Is Mauna Loa about to blow after 38 years of silence?

A couple of weeks of elevated seismic activity and ground swelling on the mountain suggest that might be the case.

Authorities are taking it seriously.

On Wednesday, Hawai‘i Volcanoes National Park closed the Mauna Loa summit backcountry due to the mountain’s heightened unrest.

On Thursday the Hawaiian Volcano Observatory started issuing daily volcano online updates instead of the usual weekly status reports. The erupting Kilauea is the only other Hawaii volcano with daily updates.

“Mauna Loa will erupt again,” said Ken Hon, scientist-in-charge at the Hawaiian Volcano Observatory. “As long as there is heightened unrest, it is more likely to erupt. But it could be weeks or months — or it could eventually die off.”

That’s what happened the last time there was elevated seismic activity and summit expansion on Mauna Loa, in late January to late March 2021.

A swarm of earthquakes and ground swelling also happened in 2019. That prompted HVO to change the alert level of Mauna Loa from “normal” to “advisory,” which means the volcano is showing signs of “elevated unrest” compared with the usual long-term background levels.

The world’s largest active volcano appears to be coming to life again. There were five to 10 earthquakes a day in June and 10-20 earthquakes in July and August. And more than 100 earthquakes were reported on two days in late September.

According to HVO, 40-50 small earthquakes have been rattling the mountain every day at a rate of two to three per hour. On Thursday the observatory reported a daily total of 44 earthquakes below the summit and the mountain’s upper-elevation flanks.

Hon said it’s hard to say what will happen this time. Before 1950, the record shows that Mauna Loa erupted every five to six years. But there have been only two eruptions since 1950, and a lot of time has passed since the 1984 eruption.

With no recent track rec­ord, scientists don’t have a lot of information to determine what normal behavior for the volcano is, Hon said.

If an eruption does occur, it could happen quite fast. In 1984 the number of earthquakes shaking the mountain went from a few per hour to two to three a minute within a span of four hours.

“We’re hoping for a longer lead time, but it might be just a number of hours,” the scientist said.

In 1984 the astronomers atop Mauna Kea reported that the continuous shaking from tremors made it impossible for them to use their instruments. And when the volcano did erupt, lava flows moved within 4 miles of Hilo.

Researchers at the University of Miami in Florida published a paper in 2021 that warned Mauna Loa could be nearing its next eruption, and evidence suggests that a magnitude-6 earthquake or greater could set it off.

Similar earthquakes did occur prior to the past two Mauna Loa eruptions, Hon said, but it’s been a long time since the volcano has experienced such a large temblor.

The geological record shows that Mauna Loa has erupted 33 times since 1843, with intervals between eruptions ranging from months to decades.

Scientists say that when Mauna Loa does erupt, it tends to generate 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.

Hawai‘i Volcanoes National Park spokeswoman Jody Anastasio said the Mauna Loa summit area was closed “out of an abundance of caution, knowing the challenging response time, extreme altitude, and specialized resources required for a summit rescue in the event of an eruption.”

“Conducting a rescue operation at almost 14,000 feet is incredibly challenging and hazardous in normal conditions,” she said in an email.

In the past couple of years, HVO has been urging people who live on the slopes of Mauna Loa to put together a “go bag” with essentials and important documents such as a birth certificate, deeds, legal papers and medications, in case of an evacuation order. Also recommended is a family communication plan that will allow family members to keep in contact during a crisis.

Hawaii County Civil Defense Administrator Talmadge Magno said his agency also recommends taking such steps in order to be prepared.

“We’re always talking about preparedness and the need to understand the activity levels of Mauna Loa and Mauna Kea,” Magno said. “You need to understand that if you live or work on the slopes of Mauna Loa or Mauna Kea, you have to deal with an active volcano.”

In 2021 state lawmakers directed the Hawaii Emergency Management Agency to develop an evacuation plan for Hawaii County in case of a Mauna Loa eruption.

Magno said his office has been helping the state agency with the plan and is scheduled to meet with a state planner next week. The first draft of the evacuation plan is due to the Legislature in January.

What happens if the mountain erupts before then?

“We’re ready to respond,” he said.