Scientists have solved a decades-old mystery behind a series of unusually predictable earthquakes deep beneath the Pacific Ocean, uncovering natural “brake zones” that repeatedly stop ruptures from growing larger.

The discovery comes from a fault line called the Gofar transform fault, located roughly 1,000 miles west of Ecuador along the East Pacific Rise. For at least 30 years, the fault has been producing magnitude 6 earthquakes every five to six years in nearly identical locations, a rare pattern in earthquake science where most major quakes remain highly unpredictable.

Researchers now say structurally complex regions inside the fault act as barriers that limit how far earthquake ruptures can travel. These zones repeatedly halt the earthquakes at nearly the same points during every seismic cycle.

“We’ve known these barriers existed for a long time, but the question has always been, what are they made of, and why do they keep stopping earthquakes so reliably, cycle after cycle?” said seismologist Jianhua Gong, lead author of the study and Assistant Professor of Earth and Atmospheric Sciences at Indiana University Bloomington.

To understand the mechanism, researchers analyzed data from two major ocean-floor monitoring experiments conducted in 2008 and between 2019 and 2022. Scientists deployed ocean bottom seismometers along separate segments of the Gofar fault, capturing tens of thousands of tiny earthquakes before and after two magnitude 6 events.

Faults hit invisible barriers

The data revealed a strikingly consistent pattern. In the days leading up to major earthquakes, the barrier zones experienced bursts of small seismic activity. Immediately after the larger rupture occurred, the same regions suddenly became quiet again.

Researchers concluded that the barriers are not passive sections of rock but active fault regions with complicated geometry. In these zones, the fault splits into multiple strands with sideways offsets between them, creating small extension gaps inside the fault system.

The team also found evidence that seawater penetrates deep into these fractured zones. Together, the fault geometry and trapped fluids create a process called “dilatancy strengthening.”

When a major rupture reaches the barrier, the rapid movement sharply lowers pore pressure inside the water-saturated rock. That temporary pressure drop effectively locks the fault zone and slows the rupture before it can continue spreading.

“These barriers are not just passive features of the landscape,” Gong explained. “They are active, dynamic parts of the fault system, and understanding how they work changes how we think about earthquake limits on these faults.”

Ocean quakes stay smaller

The findings could help explain a long-standing puzzle in global earthquake records: why many large underwater earthquakes fail to grow as large as geologic conditions suggest they should.

Transform faults like Gofar exist across the world’s ocean floors, where tectonic plates slide horizontally past each other. Scientists now believe similar barrier zones may act as natural rupture-limiting systems on many of these underwater faults.

That could improve earthquake models used to assess seismic hazards near coastal regions and underwater fault systems worldwide.

The researchers also say the study highlights how fluids inside faults can strongly influence earthquake behavior, something scientists are increasingly recognizing as a key factor in seismic activity.

The Gofar fault itself poses little direct danger because it lies far from populated coastlines. However, the mechanism discovered there may apply to many other underwater fault systems globally.

The study was published in the journal Science.