Submerged mountains help soften ‘slow slip’ earthquakes | Sciences

In 2001, geoscientists reported an entirely new type of subduction zone earthquake, the seam where a tectonic plate of ocean crust sinks beneath the continent. It was previously thought that subduction zones behave in one of two ways: either creep steadily and smoothly, with no tremors at all, or stick around for decades or centuries and then catastrophically explode in the world’s largest earthquakes. But geoscientists now know that subduction zones often take a middle course: GPS sensors have shown that they can slide in quiet, almost imperceptible earthquakes that last for weeks or months. What they didn’t know was why.

Evidence from the Hikurangi subduction zone in New Zealand now suggests that “slow slide” events often depend on seamounts, underwater volcanoes that stud the seafloor in droves. You might expect a mountain swallowed by mistake to be a sticking point. But it turns out that many seamounts provide lipids, says Nathan Pang, a marine geophysicist at the University of Texas (UT) at Austin and lead author of a new 3-D survey of Hikurangi, published earlier this month in Nature Geoscience. “A lot of intuition doesn’t seem to apply here,” he says. “You think he’s doing one thing, and he’s doing another.”

Slow sliding events aren’t just an academic curiosity, adds Laura Wallace, a geodesictist at the University of Austin in Austin. “Understanding where slow slip occurs versus where fault closures are important for assessing earthquake risk,” she says. Even more mysteriously, two recent subduction zone earthquakes, including the giant Tohoku earthquake in Japan in 2011, were preceded by—and possibly caused by—slow slip events. But the relationships are ambiguous. “We’re very confused about this issue,” says Emily Brodsky, an earthquake physicist at the University of California, Santa Cruz.

The Hikurangi subduction zone, which plunges beneath the east coast of New Zealand’s North Island along a trench into the seafloor, is a hot spot for slow-slide research because the motion occurs in a shallow area only a few kilometers from the sea floor. In 2018, the Marcus Langseth, a US seismic imaging vessel, mapped part of the fault with a degree of detail that provided “an unprecedented look at the subduction zone,” Pang says.

Langseth traversed the area, pulling long strings from the water to capture the reflections from nearly 150,000 air rifle blasts, which pierce the water and bounce off layers of sediment and rock beneath the sea floor. In addition, Japanese scientists covered the sea floor with 97 ocean floor seismometers. The campaign generated a torrent of data, one that took the contractor more than a year to piece together. When the results first came out at the 2022 conference, “People’s jaws were on the floor,” Wallace said.

The 3D images revealed a two-kilometer-high seamount wedged into the subduction zone, 4.5 kilometers below the sea floor. “We caught her in the act of bingeing,” Bangs says. “We can see the structures you create and how they are internalized.” It was not only the seamount itself, but also the shadow it cast. Like a bulletproof runner before a run, the seamount protected a pile of water-soaked seafloor sediment about 20 kilometers long, preventing it from being swept away from the overlying plate. That water can lubricate parts of plate boundaries at depths that you wouldn’t be able to reach, Kristin Chesley, a marine geophysicist at the Woods Hole Oceanographic Institution, says, preventing complete subduction.

The sediment shadow is a “massive result,” Chesley says. But it is not the only way for seamounts to smuggle water into the depths. Chesley participated in electromagnetic surveys of Hikurangi in 2018 and 2019. The team’s array of seafloor electrodes, which are sensitive to water conductivity, captured a flexible seamount that appears to have a conductive interior covered in a thin resistive shell. The team concluded that this indicated that the volcanic rocks within the seamount were also rich in water, confined by an impermeable mantle. “This stuff is wet and weak,” says Aki Fagering, a geologist at Cardiff University.

Perhaps more important than the water carried by seamounts is the slippery sediment that falls off them as they slowly erode over millions of years. In 2018, Wallace co-led a marine expedition to Hikurangi, which pulled up a lot of mud deposits rich in the mineral smectite, often used as a lubricant in drilling mud by the oil and gas industry. In lab experiments aimed at recreating the pressure and heat of a subduction zone, these clays not only become hard enough to snap and rupture in large earthquakes, but they can host slow slip, Wallace and her co-authors found in a paper published earlier this year in Science.

It remains to be seen whether seamounts play a similar role in other subduction zones. Many of the Hikurangi seamounts were previously exposed above sea level, which could lead to more erosion and more mud formation. The shallow subduction zone may also encourage slow sliding in a way that deeper regions do not, Fagering says.

This month, American scientists are visiting colleagues in Chile to discuss a planned collaboration, called SZ4D, that would study slow-slide events in the Chilean subduction zone. But this project, if funded, is still years away. In the meantime, the Bangs’ 3D seismic data is now open for other scientists to use, and sensors in the wells drilled at Hikurangi have picked up several new slow slip events. Bangs says more ideas are coming. “It will really open our eyes to what’s going on here.”

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