More than 31 years after Mauna Loa last erupted, sending lava within 4.5 miles of Hilo, the largest active volcano in the world is showing signs of unrest.
Scientists at the U.S. Geological Survey Hawaiian Volcano Observatory Thursday elevated the Volcano Alert Level for Mauna Loa from “normal” to “advisory.” There are four levels of alerts: “normal” — or background level, “advisory,” “watch,” and “warning” to inform the public about a volcano’s status.
The change in status indicates the volcano is showing signs of unrest that are above known background levels, but it does not mean an eruption is imminent or certain, officials said.
Normally, Mauna Loa sees up to 10 earthquakes per week, and recently that has escalated to 40 earthquakes per week. Prior to eruptions in 1975 and 1984, about 100 earthquakes were recorded each day.
“We expect the seismicity to grow steadily and be more consistent and persistent, and even the rates to change before we forecast an eruption,” predicted Frank Trusdell, an HVO geologist who has studied Mauna Loa extensively and was present for its last eruption in 1984.
The mountain saw a period from 2004-05 until 2009 when the volcano remained in a state of inflation, but did not erupt. When the inflation slowed to background levels in early 2010, the status of Mauna Loa was returned to “normal.”
“Mauna Loa is not a dead volcano, it is still active,” said Trusdell. “People should take the time to understand the hazards from Mauna Loa and what they present, and people should know an eruption is not imminent and HVO is monitoring it, and if there is any change that we will alert the emergency managers and notify the public.”
Since mid-2014, monitoring instruments have measured inflation on Mauna Loa consistent with recharge of the volcano’s shallow magma storage system, as well as elevated rates of shallow earthquakes of less than 2.5-magnitude beneath the summit, upper Southwest Rift Zone and west flank, Trusdell said.
Asta Miklius, a geophysicist who manages the observatory’s deformation monitoring network, said the current bout of inflation appears to be occurring under Mauna Loa’s summit and upper Southwest Rift Zone. The volcano’s magma storage system, estimated to be about 2 miles beneath the surface, consists of two reservoirs, a spherical-shaped reservoir near the southeast wall of the caldera and a tabular reservoir, which runs the length of the caldera toward the upper Southwest Rift Zone.
“The magma is infilling the shallow reservoir system and is causing enough stress to cause these shallow earthquakes,” she said. “It just fits the definition of a volcano that is in unrest, which doesn’t necessarily mean that we’re building up toward eruption.”
Though the magma reservoirs may be located primarily beneath the caldera, it is not indicative of where lava might break through. However, based on data from 1975, when Mauna Loa erupted at its summit, and 1984, Trusdell said Mauna Loa’s next eruption, if it were to occur, would start at the summit.
“Even when the summit phase starts, we still, at that point, cannot say whether or not it’s only going to stay in the summit or if one of the rift zones might be involved,” he said. “We have to watch the migration of earthquakes which would suggest where molten material is migrating.”
Past Mauna Loa eruptions have sent lava toward Hilo, South Kona, Ka‘u and Kiholo Bay, as well as other areas in between.
Trusdell said scientists expect to see earthquake counts pick up “so that we are looking at hundreds of earthquakes per day,” with increased inflation, volcanic tremors and earthquake swarms before Mauna Loa enters its next eruption phase.
“As we get closer, we will see characteristic signatures of volcanic tremor and earthquake swarms — those both say that the molten material is migrating,” he explained. “At that point, when the earthquakes start building to hundreds per day, we will issue press releases and let emergency managers know that Mauna Loa looks like it’s starting to get closer to an eruption. Then, when we see more seismic signatures, we would forecast an eruption would be imminent and to expect the summit phase to commence.”
Hawaii Island is comprised of five volcanoes, Kohala, Mauna Kea, Mauna Loa, Hualalai and Kilauea — the latter three remain active today. Kohala last erupted 60,000 years ago and Mauna Kea 4,500 years ago. Mauna Loa last erupted in 1984 and Hualalai in 1800-01, followed by a failed eruption with earthquakes, flames and gas but no lava, in 1929. Kilauea has been erupting since 1983.
Mauna Loa’s 1984 eruption occurred between March 24 to April 15, and was preceded by three years of increased earthquake activity. But, in contrast to pre-1984 activity, the energy released by recent earthquakes remains comparatively low, Trusdell said.
In 1950, lava erupted from a fissure on Mauna Loa’s Southwest Rift Zone, at an elevation of almost 10,000 feet, crossing Highway 11 in three places and destroying about two dozen structures before reaching the South Kona shoreline about three hours later, according to the USGS.
In all, Mauna Loa has erupted 33 times since 1843, which is when written records of volcanic eruptions began, following European contact. Of those eruptions, about half stayed within the caldera, flowing at most 6.2 miles to 9.3 miles from the summit. Outside the caldera, an estimated 25 percent occurred along the Northeast Rift Zone and 20 percent occurred along the Southwest Rift Zone, according to Trusdell.
Eruptions from radial vents are less frequent, occurring 6 percent of the time, but are more hazardous because of the potential proximity to inhabited areas. Radial vents, or eruptive fissures located outside the rift zone or summit, are found on the north and northwest flanks of Mauna Loa, including underwater. Since 1843, three radial eruptions have been recorded including one in 1852 that propagated toward Pohakuloa, a submarine eruption in 1877 in Kealakekua Bay and an eruption at the 11,000-foot elevation on the northwest flank of Mauna Loa that sent lava all the way to Kiholo Bay — 31 miles — in just eight days in 1859.
Over the past 3,000 years, Trusdell said, Mauna Loa has erupted once every six years. Since written records began, the volcano has erupted about once every five years.
“It’s possible that the increased level of activity at Mauna Loa could continue for many months, or years, without leading to an eruption,” said Tina Neal, HVO’s scientist-in-charge, in a prepared statement. “It is also possible that the current unrest could be a precursor to the next eruption of Mauna Loa. But at this early stage, we cannot determine precisely which possibility is more likely.”
HVO continues to closely monitor Mauna Loa, and will notify Hawaii County Civil Defense, the National Park Service and other emergency managers, as well as the public, if significant changes are detected.
“We work very closely with HVO and if we get to a point where they feel the activity has increased to point where we need to start worrying about a possible eruption, then we will take the necessary actions to alert the public to make sure everyone is informed,” said John Drummond, Civil Defense administrative officer. “There is no need for any alarm. It’s just basically saying it (Mauna Loa) is acting a little out of the normal.”
Meanwhile, Kilauea Volcano’s summit lava lake remained active Thursday. At the East Rift Zone, the lava flow northeast of Puu Oo remains active within 5 miles of the vent and does not currently pose a threat to communities. Normal levels of seismic and deformation activity continue across the volcano, scientists said.
