What triggered last year’s devastating eruption of Kilauea remains a mystery, but a new report by scientists who witnessed the historic event suggests a possible rupturing of a barrier within the volcano’s middle East Rift Zone may have opened the way for magma to reach populated areas in Lower Puna.

The three-month-long eruption that claimed more than 700 homes was the largest lower East Rift Zone eruption and caldera collapse in at least 200 years. Data collected during the event already is yielding new insights into caldera collapses, magma transport, small-scale explosive volcanism, high-rate lava discharges and other processes, according to the first scientific summary of the eruption, published Jan. 25 in Science magazine.

The report notes that in mid-March, tiltmeters at the Puu Oo vent on Kilauea’s south flank, which had been continuously erupting since 1983, began recording inflation caused by accumulating magma. Historically, such activity had resulted in the opening of new eruptive vents nearby. In the 2018 eruption, when Puu Oo collapsed April 30, highly pressurized magma traveled laterally downrift toward the lower East Rift Zone (LERZ) about 12.4 miles away.

It was the farthest intrusion of substantial magma into the LERZ since 1960, the report said.

Seismologist Brian Shiro, one of 31 Hawaiian Volcano Observatory members listed among the research article’s co-authors, said he suspects a magnitude-6.9 earthquake on May 4 caused a seaward slip of the flank that opened and stretched the rift zone, allowing the molten rock to flow more freely.

He recalled tracking the summit magma as it pushed through rock on its way to the LERZ.

“Earthquakes were one of the major indicators where magma was at any given point,” he said. “At that point we didn’t know for sure where there was going to be an eruption. But then the earthquakes stopped under Leilani Estates and, sure enough, the cracks and steam began.”

Shiro said he wonders if the eruption would have persisted as long had it not been for the earthquake.

Meanwhile, the caldera at Halemaumau Crater experienced 62 collapses between May and August, demonstrating the strong connection between the summit and the LERZ eruption, the report said. Both the caldera collapses and the vigorous fissure 8 flow abruptly stopped Aug. 4.

In a tidy finding, the scientists estimated the total lava output from the eruption was roughly equivalent to the volume of the caldera collapse — a little more than 0.8 cubic kilometers.

With major events happening both in Lower Puna and at Kilauea’s summit, the eruption was challenging for the scientists scrambling to keep up with it, according to HVO geophysicist Ingrid Johanson, another of the article’s co-authors.

“For me, the grand theme of the eruption was interconnection — in particular, the interconnection of the summit collapses and the erupting lava in the (LERZ), but also the interconnection of Puu Oo with the rift zone and of the magnitude-6.9 earthquake with the eruption,” she said.

“What’s exciting to me is how we can now broaden our view of the volcano and eventually understand how all these pieces fit together. We have many more puzzle pieces for the Kilauea system than we did a year ago.”

With the stress relief provided by the large earthquake and the summit drained of magma, the report said “it may take several years before enough magma can accumulate beneath the summit to erupt.”

Following a 1924 summit collapse that occurred under similar circumstances, there were no further eruptions outside Halemaumau for 28 years, the report noted. “If future activity at Kilauea follows a similar pattern, the next several years will see little, if any, significant eruptive activity.”

But then again, the report said, it’s possible the drop in magma at the summit may cause a pressure imbalance between the shallow and deeper parts of Kilauea’s plumbing system, drawing a higher rate of magma from the depths, “which could result in renewed eruptive activity sooner than expected.”

Fast and furious flows

Other findings in the article explain that while past eruptions have produced more lava, the high “effusion” rate — or rate of discharge — during last year’s event was sustained longer than in any other observed Kilauea eruption.

Lava poured out of fissure 8 in Leilani Estates at rates exceeding 100 cubic meters per second, feeding a fast-moving, channelized flow that emptied into the ocean at Kapoho more than 4 miles away.

The high effusion rate was due to a combination of factors, the report said, chiefly the pressure within the volcano’s plumbing system and the relatively low elevation of the eruption site, which meant gravity was at play, pulling magma downrift from the summit.

“Nobody knew that Kilauea could produce such fast-flowing lava,” Shiro said. “Past eruptions have seen lava oozing from slower eruptions. Here we’ve seen firsthand that’s not always the case.”

The clockwork timing of the 62 summit collapses was perhaps the most stunning observation from the 2018 eruption.

“The consistency of the collapse event timing between each event — about 30 hours on average — was surprising, as was the remarkably consistent energy release — about a magnitude-5.3 earthquake,” Shiro said. “It shows how well-connected the volcano’s rift plumbing system is to the summit magma reservoir and how constant the magma withdrawal rate must have been to produce this regular clock-like pattern of collapse in the summit caldera.

“We never really knew how calderas formed. Now we know they form in steps, not all at once.”

Shiro said he wants to further investigate the role the large earthquake had on the eruption, why the eruption halted so abruptly, what the underground magma is doing now and when might it refill the system enough for another eruption. He also is wondering what the events may mean for the Mauna Loa Volcano, which looms over residential areas developed since its last eruption in 1984.

Johanson, too, was intrigued by the sudden cessation of the lava flow and summit collapses. Over the past four years, she’s been studying the underground storage bodies at Kilauea and Mauna Loa with a view toward understanding how material moves along their shallow magma systems.

“I was not expecting it to quit as quickly as it did,” she said. “This observation is also one of the most exciting; it’s a very clear behavior that any future model of magma transport during the eruption will have to address. In that way, how the eruption ended may be the key to understanding how it kept going so long.”

New technology at work

The newly published research article makes much of how thorough study of Kilauea Volcano over the years allowed scientists to anticipate flow patterns and other activity during the 2018 eruption, and how newer technologies assisted in the latest observations.

These include unoccupied aircraft systems, commonly known as drones, infrasound arrays that measure sound in the air, inexpensive webcam networks and real-time global navigation satellite systems.

The infrasound arrays are “really good” at detecting explosions and new fissure eruptions, Shiro said.

“HVO had dabbled in the use of infrasound for the past few years, but it was the 2018 eruption that made us fully embrace it,” he said. “We installed a new infrasound array at Nanawale Estates and used it to monitor the LERZ eruption, and, thankfully, we had just rebuilt two arrays near the summit just a month before the eruption. This enabled us to set up alarms that could notify us when new events happened.”

For Johanson, the 2018 eruption provided her first real test of using global positioning satellites for hazard monitoring after 10 years of preparation. “This eruption was the first operational use of real-­time GPS for monitoring a volcanic eruption in the U.S., and they proved critical,” she said.

“They provided minute-by-­minute information on all aspects of (the eruption), including measuring the growth of the intrusion in the (LERZ), quickly measuring the amount of down-dropping in each summit collapse event, and rapidly estimating the geodetic size of the magnitude-6.9 earthquake. These were one of the workhorse data streams for me during the eruption, and it was incredibly gratifying to see a decade of preparation pay off.”

A computer upgrade in the months before the eruption also proved timely, according to Shiro.

“Thanks to those upgrades we were able to keep up with the earthquakes. If we had been on our older system it wouldn’t have been able to handle it,” he said.

“We were able to scale up really quickly to the huge number of earthquakes — more than 60,000. That’s the number we would normally get over three years and it happened in three months. Each one of those takes up memory and storage and overhead, and, really, the fact that we had the latest and greatest computer systems allowed us to stay on top of things as much as we did.”

Powerful personal impact

Beyond the data bonanza outlined in the scientific summary, both Shiro and Johanson said the 2018 eruption was likely a once-in-a-lifetime professional opportunity for them and a deeply personal experience. In mid-May, hazardous conditions forced HVO staff to evacuate their facility overlooking Halemaumau Crater. The damage was extensive, and it’s unclear when they might be able to return.

During the eruption, Shiro, now working from an office near Hilo Harbor, was responsible for locating earthquakes, determining their magnitudes, interpreting patterns, installing new seismic stations and communicating his findings with other agencies. He literally had a front-row seat to some of the action.

“Feeling the earthquakes of the first several collapses and seeing the plume eruptions right outside our window, the magnitude-6.9 earthquake and all the aftershocks, and riding those out every few minutes and ducking and running under our desks, and the books rattling on the bookshelves and stuff. … It was emotionally and physically exhausting, like a sprint that seemed to have no end,” he said.

“I remember sleeping in my office and working almost around the clock those first two weeks before we evacuated HVO. That evacuation led to a severe sense of loss for our former workplace.”

Johanson said she found it impossible to separate her work from the implications of the disaster for the residents of Lower Puna and Volcano village.

“I felt an incredible responsibility to perform these analyses quickly and get accurate results in the hands of the people that needed them,” she said. “Living up to that responsibility was really what drove me during the eruption and collapse sequences. My personal fear and sadness had to be channeled into making sure other people had the information they needed to make them feel less of their own fear or sadness.”

The report concludes that the long-term observations gathered during HVO’s century-long presence at Kilauea Volcano were critical to the successful monitoring of the 2018 eruption and forecasting some of the hazardous activity.