For decades, a mysterious swarm of earthquakes has rumbled beneath the small town of Pahala near the southern coast of the island of Hawaii. By 2015, the rate of subterranean trembles had ticked up from about seven to 34 quakes per week. And the year after Kilauea's 2018 eruption—the largest Hawaii has seen in centuries—the quakes reached a feverish pitch.
Nearly 500 earthquakes shook underneath Pahala every week, and the heightened activity hasn't let up. "We're like earthquake central down here," says Lou Daniele, general manager at Ka'u Coffee Mill in Pahala. "It's just become a constant part of daily life."
Now scientists have discovered the source of this geologic ruckus: a stack of interconnected features some 22 to 26 miles underground is slowly swelling with molten rock. As pulses of magma intrude into the pancake-shaped structures, known as sills, a cascade of earthquakes rumbles along their length. These pulsing magmatic roots may even provide a conduit that ushers molten rock toward Kilauea and Mauna Loa, two of the largest and most active volcanoes in the world.
"We were freaking out," says John Wilding, a graduate researcher at the California Institute of Technology and lead author of a new study describing the geologic features in Science. "No one had ever directly observed magmatic activity at this scale before."
The researchers used machine learning algorithms to search for earthquakes in seismic data from the Hawaii Volcano Observatory's network of sensors, picking out trembles so small that previous methods missed them. The result is a stunningly detailed portrait of Hawaii’s fiery underworld, which promises to help scientists sort through the geologic processes that drive the island’s volcanoes.
"This is probably going to be the future for volcano science," says Matt Burgess, a former seismic analyst in Hawaii who has studied the deep earthquakes below Pahala.
Mysterious rumbles from the deep
The Pahala earthquake swarm has been rumbling since at least 1970. The quakes are located in the mantle, the layer of our planet between the crust and core, and most of them are too small and deep to joggle the surface with much force. Instead, the trembles feel more like a rolling or swaying of the ground. Sometimes Ka'u Coffee's Daniele only realizes that something's amiss because ripples appear on the surface of his coffee. But in recent years the rattles beneath Pahala have become relentless.
"The seismicity was just continuing to go up and up," says Ninfa Bennington, a volcano seismologist with the USGS Hawaiian Volcano Observatory who has been tracking the recent surge in activity.
Pahala is thought to sit above the head of a searing column of rock, called a hotspot, that built the Hawaiian Islands. As the Pacific Plate shifts across the stationary hotspot, new volcanoes—and eventually new islands—are born. The 15 volcanoes of the Hawaiian islands are the youngest in a chain of more than 129 that the hotspot has created, most of which have fallen silent and are now hidden beneath the waves.
Past studies have identified likely sources of molten rock below the earthquake swarm and suggested that an upward pulse of magma could be driving the deep rattles. Other studies have detailed the shallow volcanic plumbing. But exactly how molten rock flows up from the depths of the mantle isn't known.
"We're essentially missing this big piece," Bennington says.
The swarm of earthquakes was a chance to get a closer look at the island's fiery underbelly. While earthquakes can come from many sources, magma or fluid moving through cracks generates telltale seismic rumbles. And as the molten rock shifts, it can stress the ground nearby, causing it to crack and shift, which scientists can also spot in the earthquake data.
By plotting all these quakes in three dimensions—a bit like geologic pointillism—scientists have now sketched out a web of subterranean structures where magma may flow toward the surface, charging volcanic eruptions.
A seismic treasure trove
Amid the surge of earthquake activity in Hawaii, Wilding joined geophysicist Zachary Ross's research group at Caltech. Ross had been developing methods that detect earthquakes using machine learning algorithms, which can pick out surprisingly small quakes and give stunning views into the spidery web of underground fault zones.
The team applied these methods to 3.5 years’ worth of Hawaiian seismic data, recorded between 2019 and 2022. The system identified nearly 200,000 earthquakes of the swarm, illuminating the stacked sill structures in the upper mantle. The extreme detail even allowed the scientists to track magma as it trickled into a sill, kicking off a cascade of quakes.
When Ross first saw the detail of the geologic structures on his computer screen, he was dumbfounded. "It was kind of just like, oh my God, what are we looking at here?" he says. "It's just shocking."
He describes the complex of sills as the "gateway into the system," providing a means to transport magma horizontally away from the area beneath Pahala. These underground features don't contain empty space, instead representing a weak zone in the rock where magma has intruded and spread as a molten sheet. The complex links up with a zone of fractures that leads to Kilauea as well as an area that the team believes is connected to Mauna Loa.
There may be more than one route that molten rock follows to the surface, Ross says. He speculates that the sills might even be part of a broader layer of structures under the island that shuttles magma to the different volcanic peaks.
The timing of the deep earthquakes is another hint that the sill structures are connected to volcanoes at the surface. The same day Kilauea erupted in 2020, the sills rumbled with quakes. A similar eruption and spike in deep earthquakes happened in 2021, and Wilding says these eruptions may have released pressure in the magmatic plumbing, drawing more molten rock up from the depths.
Magma highways
The latest study opens new windows into our planet's fiery depths, exciting scientists about what might come next. "My mind was just blown by the incredible richness of the new earthquake catalog that they developed and how much detail it shows," says volcanologist Diana Roman of Carnegie Science, who was not part of the study team. "I want more."
She and other researchers are eager for the team to extend the catalog back to 2015 or earlier for an even more detailed look at the system and its series of fiery fits. A longer catalog might also help explain the 2019 surge in earthquakes, which struck after Kilauea's eruption the year before piped out some 200 billion gallons of lava.
Roman notes that whether the nearby volcanoes directly tap the magma in the sills remains unknown. She and Burgess published a study last year that suggests an indirect link between eruptions and the swarms of deep quakes. The inflation of magma reservoirs deep under Pahala could compress nearby channels of molten rock that lead to Mauna Loa and Kilauea, like squeezing a tube of toothpaste. Or, Roman says, both processes could be at play.
This summer, Bennington and her colleagues plan to deploy an extensive network of seismic sensors across Kilauea to further fill in the subterranean picture. She says the new study has made her particularly keen to look for signs of magma in the proposed pathways connecting the sills and the nearby volcanoes.
"Each study puts a new piece in … the puzzle," she says. "They add something really amazing here."
