Scripps geophysicist helps solve mystery of deep Pacific earthquake system

This kind of predictability is almost unheard of in seismology. For many years, researchers have known that this pattern exists without fully understanding why.

Now they do. In a new study published in the journal Science, scientists, including Wenyuan Fan, a geophysicist at the Scripps Institution of Oceanography at the University of California, San Diego, have revealed the physical mechanism behind this remarkable behavior. The answer, it turns out, lies in a pair of unusual zones within the fault itself, zones that act like built-in brakes in measuring the strength of the earthquake.

“We’ve known these barriers have been around for a long time, but the question has always been: What are they made of, and why do they continue to reliably stop earthquakes, cycle after cycle?” said seismologist Jianhua Gong, co-author of the study and a former postdoctoral researcher at Scripps Oceanography.

Researchers from several universities participated in the field work led by Jinhua Gong (front row, red jacket) and Wenyuan Fan (right, blue hat) aboard the Scripps oceanographic research vessel Roger Revell. (Image credit: Scripps)

Jung and Fan, along with colleagues from Woods Hole Oceanographic Institution, the USGS, Boston College, the University of Delaware, Western Washington University, the University of New Hampshire, and McGill University, studied the Jóvar Transform Fault, which lies along the East Pacific Ridge off the west coast of Ecuador, with the goal of solving a 30-year-old mystery of why some underwater faults trigger earthquakes as large as clockwork.

The Juvar Fault is a long underwater mountain range where the Pacific and Nazca tectonic plates, two massive slabs of rock that make up the Earth’s outer crust, rub against each other at a rate of about 140 millimeters per year, which is about the speed at which a fingernail grows. Transform faults are boundaries where plates slide horizontally past each other, and the Govar Fault is one of the most studied faults of its kind anywhere under an ocean on Earth on the global seafloor.

What makes Govar unusual is that large earthquakes occur over and over again in the same locations, and stop at the same locations. Between these earthquake zones are areas of faults that appear to quietly absorb pressure, without causing major earthquakes. Scientists call these extensions “barriers.” But until now, no one has been able to fully explain why it works.

To find out, the research team relied on data from two major ocean floor experiments, one conducted in 2008 and the other running from 2019 to 2022, in which scientists lowered instrument packages, known as ocean floor seismometers, which are earthquake detectors placed directly on the sea floor, along two separate segments of the Govar Fault. The instruments recorded tens of thousands of small earthquakes in the weeks and months surrounding two large magnitude 6 events, building a highly detailed picture of how a fault behaves before, during and after a major rupture.

“This is the most ambitious offshore experiment to study oceanic transform faults to date,” Fan said.

What scientists found in the two buffer zones was strikingly similar. In the days and weeks before each major earthquake, the barriers lit up with intense small seismic activity, then fell almost completely quiet immediately after the big event. This pattern, which was repeated across two different sections of the fault 12 years apart, suggests a common physical mechanism at work.

The team concluded that the barriers are not just inert stretches of rock. They are structurally complex areas where the fault splits into multiple branches, with small lateral displacements of 100 to 400 meters between them, creating areas of local extension, like a slight gap in a continuous fault.

This geometry, combined with evidence of seawater seeping deep into the rift, promotes a process scientists call “enhancing expansion.” That is, when a large seismic rupture reaches the barrier, the sudden movement causes the fluid-saturated porous rock to temporarily trap the pore pressure, which is the pressure of the fluids trapped within the rock, to drop sharply, effectively closing the brakes on the rupture before it grows larger.

“These barriers are not just negative features of the landscape,” said Gong, who is now an assistant professor of earth and atmospheric sciences in the College of Arts and Sciences at Indiana University Bloomington. “They are active, dynamic parts of the fault system, and understanding how they work changes the way we think about the limits of earthquakes on these faults.”

The Govar Fault is located far from populated coasts, so its earthquakes do not pose a significant direct danger to people. But the findings have implications far beyond this distant rift system.

Transform faults, like the Govar Fault, are found throughout the world’s ocean floors, and are responsible for a puzzling feature that scientists have long noted in global earthquake records. Large underwater earthquakes tend to remain smaller than geological conditions allow, as if some feature or process continually places a ceiling on their size.

The new findings suggest that buffer zones like those at Jóvar, formed by the same combination of complex fault geometry and seawater intrusion, may be widespread across the ocean floor, acting as a global system of natural brakes that limit the maximum magnitude of earthquakes along these boundaries.

This insight could help improve seismic models used to assess seismic hazards along underwater faults around the world, including those near coastal population centers.

The research was funded by the US National Science Foundation and the Natural Sciences and Engineering Research Council of Canada.

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