Kilauea has been Hawaii’s star volcano in recent decades, but neighbor Mauna Loa is no slouch in that department either, having erupted every seven years on average, according to the geological record.

The last time the world’s largest active volcano came to life, however, was in 1984, when lava flows moved within 4 miles of Hilo.

Seems like Mauna Loa is a little overdue.

Researchers at the University of Miami in Florida have published a paper that says Mauna Loa could be nearing its next eruption and that a magnitude-6 earthquake or greater could set it off.

Not only does the paper warn of the possibility of a strong earthquake; it says the rattler, if it does occur, will be on the Kona side of the mountain.

“If the rate (of magma influx) continues at the current rate, it should be soon,” said Falk Amelung, professor of geophysics at Miami’s Rosenstiel School and one of the co-authors of the paper, along with lead author Bhuvan Varagu, a doctoral candidate who is one of Amelung’s students.

To reach their conclusion, the researchers examined a decade’s worth of data taken from satellites and from global positioning system stations on the ground to model magma movements over time.

The study, a continuation of work by Amelung, a former University of Hawaii researcher, in 2007, was published last month in Nature’s Scientific Reports.

It’s been two years since the the U.S. Geological Survey’s Hawaiian Volcano Observatory changed the alert level of Mauna Loa from “normal” to “advisory” following a steady increase in earthquakes and ground swelling. Advisory means the volcano is showing signs of “elevated unrest” compared with the usual long-term background levels.

In recent months HVO researchers, armed with increasingly sophisticated instruments, have recorded small but unusual changes in Mauna Loa that reflect a restless mountain:

>> In March the agency reported that the direction of deformation, or ground swelling, at the summit had reversed twice in five months, switching back and forth from an outward (expansive) direction to an inward (contractional) one.

>> A small, shallow magnitude-3.2 earthquake on March 6 occurred in the summit caldera, Mokuaweoweo, causing a 4-inch collapse in a small portion of the crater. The incident led scientists to believe that the fault may have slipped all the way up to the surface.

>> In late March a tiltmeter near the summit recorded for the first time a change in tilt, or angle of the slope, linked to changes in Mauna Loa’s volcanic system.

Also in March, HVO in its weekly Volcano Watch column urged Hawaii island residents to prepare for an eruption at Mauna Loa by making a personal eruption plan.

The column suggested packing “go” bags containing essential items, including important documents such as a birth certificate, deeds, legal papers and medications, in case of an evacuation order.

HVO said preparation was especially important for those on the west side of the island because flows from Mauna Loa’s Southwest Rift Zone flows can reach — and have reached — populated areas in a matter of hours.

Also in March state lawmakers formally requested that the Hawaii Emergency Management Agency develop an evacuation plan for Hawaii County in the event of an eruption of Mauna Loa.

Under a state Senate resolution, plans will be developed for all parts of the island except for North and South Kohala and parts of Hamakua.

Ingrid Johanson, HVO research physicist, said the mountain has been relatively quiet in the past couple of months, both in terms of deformation and seismicity.

“I would still consider it to be in a period of unrest,” Johanson said. “In some ways the current lack of deformation is just as intriguing as the previous activity.”

The Miami researchers found lots of unrest in their examination of the mountain. They estimated that between 2014 and 2020 enough new magma to fill 44,000 Olympic-size swimming pools intruded into a vertical “dike-like magma body” under and south of the summit caldera, with the upper edge of the magma just over 1.5 miles beneath the summit.

They were able to determine that in 2015 the magma began expanding southward, where the topographic elevation is lower and the weight of the mountain isn’t as oppressive. But then the influx of magma inexplicably waned in 2017 and returned to its previous 2014-2015 horizontal state.

Such changes of a magma body have never been observed before.

“Our hypothesis is this is related to less magma coming in. So why did the magma supply get reduced? We don’t know. It could be related to Kilauea. But that’s just speculation,” Amelung said.

The researchers also found that there was movement of the ground not linked to seismic activity along the horizontal fault under the eastern flank, while no such movement was detected under the western flank.

“The Kona flank isn’t doing it. There’s nothing,” he said.

Amelung said the finding suggests that an earthquake is due on the Kona side, since such motions under the mountain flanks are essential features of volcano growth.

There are uncertainties, however, including the size of the quake. The size could depend on the size of the fault patch that ruptures or on how much time passes until the actual earthquake strikes. It’s possible that the longer the wait, the bigger the earthquake, he said.

Amelung said an earthquake would act to literally shake up the system, releasing gases comparable to shaking a soda bottle, and generating the kind of pressure needed to break the rock above the magma.

But with no satellite data available to determine movements prior to 2002, it’s difficult to say exactly what lies ahead for the volcano.

“We will continue observing, and this will eventually lead to better models to forecast the next eruption site,” Amelung said.

Johanson said the potential interaction between an earthquake on the horizontal fault under Mauna Loa and an eruption is something HVO has considered for some time.

As the paper notes, there have been two magnitude-6 earthquakes just before significant eruptions — in 1950 and 1984.

But there’s a chicken- or-egg aspect to them, Johanson said. While depressurizing a magma chamber could cause magma to rise like soda in a bottle, increasing pressure due to magma influx imparts stresses on the surrounding rock, making earthquakes more likely.

“So while an earthquake might tip off an eruption, the earthquake itself would likely only occur if Mauna Loa was getting ready to erupt anyway,” she said. “My expectation is that we will see additional signs that Mauna Loa is building to an eruption before such an earthquake occurs.”

Hawaii experiences, on average, one magnitude-6 or greater earthquake every 10 years, according to Hawaiian Volcano Observatory data. The last one, a 6.9-magnitude temblor on May 4, 2018, struck just hours after the start of Kilauea’s Lower Puna eruption.

Mauna Loa, meanwhile, has erupted 33 times since 1843, with intervals between eruptions ranging from months to decades, according to HVO.

Scientists say that when Mauna Loa does erupt, it tends to produce lots of fast-moving lava that can flow into communities on both sides of the island.

Hilo has been threatened by seven Mauna Loa lava flows over the past century and a half, according to HVO, while flows have reached the south and west coasts of the island eight times. In 1950 it took only three hours for the lava to reach the Kona Coast.

BE PREPARED

>> An example of a “go” bag checklist can be found at bit.ly/3qbLxmp.