A tear off the Pacific Northwest coast is helping scientists understand earthquake risk

Scientists have long known about a major fault off the Pacific Northwest coast that could potentially cause a major earthquake in the decades to come.

Now a new study has shed light on the dynamics involved.

The Cascadia Subduction Zone is a 700-mile fault stretching from Northern California to British Columbia and separates the Juan de Fuca and North America tectonic plates, according to the Pacific Northwest Seismic Network (PNSN).

Tectonic plates, which are massive slabs of solid rock that float on the Earth's mantle, can cause earthquakes and volcano formations at the surface as they shift and subduct – the process during which the plates collide and one is forced beneath the other, typically denser plate.

The Juan de Fuca plate, located in the northeast Pacific Ocean off the West Coast of North America, is moving eastward toward the North American plate at a rate of about four centimeters per year, according to the PNSN. As it moves, Juan de Fuca is slowly subducting beneath the North American plate, a major tectonic plate that covers most of North America, Greenland Cuba and the Bahamas, as well as parts of Iceland, Siberia and northern Japan.

This activity makes the Cascadia Subduction Zone capable of producing earthquakes of up to 9.0 magnitude, according to the PNSN. In the Pacific Northwest, the last known so-called "megathrust" earthquake – considered by geologists to be the largest and most powerful type – likely happened in January 1700, seismologists say.

New research points to a mechanism within the Cascadia fault that could be accelerating a break. Scientists at the University of Washington discovered highways within the fault that transport migrating fluid, which facilitates further weakening and stress on the fault, according to a paper published earlier this year in Science.

The hot fluid, typically composed of water and dissolved minerals and gases, is generated during episodic tremor and slip. The latter is a phenomenon in which tectonic plates slip or move forward by several inches, slowly enough that it doesn't cause a felt earthquake in the way a sudden slip does. The movement creates pathways that allow the migrating fluid to be released onto the seafloor.

The first-of-its-kind study that revealed this mechanism used data from the Regional Cabled Array – an underwater tectonic observatory comprised of more than 550 miles of electro-optical cables and measuring instruments off the shores of Washington and Oregon – to measure seismic noise hidden within Cascadia, Maleen Kidiwela, a postdoctoral marine geology and geophysics researcher and lead author of the study, told ABC News.

Unlike the subduction zones found in Chile and Japan, Cascadia is relatively silent.

"One big thing that we see in Cascadia is that we don't have [many] earthquakes, so we really don't know what's going on in the in that region, in that offshore part of the subduction zone," Kidiwela said.

By measuring noise via the underwater seismometer networks, the researchers were able to detect strain accumulating in the northern part of the Cascadia Subduction Zone.

"When it eventually ruptures, it would rupture differently than the central part of Cascadia," Kidiwela said. "In the north, we would expect potential for quite a bit larger magnitude events compared to the central part of Cascadia."

The earthquakes would occur offshore and likely would not be felt on land. However, they could be strong enough to trigger a tsunami, Kidiwela said.

The USGS found last year that there's a 10% to 15% chance that a megathrust earthquake in the Pacific Northwest could occur within the coming decades, but the dynamics of fluid migration captured in the recent study could influence the severity of the eventual earthquake, Kidiwela said. That probability also doesn't capture the full hazard in southern Cascadia, where evidence suggests that partial-margin ruptures of magnitude 8 and lower could occur, and even sooner, Kidiwela said.

"We have a 15% chance of a full margin rupture in the next 50 years, which is about a magnitude 9," he said.

Additionally, scientists last year observed a massive portion of the Juan de Fuca plate off the coast of Vancouver Island tearing apart for the first time.

Using data collected from the 2021 Cascadia Seismic Imaging Experiment, researchers observed tears slicing through the oceanic plate, including a massive slab that had dropped by more than three miles – evidence of a dying subduction zone, according to a study published in October

"This is the first time we have a clear picture of a subduction zone caught in the act of dying," said Brandon Shuck, a geologist at Louisiana State University and lead author of that study.

The fault has not completely torn through the plate yet, but it's close, Shuck said in a statement.

Uncertainty related to the extent of ruptures and the recurrence of past subduction megathrust earthquakes has lead to ambiguity in earthquake and tsunami hazard assessments, hindering the ability for emergency planners to prepare for future events, according to the U.S. Geological Survey.

The risk of increased earthquakes in Cascadia is so high that the National Science Foundation in 2023 provided a $15 million grant to the University of Washington to bolster earthquake preparedness.

The project involves implementing more monitoring instruments offshore, including early-warning systems, Kidiwela said.