Swell mission to sort out unusual earthquake patterns in New Zealand

The research ship swayed in the southwest Pacific like a toy boat in a bathtub. Three scientists watched numbers roll across the screen as minutes ticked in another 12-hour shift. The instruments collecting this raw data were published days ago in a line on the ocean floor. Now, the ship was towing a fish-shaped transmitter just above the sea floor to pick up the signals of the distant devices and send them back to the ship. Things were going well. So they weren’t.

“When you are at sea, you pray that everything is going well. It was the researchers’ job to collect data that might answer a key question about a seismic zone,” says Christine Chesley, a marine geophysicist at Columbia University who is part of the science team aboard the expedition. An unusual called the Hikurangi Fringe: In this relatively small region, why are some earthquakes huge, but others so sneaky that only the most sensitive instruments detect them?Scientists may discover that the answer seems to be a surprising source of lubrication for the planet’s mossy plate tectonics.

But first, the team needed the data. The sea floor off the east coast of New Zealand’s North Island, like most of the world’s ocean floor, is poorly planned. Which is anything but flat. Rugged peaks and dramatic canyons hide under thousands of feet of water. The trio who were watching the screen saw the numbers changing, indicating a sudden shift in the features of the sea floor. Their transmitter was approaching something enormous. They sounded the alarm: the transmitter had to be pulled out to avoid hitting something – a steep cliff signaled an undersea mountain no one knew existed. This meant pulling on the cable that was pulling a piece of expensive equipment. The cable was about three miles long.

The science team deployed a shark-shaped transmitter, towed behind the boat by about three miles of cable, to capture data from remote receivers on the sea floor. Samer Nayef

“You can really only pull the transmitter out that fast,” says Georgia Tech marine geophysicist Samer Naif, the mission’s chief scientist. “When the seas get a little rougher, and you start getting close to the tension line for what the cable is designed for, it can get really scary.”

“You scramble, I just hope you succeed. We missed it by about 10 or 15 metres,” he adds, his voice still strained, as he recalls how the team barely avoided smashing expensive equipment down an uncharted cliff.

The incident, says Naif, was one of the “hairiest” moments during the 29-day expedition, though the crew also encountered some of the area’s infamous weather. “It was never dangerous, but it definitely got exciting,” says Naif. “Over the course of a month, people learn what kind of bloating is the maximum for their function.”

Marine geophysicists from around the world challenge these waters because of the Hikurangi Fringe: Beneath the bulging and treacherous seafloor mountains, the Pacific plate sinks beneath the Australian plate, creating New Zealand’s largest subduction zone. The southern region of Hikurangi is capable of unleashing massive earthquakes — “the ones that make the news,” Chesley says, adding, “Those are the ones that geophysicists and geologists have taken care of for a long time.” But in the north, there is a different kind of seismic activity, one that is still relatively unstudied.

Over the course of about a month, the team deployed and retrieved remote receivers, which were sitting on the ocean floor to collect data on the electromagnetic properties of the rock below. Samer Nayef

“In the past 20 years, there’s been a whole new class of earthquakes that have been discovered called slow-slip events,” says Chesley, who is doing her PhD at the Lamont-Doherty Earth Observatory in Columbia. She’s the lead author of a research paper, recently published in Nature, looking at these “silent” earthquakes in Hikurangi. If the Southern Hikurangi quakes are a pot boiling over your stove, the Northern Slope events are a tea kettle that gently releases steam so as not to whistle.

Naif, one of the paper’s co-authors, says slow-slide events “accumulate every few years and release energy slowly. This actually reduces potential risks: Instead of releasing that energy in seconds, it’s releasing it over weeks.”

Based on imaging data from their expedition, Chesley and her colleagues have found a possible explanation for the North’s sneaky Hikurangi tremors: a seamount. These extinct underwater volcanoes are scattered on the sea floor around the world, but geophysicists haven’t figured out what happens when the plate they’re sitting on is pulled up from under another plate during subduction.

Waves of the Southwest Pacific Ocean occasionally hit the research ship during the month-long expedition. Keri K / Courtesy of Samer Nayef

Some scientists believe that the force of the massive conveyor belt-like subduction process crushes or shears the seamount. Others suspect that the seamount, being pulled from below, is strong enough to damage the plate above it, reducing its ability to gather enough energy to trigger a major earthquake. Instead, the weak plate releases energy in smaller seismic events, which is what the team found. “The ideas were there,” says Naif. “We were only able to film it for the first time and give it some credibility.”

However, what the team was surprised to discover was one of the ways in which the seamount appears to weaken the upper plate and create a slow-slip event: the seamount has been loaded with water. The fluid, stored deep in the rock, acts like a lubricant and reduces friction as the entire mass has been lowered.

“It was one of the most amazing and amazing results,” Chesley says. The team was able to compare their data with seismic records from a slow-slip event in 2014 and found that the silent quakes occurred in the same place where imaging suggested the water-filled boulders had collapsed.

“These silent earthquakes seem to be affected a lot by the amount of water you can add to the slip system, and these seamounts seem to affect the amount of water that is transferred into that system,” Chesley says.

The east coast of New Zealand’s North Island is a dynamic place above and below the waves. Samer Nayef

University of Southampton geophysicist Kate Reichert, who wrote a comment in Nature about the new paper but was not involved in the research, says the results are not definitive, and that several alternative scenarios could explain the difference between the southern Hikurangi’s violent tremors and the silent northern tremor. . But she adds, “It’s an important step.” She says the team’s approach, which combines seismic and electromagnetic data, is of particular interest. “This kind of approach, given two independent types of observation, is a powerful way to increase our understanding,” she says.

Chesley and colleagues will continue to process data from their expedition, which included surveys of areas along the entire Hikurangi margin. They hope to uncover additional evidence to explain unusual earthquake patterns that may eventually lead to improved risk assessments for subduction zones outside New Zealand. As Naif points out: “The vast majority of people live along coasts, and a lot of these areas are subduction zones.”

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