Prolonged rainfall triggered Kilauea eruption, study finds

It was excessive and prolonged rainfall that triggered the 2018 eruption of Kilauea Volcano, according to new research due out today.

The study, published in the journal Nature, says nearly 90 inches of rain fell on Kilauea in the first quarter of 2018, leaving the volcano’s subsurface rock weakened and more vulnerable to eruption.

“Under pressure from magma, wet rock breaks easier than dry rock. It is as simple as that,” said Falk Amelung, professor of geophysics at the University of Miami Rosenstiel School and co-author of the study along with UM researcher Jamie Farquharson.

But researchers at the Hawaiian Volcano Observatory say it isn’t quite as simple as that. They say Kilauea’s magma system was showing signs of increasing pressurization for weeks and that the volcano likely blew its top without any outside triggering mechanism.

“It’s a much more complicated system,” said Ingrid Johanson, HVO research physicist.

Johanson said a group of researchers from the observatory is working on a formal response to the study and plans to submit it to the journal Nature following peer review.

In addition, Thursday’s Volcano Watch column, published on the HVO website each week, will respond to the study’s conclusions.

Amelung admitted that the study did not report there was between 2 and 3 centimeters of inflation at the summit in the two weeks prior to the eruption. He said a correction has been submitted and will be printed in the journal.

“It was such a short time period, and we just missed it,” he said.

But that fact, he said, doesn’t change the conclusion of the study: that heavy and prolong rainfall likely triggered the eruption.

“It rained for months prior to the eruption,” he said. “If it hadn’t have happened, who knows? Maybe the eruption would have happened later or maybe not at all.”

The team used a combination of ground-based and satellite measurements of rainfall before modeling the fluid pressure within the volcano’s structure.

They found that nearly 90 inches of rain fell over Kilauea during the first quarter of 2018, compared with a first-quarter 19-year average of 35 inches.

What’s more, fluid pressure penetrated 1.9 miles into the ground, according to the model, and the pressure was at its greatest point in almost a half-century immediately prior to the eruption.

In their paper, the scientists propose that the weight of the water helped to generate the movement of magma beneath the volcano.

Stretching from May 3 to Sept. 4, 2018, the eruption destroyed 716 homes in Lower Puna, forced the evacuation of 2,000 residents, covered 30 miles of roads, isolated 1,600 acres of farmland and caused damage estimated at more than $800 million.

The largest lower East Rift Zone eruption and caldera collapse in at least 200 years produced roughly 1 billion cubic yards of molten rock — enough lava to fill at least 320,000 Olympic-size swimming pools.

Less than a month before the Kilauea eruption, torrential downpours struck the Hawaiian Islands, causing substantial damage on Kauai and Oahu, with at least 786 properties affected by the flooding.

A gauge at Waipa about a mile west of Hanalei recorded 49.69 inches of rain during a 24-hour period ending at 12:45 p.m. April 15, 2018, breaking the previous U.S. 24-hour rainfall record of 43 inches set in Alvin, Texas, in 1979.

Amelung said he and Farquharson didn’t see any significant inflation at Kilauea in the year prior to the eruption, and that prompted them to turn their attention to alternative explanations.

“When we investigate Kilauea’s historical eruption record, we see that magmatic intrusions and recorded eruptions are almost twice as likely to occur during the wettest parts of the year,” Farquharson said in a news release.

The authors said they hope to investigate whether rainfall-triggered eruptions are a phenomenon elsewhere and whether some kind of advanced warning of associated volcanic hazards can be developed.

Should Big Island residents fear rain?

Amelung said no. One or two days of rain aren’t likely to start anything. Weeks of heavy rain maybe, but the volcano also would have to be pressurized and essentially primed to erupt, he said.

“(Kilauea) is more likely to go off in rainy times than dry times,” he said.

Farquharson noted that it’s been shown that the melting of ice in Iceland led to changes in volcanic productivity.

“As ongoing climate change is predicted to bring about changes in rainfall patterns, we expect that this may similarly influence patterns of volcanic activity,” he said.