‘The tick-tick quake’: Scientists crack the code of the world’s most frequent earthquakes in the Pacific
Deep beneath the eastern Pacific Ocean, roughly a thousand miles off the coast of Ecuador, the seafloor has been keeping time. Every five to six years, in almost the same locations, at almost the same intensity, a magnitude 6 earthquake strikes with a regularity so precise that scientists reached for the word “clockwork” to describe it. That pattern has been documented for at least three decades. It is, by the standards of earthquake science, extraordinary. Most faults are chaotic systems the timing, location, and size of their ruptures resist prediction in almost every meaningful way. The Gofar transform fault, by contrast, behaves more like a metronome than a geological feature. And yet, until now, nobody could fully explain why.A new study published in the journal Science has finally supplied the answer. Led by seismologist Jianhua Gong of Indiana University Bloomington, and drawing on collaborators from the Woods Hole Oceanographic Institution, Scripps Institution of Oceanography, the U.S. Geological Survey, and several other institutions, the research identifies the precise physical mechanism that both drives the regularity and
A fault that keeps its own schedule
The Gofar fault is a transform fault, a boundary where two tectonic plates slide horizontally past each other rather than colliding or pulling apart. It sits along the East Pacific Rise, one of the most geologically active spreading zones on Earth. The Pacific and Nazca plates grind past each other here at about 140 millimetres per year, roughly the rate at which a fingernail grows. It’s a slow, relentless accumulation of stress.What makes Gofar unusual is not just that it produces large earthquakes and transform faults do that regularly, but that its large earthquakes keep happening in the same spots, stopping at the same spots, and doing so on a remarkably consistent schedule. In between those earthquake zones lie stretches of fault that seem to absorb tectonic stress quietly, without generating major ruptures. Scientists have long called these stretches “barriers,” but the question of what actually made them work had lingered unanswered for decades.“These barriers are not just passive features of the landscape. They are active, dynamic parts of the fault system, and understanding how they work changes how we think about earthquake limits on these faults.” said Jianhua Gong, Lead Author, Indiana University Bloomington.
Two expeditions, tens of thousands of quakes
To get at the answer, the research team turned to an unusually rich dataset assembled over more than a decade of fieldwork. In 2008, scientists deployed ocean bottom seismometers, sensitive earthquake detectors placed directly on the seafloor along segments of the Gofar fault. They returned from 2019 to 2022 for a second major experiment, placing instruments along two separate fault segments and recording the seismic activity surrounding another major magnitude 6 event.The instruments captured tens of thousands of tiny earthquakes occurring in the weeks and months before and after each large rupture. The resulting picture of fault behaviour across two different segments, separated by twelve years, was extraordinarily detailed, and it revealed something striking: both barrier zones behaved in almost exactly the same way.In the days and weeks before each major earthquake, the barriers became intensely active, generating swarms of small quakes. Then, immediately after the big event struck, they went almost completely silent. The same pattern, repeated across space and time, pointed unmistakably to a single shared mechanism.The barrier zone sits between two seismically active fault segments. Its complex, multi-strand geometry and fluid-saturated rock create a “dilatancy strengthening” effect that arrests incoming ruptures.
Implications far beyond one remote fault
The Gofar fault sits far from any coastline, and its earthquakes pose no direct hazard to human populations. But the findings carry implications that extend well beyond this one remote stretch of Pacific seafloor.Transform faults exist across the world’s ocean floors, and seismologists have long puzzled over a curious feature of the global earthquake record: large underwater earthquakes on transform faults tend to stay smaller than the underlying geology would seem to allow. Something consistently appears to be capping their size. The Gofar study suggests that barrier zones like the ones discovered there, formed by the same combination of complex fault geometry and seawater infiltration, may be widespread across oceanic transform faults worldwide, acting as a global system of natural brakes that limit maximum earthquake size along these boundaries.It resolves a 30-year mystery in seismology about why oceanic transform fault earthquakes rarely exceed certain magnitudes.
