For decades, a mysterious swarm of earthquakes has rumbled beneath the small town of Pahala near the south coast of the island of Hawaii. By 2015, the rate of subterranean tremors had increased from about seven to 34 earthquakes per week. And the year after Kilauea’s 2018 eruption, the largest Hawaii has seen in centuries, earthquakes reached a feverish climax.
Nearly 500 earthquakes have rocked Pahala each week, and the increased activity has not stopped. “We are like a central earthquake here,” says Lou Daniele, general manager of Ka’u Coffee Mill in Pahala. “It just became a constant part of everyday life.”
Scientists have now discovered the source of this geological ruckus: a pile of interconnected features about 22 to 26 miles underground is slowly swelling with molten rock. As pulses of magma enter the pancake-like structures, called sills, a cascade of earthquakes rumbles along their length. These pulsating magmatic roots may even provide a conduit that brings molten rock to Kilauea and Mauna Loa, two of the largest and most active volcanoes in the world.
“We panicked,” says John WildingCalifornia Institute of Technology graduate researcher and lead author of a new study describing the geological features Science. “No one had ever directly observed magmatic activity on this scale before.”
Researchers used machine learning algorithms to search for earthquakes in seismic data from the Hawaii Volcano Observatory array of sensors, detecting tremors so small that previous methods missed them. The result is an incredibly detailed portrait of Hawaii’s fiery underworld that promises to help scientists sort out the geological processes that drive the island’s volcanoes.
“This will probably be the future of volcano science,” says Matt Burgessa former seismic analyst in Hawaii who has studied deep earthquakes under Pahala.
Mysterious rumblings from the depths
The Pahala earthquake swarm has been rumbling since at least 1970. Earthquakes are located in the mantle, the layer of our planet between the crust and the core, and most of them are too small and too deep to shake the surface with great force. Instead, the tremors are more like the ground rolling or swaying. Sometimes Daniele from Ka’u Coffee doesn’t realize something is wrong because ripples appear on the surface of her coffee. But in recent years, rattles under Pahala have become relentless.
“The seismicity just kept going up and up,” says Ninfa Benningtona USGS Hawaiian Volcano Observatory volcano seismologist who has been tracking the recent increase in activity.
Pahala is believed to sit above the head of a column of hot rock, called a hotspot, which built the Hawaiian Islands. As the Pacific plate moves through the stationary hotspot, new volcanoes – and possibly new islands – are being born. The 15 volcanoes of the Hawaiian Islands are the youngest in a chain of over 129 that the hotspot created, most of which have fallen silent and are now hidden under the waves.
Previous studies have identified probable sources of molten rock under the swarm of earthquakes and suggested that an upward pulse of magma could drive the deep rattles. Other studies have detailed shallow volcanic plumbing. But it’s unclear exactly how the molten rock rises from the depths of the mantle.
“We’re basically missing that big chunk,” Bennington says.
The swarm of earthquakes provided an opportunity to take a closer look at the fiery underbelly of the island. While earthquakes can originate from many sources, magma or fluid moving through fissures generates telltale seismic rumbles. And as molten rock moves, it can put pressure on nearby ground, causing it to crack and shift, which scientists can also spot in earthquake data.
By plotting all these earthquakes in three dimensions – much like geological pointillism – scientists have now sketched out a network of subterranean structures where magma can flow to the surface, charging volcanic eruptions.
An earthquake treasure
Amid increased seismic activity in Hawaii, Wilding joined the geophysicist by Zachary Ross research group at Caltech. Ross had developed methods that detect earthquakes using machine learning algorithms, which can detect surprisingly small earthquakes and give unobstructed views of the spider’s web of underground fault zones.
The team applied these methods to 3.5 years of Hawaiian seismic data, recorded between 2019 and 2022. The system identified nearly 200,000 earthquakes from the swarm, illuminating stacked sill structures in the upper mantle. The extreme detail even allowed scientists to track magma as it flowed down a sill, setting off a cascade of earthquakes.
When Ross first saw the detail of the geological structures on his computer screen, he was stunned. “It was kind of like, oh my God, what are we looking at here?” he says. “It’s just shocking.”
He describes the sill complex as the “gateway to the system”, providing a means to transport magma horizontally away from the area below Pahala. These subterranean features do not contain empty space, but rather represent a weak area in the rock where magma entered and spread as a molten sheet. The complex is connected to a rupture zone which leads to Kilauea as well as an area the team says is tied to Mauna Loa.
There may be more than one route that the molten rock follows to the surface, Ross says. He speculates that the sills may even be part of a larger layer of structures beneath the island that transport magma to the various volcanic peaks.
The timing of deep earthquakes is another clue that sill structures are connected to volcanoes on the surface. On the very day Kilauea erupted in 2020, the sills rumbled with earthquakes. A similar eruption and spike in deep earthquakes occurred in 2021, and Wilding says those eruptions may have released pressure in the magma plumbing, drawing in more molten rock from deep below.
magmatic highways
The latest study opens new windows into the fiery depths of our planet, exciting scientists about what might happen next. “I was blown away by the incredible richness of the new catalog of earthquakes they have developed and the amount of detail it shows,” says the 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 to 2015 or earlier for even more detailed insight into the system and its series of fiery attacks. A longer catalog could also help explain the 2019 earthquake surge, which hit after Kilauea erupted the previous year. 200 billion gallons of lava.
Roman notes that whether nearby volcanoes directly mine magma in the sills remains unknown. She and Burgess published a study last year this suggests an indirect link between eruptions and deep earthquake swarms. Inflation of magma reservoirs deep below Pahala could compress nearby channels of molten rock that lead to Mauna Loa and Kilauea, like squeezing a tube of toothpaste. Or, says Roman, both processes could be at play.
This summer, Bennington and his colleagues plan to deploy a large array of seismic sensors across Kilauea to further complete the picture underground. She says the new study has made her particularly keen to look for signs of magma in proposed pathways connecting sills and nearby volcanoes.
“Each study puts a new piece in … the puzzle,” she says. “They add something really amazing here.”
