A MERMAID undergoes testing off Japan's coast in 2018.

ALEX BURKY/PRINCETON UNIVERSITY

These ocean floats can hear earthquakes, revealing mysterious structures deep inside Earth

A versatile, low-cost way to study Earth's interior from sea has yielded its first images and is scaling up. By deploying hydrophones inside neutrally buoyant floats that drift through the deep ocean, seismologists are detecting earthquakes that occur below the sea floor and using the signals to peer inside Earth in places where data have been lacking.

In February, researchers reported that nine of these floats near Ecuador's Galápagos Islands had helped trace a mantle plume—a column of hot rock rising from deep below the islands. Now, 18 floats searching for plumes under Tahiti have also recorded earthquakes, the team reported last week at the European Geosciences Union (EGU) meeting here. "It seems they've made a lot of progress," says Barbara Romanowicz, a geophysicist at the University of California, Berkeley.

The South Pacific fleet will grow this summer, says Frederik Simons, a seismologist at Princeton University who helped develop the floats, called MERMAIDs (mobile earthquake recorders in marine areas by independent divers). He envisions a global flotilla of thousands of these wandering devices, which could also be used to detect the sound of rain or whales, or outfitted with other environmental or biological sensors. "The goal is to instrument all the oceans."

For decades, geologists have placed seismometers on land to study how powerful, faraway earthquakes pass through Earth. Deep structures of different density, such as the cold slabs of ocean crust that sink into the mantle along subduction zones, can speed up or slow down seismic waves. By combining seismic information detected in various locations, researchers can map those structures, much like 3D x-ray scans of the human body. Upwelling plumes and other giant structures under the oceans are more mysterious, however. The reason is simple: There are far fewer seismometers on the ocean floor.

Such instruments are expensive because they must be deployed and retrieved by research vessels. And sometimes they fail to surface after yearlong campaigns. More recently, scientists have begun to use fiber optic communication cables on the sea floor to detect quakes, but the approach is in its infancy.

MERMAIDs are a cheap alternative. They drift at a depth of about 1500 meters, which minimizes background noise and lessens the energy needed for periodic ascents to transmit fresh data. Whenever a MERMAID's hydrophone picks up a strong sound pulse, its computer evaluates whether that pressure wave likely originated from seafloor shaking. If so, the MERMAID surfaces within a few hours and sends the seismogram via satellite.

The nine floats released near the Galápagos in 2014 gathered 719 seismograms in 2 years before their batteries ran out. Background noise, such as wind and rain at the ocean surface, drowned out some of the seismograms. But 80% were helpful in imaging a mantle plume some 300 kilometers wide and 1900 kilometers deep, the team described in February in Scientific Reports. The widely dispersed MERMAIDs sharpened the picture, compared with studies done with seismometers on the islands and in South America. "The paper demonstrates the potential of the methodology, but I think they need to figure out how to beat down the noise a little more," Romanowicz says.

Since that campaign, the MERMAID design was reworked by research engineer Yann Hello of Geoazur, a geoscience lab in Sophia Antipolis, France. He made them spherical and stronger, and tripled battery life. The floats now cost about $40,000, plus about $50 per month to transmit data. "The MERMAIDs are filling a need for a fairly inexpensive, flexible device" to monitor the oceans, says Martin Mai, a geophysicist at King Abdullah University of Science and Technology in Thuwal, Saudi Arabia.

Between June and September of 2018, 18 of these new MERMAIDs were scattered around Tahiti to explore the Pacific Superswell, an expanse of oddly elevated ocean crust, likely inflated by plumes. The plan is to illuminate this plumbing and find out whether multiple plumes stem from a single deep source. "It's a pretty natural target," says Catherine Rychert, a seismologist at the University of Southampton in the United Kingdom. "You'd need a lot of ocean bottom seismometers, a lot of ships, so having floats out there makes sense."

So far, the MERMAIDs have identified 258 earthquakes, Joel Simon, a graduate student at Princeton, told the EGU meeting. About 90% of those have also been detected by other seismometers around the world—an indication that the hydrophones are detecting informative earthquakes. Simon has also identified some shear waves, or S-waves, which arrive after the initial pressure waves of a quake and can provide clues to the mantle's composition and temperature. "We never set out to get S-waves," he said. "This is incredible." S-waves can't travel through water, so they are converted to pressure waves at the sea floor, which saps their energy and makes them hard to identify.

In August, 28 more MERMAIDS will join the South Pacific fleet, two dozen of them bought by the Southern University of Science and Technology in Shenzhen, China. Heiner Igel, a geophysicist at Ludwig Maximilian University in Munich, Germany, cheers the expansion. "I would say drop them all over the oceans," he says.