Quakes and bombs: When Hawaii’s Mauna Loa, the world’s largest volcano, erupted
Christine Hitt, SFGATE
Oct. 16, 2022
On the Big Island of Hawaii, eruptions are inevitable. It has four active volcanoes — Hualalai, Maunakea, Mauna Loa and the highly active Kilauea — cycling through periods of sleep and unrest.
Most are slumbering — some for decades, others for hundreds or thousands of years — but magma still flows through their underground chambers. These movements can trigger thousands of small earthquakes per year, telling scientists that the volcanoes are indeed still alive.
For a long time, from 1983 to 2018, the volcanic eruptions on the Big Island have been contained to the eastern corner of the island at Kilauea volcano, leaving the rest of the island unaffected by its flows. Locals and tourists grew accustomed to safely watching Kilauea’s lava flow emanating from the east rift zone at Puu Oo crater and visiting the summit of Kilauea to see the red glow from Halemaumau crater’s lava lake.
But in 2018, the status quo changed when Puu Oo collapsed and the Halemaumau lava lake drained, marking the end of that 35-year-long eruption.
Although Kilauea has since resumed its summit eruption at Halemaumau crater, an increasing number of earthquakes have turned attention to Kilauea’s big brother, the massive Mauna Loa volcano.
"Mauna Loa has erupted 33 times since 1843 — averaging once every five years," the National Parks Service says on its website. "Over a longer period of time, the past 3,000 years, [it is] estimated to have erupted once every six years."
The last eruption at Mauna Loa, however, hasn’t been since 1984, nearly 40 years ago, so public anticipation of an eruption builds each time the volcano stirs.
Located right in the middle of the island, Mauna Loa (Hawaiian for “Long Mountain”) is the world’s largest volcano. It covers half of the Big Island at 60 miles long and 30 miles wide, and rises 13,679 feet above the Pacific Ocean. Its large summit caldera, Mokuaweoweo, alone is 3 miles long.
Earlier this month, the U.S. Geological Survey Hawaiian Volcano Observatory changed its advisory level from green to yellow as a precaution due to the volcano’s heightened unrest. It’s the second out of four levels in the volcano alert system. Hawaii Volcanoes National Park also closed its backcountry trails to Mauna Loa’s summit.
The volcano is not erupting, but its earthquakes have escalated. Two months ago, the volcano was seeing about 20 earthquakes per day. That increased to 40 to 50 earthquakes per day from mid-September to October, with two days reaching peaks of more than 100 earthquakes.
A river of lava during the 1984 eruption of Mauna Loa.
A river of lava during the 1984 eruption of Mauna Loa.
National Park Service
“The unrest is likely caused by renewed input of magma into Mauna Loa’s summit reservoir system,” the advisory said. “As the reservoir expands it is triggering small earthquakes directly beneath Mokuaweoweo caldera in a region just to the northwest of the caldera.”
Geologists use many signals, such as earthquakes, ground deformation, satellite imagery, temperature data and visual observation, to assess whether a volcano is about to erupt, according to Frank Trusdell, geologist with the USGS Hawaiian Volcano Observatory. Though they cannot pinpoint an exact day and time an eruption will occur, they are able to use these factors to determine how likely it is to happen.
“In this case, we have heightened seismicity, and the deformation, the ground is changing but not at a rapid rate to cause alarm,” says Trusdell. He does not believe these mean an imminent eruption is expected.
As a comparison, there were hundreds of earthquakes per day sustained for weeks prior to Mauna Loa’s last eruption in 1984. Earthquakes reached more than 1,000 per day prior to the 1975 eruption. Today’s earthquake pattern isn’t anywhere close to those, but there have been multiple episodes of increased seismic activity in the past 20 years, starting in 2004. The last time the alert system was elevated to yellow was in 2019.
Additionally, there is historical evidence that Mauna Loa tends to have diminished activity while Kilauea is active and vice versa, says Trusdell. So it could be theorized that the end of Kilauea’s 35-year-long eruption and diminished state could mean that Mauna Loa
Mauna Loa is known for its voluminous flows. The volcano’s gigantic size and large interior chamber, which has been accumulating magma the past few decades, means it could have a more dangerous and potentially more damaging flow than what we’re used to seeing from Kilauea.
“The amount of lava that Mauna Loa produces in a period of time can exceed the amount of lava that Kilauea produces in the same period of time by sometimes as much as a factor of three,” says Trusdell.
During the 2018 Kilauea eruption, fissures erupted from the volcano's lower east rift zone. On the lower right, Fissure 8, now called Ahuailaula, produced a large, fast-moving lava river that reached the ocean.
During the 2018 Kilauea eruption, fissures erupted from the volcano's lower east rift zone. On the lower right, Fissure 8, now called Ahuailaula, produced a large, fast-moving lava river that reached the ocean.
USGS/M. Patrick
If you remember the height of Kilauea’s 2018 eruption, when Fissure 8, now called Ahuailaau, sent a lava river speeding to the ocean, that’s a normal output of lava for Mauna Loa, says Trusdell. And using that as its baseline, Mauna Loa has the ability to produce three times that. With three times more lava, the flows go much faster — and much farther.
Mauna Loa’s central location can also send flows in many different directions.
“When the [Kilauea] eruption was happening down in the lower east rift zone, the rest of the Big Island could say, ‘Oh, well it’s just over there. It’s not going to impact us. We can go to Hilo, we can go to Kona,’” says Trusdell. “But if Mauna Loa sends down a lava flow and it cuts that one highway that goes around the island, then the impacts to commerce on the Big Island would be greatly impacted.
“At Kilauea, only one section of the Big Island is impacted,” he continues, “but with Mauna Loa, it can threaten most areas on the Big Island downslope. And even though it can’t impact areas like Maunakea and Kohala, if it severs the highway, those communities are not immune.”
Aside from the highway, Mauna Loa’s lava flows could destroy multimillion dollar developments. Between 1984 and 2002, there has been $2.3 billion invested in new construction on the flanks of the volcano, according to a 2012 Mauna Loa fact sheet.
The most horrifying example of Mauna Loa’s destructive power happened in 1868, following a 7.9 magnitude earthquake in the Kau District. Not only did the quake cause widespread damage, but it also produced a river of lava that reached the sea, a half-mile-wide mudslide that killed 31 people, and a wall of water that rose 60 feet high and killed 47 people.
“The kind of sea that struck the settlements above, it was ocean water joined with water coming up from the ground,” reported the 1868 Hawaiian-language Ka Nupepa Kuokoa newspaper of the incident. “These waters were not like the ocean seen on our other islands; it was terribly unusual. If it was a tsunami [kai hooee], then there would be no human toll, but what came ashore was a swirling sea [kai owili].”
The threat to Hilo
Hilo is the largest city on the Big Island and its population has grown to 44,186, per the 2020 census. The city has been threatened by Mauna Loa’s lava flows in the past — and has every reason to worry about them in the future.
Mauna Loa’s last eruption in 1984 produced a lava flow that came within 4.5 miles of Hilo. Flows also headed in the city’s direction in 1942 and 1935 before stopping. In eruptions starting in 1855 and 1880, lava flows came within today’s city limits.
Military bombs were used during the 1935 eruption to “disrupt the lava channels by diverting lava from the advancing flow,” according to the National Park Service. When the lava flow stopped six days after the bombing, it was considered a success by some proponents, but others believed it to be a coincidence.
Later, in 1984, the governor at that time decided against using bombs, but was asked by his staff to draft a plan as a worst-case scenario.
“That’s a political question when you start to use lava diversion as a strategy to mitigate impacts,” says Trusdell. “Of course there are other points of view. I don’t think the Native Hawaiian community might be in favor of such a thing.”
One thing Hilo does have in its favor is time, as it’s relatively far from the most active part of Mauna Loa’s northeast rift zone and the slopes to the city are gentle.
And while Mauna Loa hasn’t erupted for 38 years, it doesn’t mean it will erupt just because the average says it should by now.
“The long-term average is just a long-term average. It doesn’t take into the complexities of what’s actually happening,” says Trusdell. “So when we deal with probability of an eruption from a volcano like Mauna Loa, we say that every day has an equal probability of eruption as the day before and the day after.”
“All we can do is tell you like the weather, we keep our eye on it,” he continues. “We’re looking at all the different parameters. If the parameters change, we’ll update the forecast.”
Editor’s note: SFGATE recognizes the importance of diacritical marks in the Hawaiian language. We are unable to use them due to the limitations of our publishing platform. We use Maunakea as a single word as suggested by the UH Hilo School of Hawaiian Language.
|
|