buffer against movement on one segment translating to the next.

"We can't say that this definitely means only single segments will rupture, or that definitely the whole thing will go at once," said Harold Tobin, a geophysicist at the University of Washington and co-author of the study. "But this does upgrade evidence that there are segmented ruptures."

The imagery also suggests the causes of the segmentation: the rigid edge of the overriding North American continental plate is composed of many different kinds of rocks, formed at different times over many tens of millions of years, with some being denser than others. This variety in the continental rocks causes the incoming, more pliable oceanic plate to bend and twist to accommodate differences in overlying pressure. In some places, segments go down at relatively steep angles, in others at shallow ones.

The researchers zeroed in on one segment in particular, which runs from southern Vancouver Island alongside Washington state, more or less ending at the Oregon border.

The subterranean topography of other segments is relatively rough, with oceanic features like faults and subducted seamounts rubbing up against the upper plate—features that might erode the upper plate and limit how far any quake may propagate within the segment, thus limiting the quake's size. In contrast, the Vancouver-Washington segment is quite smooth. This means that it may be more likely to rupture along its entire length at once, making it potentially the most dangerous section.

Also in this segment, the seafloor is subducting under the continental crust at a shallow angle relative to the other segments. In the other segments, most of the earthquake-prone interface between the plates lies offshore, but here the study found the shallow subduction angle means it probably extends directly under Washington's Olympic Peninsula. This might magnify any shaking on land.

"It requires a lot more study, but for places like Tacoma and Seattle, it could mean the difference between alarming and catastrophic," said Tobin.

A consortium of state and federal agencies and academic institutions has already been poring over the data since it became available to sort through the implications.

As for tsunami hazard, that is "still a work in progress," said Kelin Wang, a research scientist at the Geological Survey of Canada who was not involved in the study.

Wang's group is using the data to model features of the seafloor off Vancouver Island that might generate tsunamis. (In general, a tsunami occurs when the deep seafloor moves up or down during a quake, sending a wave to the surface that concentrates its energy and gathers height as it reaches shallower coastal waters.) Wang said his results will go to another group that models tsunamis themselves, and after that to another group that analyzes the hazards on land.

Practical assessments that could affect building codes or other aspects of preparedness may be published as early as next year, say the researchers. "There's a whole lot more complexity here than was previously inferred," said Carbotte.

More information:Suzanne Carbotte, Subducting plate structure and megathrust morphology from deep seismic imaging linked to earthquake rupture segmentation at Cascadia, Science Advances (2024). DOI: 10.1126/sciadv.adl3198. www.science.org/doi/10.1126/sciadv.adl3198

Journal information:Science Advances

Provided byColumbia Climate School

Citation:Cascadia Subduction Zone, one of Earth's top hazards, comes into sharper focus (2024, June 7)retrieved 21 June 2024from https://phys.org/news/2024-06-cascadia-subduction-zone-earth-hazards.html

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Cascadia Subduction Zone, one of Earth's top hazards, comes into sharper focus (2024)

FAQs

What is the Cascadia Subduction Zone known for? ›

The Cascadia subduction zone is where the Juan de Fuca, Explorer, and Gorda tectonic plates are subducting under the North American plate. It is now thought to be capable of producing great earthquakes of magnitude 8 or 9, like those off Indonesia in 2004 and Japan in 2011.

What cities will be affected by the Cascadia earthquake? ›

Coastal Pacific Northwest

Called the Cascadia subduction zone, a big quake along this fault could affect the cities of Seattle, Tacoma, Portland, Eugene, Salem, and Olympia.

What type of fault is the Cascadia Subduction Zone? ›

The Cascadia Subduction Zone (CSZ) "megathrust" fault is a 1,000 km long dipping fault that stretches from Northern Vancouver Island to Cape Mendocino California. It separates the Juan de Fuca and North America plates.

What are the hazards in the Cascadia Subduction Zone? ›

Oregon has the potential for a 9.0+ magnitude earthquake caused by the Cascadia Subduction Zone and a resulting tsunami of up to 100 feet in height that will impact the coastal area.

Has California ever had a 9.0 earthquake? ›

North Coast

The Cascadia Subduction Zone stretches underneath the Humboldt-Del Norte county region, extending from Cape Mendocino all the way up through the Pacific Northwest. This fault zone is capable of generating a magnitude 9 (or larger) earthquake on average every 500 years. The last such event was in 1700.

What is Cascadia known for? ›

Cascadia contains the largest tracts of untouched old growth temperate rainforests in the world, including 7 of the top 10 worlds carbon absorbing forests, the worlds tallest trees, thousands of volcanoes, hot springs, rivers, lakes, inlets, island and ocean, and some of the last diminishing, though still impressive ...

What are the odds of the Cascadia earthquake? ›

Because this probability is constant, an earthquake cannot be “overdue.” Using the entire paleoearthquake record, the chance of an earthquake in the next 50 years is 50/530 = 0.094 or 9.4%. Alternatively, assuming that we are still in the recent cluster gives a probability about twice as large: 50/326 = 0.15 or 15%.

Where in the USA is the greatest chance of earthquake damage? ›

Which state has the most earthquakes (not human-induced)? California has more earthquakes that cause damage than any other state. Alaska and California have the most earthquakes (not human-induced).

What are the signs of a big earthquake coming? ›

The so-called precursor is often a swarm of small earthquakes, increasing amounts of radon in local water, unusual behavior of animals, increasing size of magnitudes in moderate size events, or a moderate-magnitude event rare enough to suggest that it might be a foreshock.

How long does a 9.0 earthquake last? ›

It is capable of producing 9.0+ magnitude earthquakes and tsunamis that could reach 30 m (98 ft). The Oregon Department of Emergency Management estimates shaking would last 5–7 minutes along the coast, with strength and intensity decreasing further from the epicenter.

When was the last Cascadia earthquake? ›

Neskowin Ghost Forest, 2012. At approximately 9 p.m. Pacific Standard Time, Jan. 26, 1700, a magnitude 8 or 9 earthquake occurred on the Cascadia Subduction Zone, a 600-mile stretch between Vancouver Island, British Columbia and Cape Mendocino, California.

How far inland will the Cascadia tsunami go? ›

The waves could be 30-40 feet (9-12 m) in height when they hit the coast but some models suggest they could reach 100 feet (30 m), and in many parts of the coast they would flood up to 10 miles (16 km) inland.

What type of plate boundary is the Cascadia plate boundary? ›

The Cascadia Subduction Zone, extending from northern California through western Oregon and Washington to southern British Columbia, is a type of convergent plate boundary. Two parallel mountain ranges have been forming as a result of the Juan de Fuca Plate subducting beneath the edge of North America.

What is the hazard of subduction zone? ›

What makes subduction zones so hazardous? The most powerful earthquakes, tsunamis, volcanic eruptions, and landslides occur in subduction zones where tectonic plates collide and one plate is thrust beneath another.

How far do aftershocks travel? ›

Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a larger event or "mainshock." They occur within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed.

How likely is the Cascadia earthquake to happen? ›

Scientists say there is a 37 percent chance of a CSZ earthquake of 7 magnitude or higher in the next 50 years.

What would happen if the Cascadia Subduction Zone rupture? ›

Subduction-zone earthquakes operate on the opposite principle: one enormous problem causes many other enormous problems. The shaking from the Cascadia quake will set off landslides throughout the region—up to thirty thousand of them in Seattle alone, the city's emergency-management office estimates.

What are subduction zones known for? ›

Since each interaction can produce natural hazards like earthquakes, tsunamis, volcanoes, and landslides, understanding each type of interaction is important. Subduction zones, since they involve oceanic plates, are known for earthquakes that produce tsunamis and are often responsible for volcanic ranges too.

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