Stress testing in the Cascadia subduction zone reveals variability that could affect how earthquakes propagate – UW News

The Nesquin Ghost Forest, pictured above, helped researchers identify the last earthquake that occurred in the Cascadia subduction zone in 1700. A new University of Washington study has revealed details about the structure of the fault that may help scientists understand what will happen when the next quake hits. Image: iStock

The Cascadia subduction zone is unusually quiet for a large thrust fault. The rift extends more than 600 miles from Canada to California, and marks the convergence of the Juan de Fuca and North American plates. While other subduction zones produce sporadic sounds when plates rub against each other, Cascadia shows very little seismic activity, fueling assumptions that the plates are locked together by friction.

The subduction zone – located miles offshore and deep underwater – is difficult to monitor. Most data are collected on shore, which limits the breadth and quality of the results. The absence of earthquakes further complicates efforts to understand their behavior and structure.

In a new study, the first to monitor marine pressure over a long period of time, researchers from the University of Washington report that the plates may not be completely closed. Based on ground motion data collected over 13 years from sensors in different regions, the study showed that the northern part of the fault is closed and quiet, but the central region appears to be more active. There, researchers noticed signs of a shallow, slow-moving earthquake, and detected pulses of fluid flowing through subterranean channels, which may relieve pressure from the fault.

The results, published February 27 in the journal Science Advances, may change expectations about how this region will respond to a large earthquake. Similar features elsewhere have stopped rupture that would have continued along the entire fault line.

“It’s preliminary, but we think that the changing fluid paths in Cascadia will change the behavior of large earthquakes on the fault,” said co-author Marine Degnole, an associate professor of Earth and space sciences at the University of Wisconsin.

The Juan de Fuca Plate is advancing toward the North American Plate at a rate of about 4 centimeters per year. But because the plates are stuck together, this movement generates pressure. Eventually, the building’s tension will exceed what the panels can withstand. When they are finally free, an earthquake will spread along the border.

Huge earthquakes, which occur at the boundary where one plate slides under another, shake the Pacific Northwest every 500 years or so. Researchers date the last fault to 1700, and estimates indicate a 10 to 15 percent chance that the entire fault will rupture, triggering an earthquake that could exceed 9 on the Richter scale, within the next 50 years. The results of this study do not change these possibilities, but the dynamics captured may affect the eventual intensity of the earthquake.

A recent study of the seafloor found that the fault can be separated into at least four distinct geological parts. Each one may be isolated from a tear in another area. In this study, researchers took a closer look at two of the regions by analyzing data from three monitoring stations, one near Vancouver Island and two off the Oregon coast.

In this map of subduction zone locking, red indicates where the plates are tightly locked and orange/yellow shows less locking. Study sites are marked with red boxes, while blue lines along the coast highlight other faults—fluid channels proposed in this study. The cross section shows fluid migration in more detail. Image: Science Advances/Kidiwela et al.

“We wanted to understand strain changes in different areas offshore,” said lead author Malin Kidewila, a doctoral student in oceanography at the University of Wisconsin. “We used seismometers to measure how seismic velocity varies under each station.”

Seismic velocity is a term used to describe the rate at which ambient noise travels through material. Since the speed of sound depends on what it moves through, tracking seismic velocity can give researchers a window into the processes happening beneath the ocean floor.

“When you compress something, you can expect sound waves to move through it faster,” Kidwela said.

The steady increase in seismic velocity observed at the northern site told researchers that the rock was compressed, supporting the theory that the two plates were locked in place.

The central region displayed a different pattern. For two months in 2016, seismic velocity decreased. The researchers attribute this dip to a slow-moving earthquake on the shallow edge of the oceanic plate, which relieved some of the pressure at the fault.

Other decreases in seismic velocity, recorded between 2017 and 2022, were associated with fluid dynamics. Subduction releases fluid from rocks and pushes it towards the surface. The study found that other faults, which run perpendicular to the subduction zone, may serve as pathways to allow trapped fluid to escape.

“During a large thrust rupture, one of the ways an earthquake propagates is through fluid pressure. If you have a way to release these fluids, it could help improve the stability of the fault, and possibly affect how the area behaves during a large earthquake,” Kidwela said.

By pulling data from just three sites, the researchers observed complex dynamics that might have been overlooked. Future work will expand this effort significantly. University of Wisconsin researchers received $10.6 million in 2023 to build an underwater observatory in the Cascadia subduction zone.

“Finding this link between fluids coming into the shallow subduction zone is unique, as is the evidence that the rift is not completely closed,” said co-author William Wilcock, a professor of oceanography at the University of Wisconsin and one of the scientists involved with the observatory. “It suggests we need more tools out there, because there may be more things that people haven’t been able to detect before.”

Additional co-authors include Quan-Fu Feng of the University of Utah.

This study was funded by the Jerome M. Barros Endowed Chair in Sensor Networks at the University of Washington and the National Science Foundation.

For more information, contact Kidiwela at [email protected].

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