In 2001, geoscientists reported a completely new kind of earthquake at a subduction zone, a seam where a tectonic plate of ocean crust dives under a continent. Subduction zones were previously thought to behave in one of two ways: either creeping along steadily and smoothly, without any tremors at all, or sticking for decades or centuries and then rupturing catastrophically in the world’s largest earthquakes. But geoscientists now know subduction zones often take a middle path: GPS sensors have shown they can slip in quiet, nearly imperceptible earthquakes that last weeks or months. What they haven’t known is why.
Evidence from the Hikurangi subduction zone in New Zealand now suggests these “slow slip” events often depend on seamounts, the underwater volcanoes that stud the sea floor in large numbers. You might expect a mountain swallowed up by a fault to act as a sticking point. But it turns out that many seamounts provide the grease, says Nathan Bangs, a marine geophysicist at the University of Texas (UT) at Austin and lead author of a new 3D survey of Hikurangi, published earlier this month in Nature Geoscience. “A lot of the intuition doesn’t seem to be applying here,” he says. “You think it’s doing one thing, and it’s doing something else.”
Slow slip events are not simply an academic curiosity, adds Laura Wallace, a UT Austin geodesist. “Understanding where slow slip is happening versus where faults are locked up is important for seismic hazard assessments,” she says. And more mysteriously, two recent subduction zone earthquakes, including the giant 2011 Tohoku quake in Japan, were preceded by slow slip events—and perhaps triggered by them. But the relationships are murky. “We are pretty confused about this issue,” says Emily Brodsky, an earthquake physicist at the University of California, Santa Cruz.
The Hikurangi subduction zone, which dives under the east coast of New Zealand’s north island along a seafloor trench, is a hot spot for slow slip research because the motion occurs in a shallow zone just a few kilometers below the sea floor. In 2018, the Marcus Langseth, a U.S. seismic imaging ship, mapped part of the fault with a degree of detail that provided “an unprecedented look at a subduction zone,” Bangs says.
The Langseth crisscrossed the region, towing long strings of hydrophones to catch reflections from nearly 150,000 airgun blasts, which penetrate the water and bounce off layers of sediment and rock under the sea floor. In addition, Japanese scientists covered the sea floor with 97 ocean-bottom seismometers. The campaign produced a torrent of data, which took more than a year for a contractor to stitch together. When the results debuted at a conference in 2022, “people’s jaws were on the floor,” Wallace says.
The 3D images revealed a 2-kilometer-tall seamount wedged in the subduction zone, 4.5 kilometers below the sea floor. “We caught it in the act of subducting,” Bangs says. “We can see the structures it’s creating and how it’s being accommodated.” Notable was not only the seamount itself, but also the shadow it cast. Like a lead blocker ahead of a running back, the seamount shielded a pile of water-logged seafloor sediments nearly 20 kilometers long, keeping it from being scoured away by the top plate. This water can lubricate parts of the plate boundary at depths it would otherwise be unable to reach, preventing a full lock up, says Christine Chesley, a marine geophysicist at the Woods Hole Oceanographic Institution.
The sediment shadow is a “huge result,” Chesley says. But it’s not the only way for seamounts to smuggle water down to the depths. Chesley participated in an electromagnetic survey of Hikurangi in 2018 and 2019. The team’s array of seafloor electrodes, sensitive to the conductivity of water, captured a subducting seamount that appeared to have a conductive interior covered by a thin resistive crust. That suggested the volcanic rocks inside the seamount were also rich in water, trapped by an impermeable cap, the team concluded. “That stuff is wet and weak,” says Ake Fagereng, a geologist at Cardiff University.
Perhaps even more important than the water carried by seamounts are the slick sediments they shed as they slowly erode over millions of years. In 2018, Wallace co-led an offshore drilling expedition to Hikurangi, which pulled up a lot of clay sediments rich in the mineral smectite, often used by the oil and gas industry as a lubricant in drilling muds. In lab experiments meant to recreate the pressure and heat of the subduction zone, these clays don’t grow stiff enough to lock and rupture in large earthquakes, but could host slow slip, Wallace and her co-authors found in a paper published earlier this year in Science.
Whether seamounts play a similar role in other subduction zones remains to be seen. Many of Hikurangi’s seamounts were once exposed above sea level, which may have led to more erosion and the creation of more clays. The shallow subduction zone might also encourage slow slip in a way that deeper zones do not, Fagereng says.
This month, U.S. scientists are visiting colleagues in Chile to discuss a planned collaboration, called SZ4D, that would study slow slip events in the Chilean subduction zone. But that project, if funded, is still years away. In the meantime, Bangs’s 3D seismic data are now open for other scientists to use, and sensors in the boreholes drilled at Hikurangi have captured several new slow slip events. More insights are coming, Bangs says. “It will really open our eyes to what is happening here.”
