Most dangerous part of this West Coast earthquake fault identified as one of Earth's biggest hazards

Imagine two huge puzzle pieces of the Earth’s outer shell slowly pushing against each other under the ocean. One piece is sliding underneath the other – this is called a subduction zone, and the Cascadia Subduction Zone is one of the most dangerous regions on Earth.

When subduction zones get stuck and then suddenly slip, it can cause a huge earthquake. These kinds of earthquakes happen along special cracks called megathrust faults.

Scientists know that in this specific region of the West Coast fault in the U.S., massive earthquakes happened in the past, and they could happen again at any time.

Dangers of the Cascadia Subduction Zone

Coastal communities from northern California to British Columbia live atop the Cascadia Subduction Zone, which spans about 600 miles offshore, where the Juan de Fuca plate creeps eastward beneath North America.

Though the movement is slow – barely the width of a fingernail each year – the stress it stores can unleash earthquakes strong enough to rattle skylines and send 100-foot tsunamis racing toward shore.

Geologists calculate that a full-length Cascadia rupture would release more energy in minutes than all human activity consumes in a year.

Geologic records show these giant events arrive roughly every 500 years, give or take a couple of hundred. The last one struck on January 26, 1700, long before skyscrapers and pipelines lined the coast.

Tree rings, coastal marsh sediments, and stories passed down by Indigenous communities all record the sudden drop of land and arrival of waves that night.

Modern scientists want to know whether the next rupture will run the length of the fault or stop at unseen boundaries, because that difference separates an alarming disaster from a catastrophic one.

Listening to the Earth in high-def

In summer 2021, a 41-day expedition aboard the research vessel Marcus G. Langseth sailed nearly the entire length of Cascadia, towing an array of powerful acoustic instruments.

The equipment emitted low-frequency sound that penetrated dense rock, and returning echoes painted detailed images of structures more than six miles below the seabed.

“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.”

Four Cascadia Subduction Zone segments

The new images reveal that the megathrust is not a single unbroken scar. Instead, at least four distinct segments lie side by side, divided by buried faults that slice toward the coastline.

Each segment shows its own slope, thickness of sediment, and patterns of cracks that either speed or slow a rupture.

These crosswise barriers can dampen shaking by blocking a break or, in the worst case, fail and let the fault unzip in one enormous event.

“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 Vancouver–Washington wild card

One segment demands special attention. Stretching from southern Vancouver Island to roughly the mouth of the Columbia River, it shows a remarkably smooth contact where the oceanic plate slides beneath North America.

Smoothness leaves fewer natural speed bumps, so stress can travel farther before stalling.

Computer models suggest that a complete break of this stretch could push shaking intensity one unit higher on the Modified Mercalli scale across Puget Sound.

In this zone, the interface also dips shallowly, which means the section capable of rupturing may extend beneath the Olympic Peninsula and closer to cities such as Seattle and Tacoma. That geometry could translate offshore motion directly into stronger shaking on land.

Lessons from Japan

When a similar fault off Japan failed on March 11, 2011, coastal defenses built for smaller quakes proved no match for the tsunami that overwhelmed Fukushima Daiichi.

Although Cascadia has its own personality, the Japanese tragedy showed how an underappreciated segment can surprise engineers and emergency planners.

That quake also highlighted how intertwined infrastructure failures – power loss, flooded roads, and chemical spills – can magnify the human toll long after the shaking stops.

With the new Cascadia maps in hand, agencies can refine computer simulations that test bridges, hospitals, and energy corridors against plausible shaking patterns and wave heights.

Those results filter into revised building codes and the Pacific Northwest’s growing earthquake early-warning network.

Earthquakes and tsunamis

While the cruise captured the roughness and rock types of the seafloor, translating those details into wave forecasts will take more number-crunching.

“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 team is already feeding the new depth profiles into models that calculate whether the seabed will lurch upward or downward during different rupture shapes.

A separate group will turn that motion into tsunami projections, and emergency managers will use the results to update escape routes posted on telephone poles from Brookings to Port Alberni.

Many coastal residents practice annual evacuation drills timed with sirens every March.

Cascadia Subduction Zone preparations

No scientist can predict the day of the next Cascadia earthquake, but probabilities guide practical decisions.

Updated hazard maps will inform where to install vertical evacuation towers, how to retrofit schools, and which stretches of highway deserve seismic upgrades first.

Residents can take simple steps today: strap water heaters, secure heavy furniture, keep an emergency kit, and learn the difference between a smartphone alert and a tsunami siren.

By treating resilience as a routine chore rather than a distant fear, communities stand a better chance of bouncing back when the locked plates finally give way.

The full study was published in the journal Science Advances.

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