Scientists have just mapped the volcanic lower region of Hawaii in amazing detail

For decades, a mysterious swarm of earthquakes has rocked beneath the small town of Pahala near the southern coast of the island of Hawaii. By 2015, the rate of earthquakes had increased from about seven to 34 earthquakes per week. And in the year since the 2018 eruption of Kilauea – the largest Hawaiian eruption in centuries – the earthquakes have reached a feverish intensity.

Nearly 500 earthquakes rock below Pahala each week, and the increased activity has not abated. “We’re like an earthquake in the middle of the area here,” says Lou Daniele, general manager at Ka’u Coffee Mill in Pahala. “It just became a constant part of everyday life.”

Now scientists have discovered the source of this geological tiff: a mound of interconnected features about 22 to 26 miles underground that slowly swells with molten rock. As magma impulses permeate pancake-shaped structures, known as sills, a chain of earthquakes rolls along them. These pulsating rock roots may provide a channel that leads molten rock toward Kilauea and Mauna Loa, two of the largest and most active volcanoes in the world.

says John Wilding, a graduate researcher at the California Institute of Technology and lead author of a new study describing the geological features of the science. “Nobody has ever directly observed magma activity of this magnitude before.”

The researchers used machine learning algorithms to search for earthquakes in seismic data from the network of sensors at the Hawaii Volcano Observatory, picking out tremors so small that previous methods missed. The result is a stunningly detailed picture of Hawaii’s fiery underworld, which promises to help scientists sort out the geological processes that drive the island’s volcanoes.

“This is potentially the future of volcanology,” says Matt Burgess, a former Hawaiian seismologist who has studied deep earthquakes below Pahala.

A mysterious rumble from the depths

The Pahala earthquake swarm has been roaring since at least 1970. Earthquakes occur in the mantle, the layer of our planet between the crust and core, and most are too small and deep to move the surface with much force. Instead, the tremor feels more like the rolling or swaying of the ground. Sometimes, Daniele of Ka’u Coffee realizes something is wrong because ripples appear on the surface of his coffee. But in recent years, the rattles under Pahala have become relentless.

“Seismicity has been continuing to rise,” says Ninfa Pennington, a volcanic seismologist at the USGS Volcano Observatory in Hawaii, who has been tracking the recent spike in activity.

Pahala is believed to sit atop a scorching rock column, called a hotspot, that built the Hawaiian Islands. As the Pacific Plate moves 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 volcanoes created by the hotspot, most of them silent and now hidden beneath the waves.

Previous studies have identified potential sources of magma below the seismic swarm and suggested that an upward pulse of magma could be driving the deep crackles. Other studies have detailed shallow volcanic plumbing. But exactly how molten rock flows out from the depths of the mantle is unknown.

“We’re basically missing out on this big hunk,” says Pennington.

The earthquake swarm had an opportunity to get a closer look at the fiery bottom of the island. While earthquakes can come from many sources, magma or fluids moving through cracks generates the telltale seismic rumble. As the molten rock shifts, it can pressurize nearby ground, causing it to crack and shift, which the scientists can also detect in the earthquake data.

By plotting all of these earthquakes in three dimensions—a bit like a geological drip—scientists have now mapped a network of subterranean structures where magma might flow toward the surface, charging volcanic eruptions.

Seismic treasure trove

Amid increased earthquake activity in Hawaii, Wilding joined geophysicist Zachary Ross’s research group at Caltech. Ross has been developing methods for detecting earthquakes using machine learning algorithms, which can pick up surprisingly small earthquakes and give stunning spider-web views of fault zones underground.

The team applied these methods to 3.5 years of seismic data in Hawaii, recorded between 2019 and 2022. The system identified nearly 200,000 earthquakes from the swarm, illuminating sill structures stacked in the upper mantle. Even the extreme detail has allowed scientists to track magma as it flows into a sill, triggering a series of earthquakes.

When Ross first saw the details of geological structures on a computer screen, he was dumbfounded. “He was kind of like, ‘Oh my God, what are we looking at here?'” He says. “It’s just shocking.”

He describes the thresholds complex as a “gateway to the system,” providing a means for moving magma horizontally away from the region below Pahala. These underground features do not contain empty space, and instead represent a weak area in the rock where magma has intruded and spread out as a molten plate. The complex is associated with an area of ​​fractures leading to Kilauea as well as an area the team believes is associated with Mauna Loa.

There may be more than one path that the molten rock follows to the surface, Ross says. The sills are expected to be part of a broader layer of sub-island structures transporting magma to the various volcanic peaks.

The timing of deep earthquakes is another hint that the sill structures are associated with volcanoes at the surface. On the same day that Kilauea erupted in 2020, earthquakes were building up. A similar volcanic eruption and spike in deep-seismic earthquakes occurred in 2021, and Wilding says those eruptions may have created pressure in the water pipes of the shale, drawing more molten rock from the depths.

Magma highways

The latest study opens new windows into the fiery depths of our planet, teasing scientists about what might happen next. “I was amazed by the amazing richness of the new earthquake catalog they have developed and the amount of detail it shows,” says volcanologist Diana Roman of Carnegie Science, who was not part of the study team. “I want more.”

She and the other researchers are eager for the team to extend the catalog to 2015 or earlier to get a more detailed look at the system and its series of fiery spells. The longer catalog may also help explain the increase in earthquakes in 2019, which hit after the Kilauea eruption the year before an outflow of about 200 billion gallons of lava.

Whether nearby volcanoes are directly tapping into magma at the thresholds, Roman notes, remains unknown. She and Burgess published a study last year that suggested an indirect link between the eruptions and deep earthquake swarms. Swelling magma reservoirs deep in Pahala can compress nearby channels of magma that lead to Mauna Loa and Kilauea, like squeezing a tube of toothpaste. Or, says Roman, both processes could play a role.

This summer, Pennington and her colleagues plan to deploy an extensive network of seismic sensors across Kilauea to fill in the subsurface picture even further. She says the new study has made her especially eager to look for signs of magma in the proposed pathways that connect the rapids to nearby volcanoes.

“Each study puts a new piece into the … puzzle,” she says. “They add something really cool here.”

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