Volcano Watch — Mauna Loa eruption gave Hilo scare

Friday, March 25, marked the tenth anniversary of the last eruption of Mauna Loa Volcano. The 1984 eruption followed a nine-year period of quiescence. Roughly three years of increasing frequency of shallow- and intermediate-depth earthquakes preceded the eruption. These earthquakes reached a maximum frequency during a swarm that occurred in mid-September, 1983, nearly six months before the eruption began. A magnitude 6.6 earthquake beneath the southeast flank of Mauna Loa occurred one month later, at 6:13 a.m., on November 16. The number of larger earthquakes (those with magnitudes larger than 1.5) increased persistently as the time of eruption approached. During the entire period following the brief summit eruption that occurred on July 5-6, 1975, and before the beginning of the 1984 eruption, the summit region of Mauna Loa inflated as magma accumulated inside the volcano. There was no clear change in gas emissions, deformation, or seismicity immediately preceding the eruption.

Observations in the days just prior to the outbreak include a March 18 report of red glow in a crack on the floor of Mokuaweoweo, the summit caldera; several reports on March 23 of steam clouds (fume?) rising above the summit; and a report on March 24 of rocks being ejected and steam issuing from the 1975 fissures on the summit floor. However, these reports did not reach scientists at the Observatory until after the eruption began.

The immediate precursors to the eruption were recorded by the seismic network operated by the U.S. Geological Survey. At 11:55 p.m. on March 24, small earthquakes began at a rate of 2-3 per minute. By 11:30, the seismic background increased, marking the onset of tremor. Just before 1:00 a.m. on March 25, the tremor amplitude increased to the point that the astronomical telescopes on Mauna Kea, 26 miles to the northwest, could not be stabilized because of the ground vibration. At 1:25 a.m., a military satellite recorded a strong infrared signal from the summit of Mauna Loa which indicated that the eruption was underway.

The eruptive fissures migrated rapidly down the southwest rift zone to the 12,750-foot elevation and across the floor of Mokuaweoweo to the northeast. By 4:00 a.m., fountains extended to the northeast rift zone and migrated in a series of fissures towards the northeast at the same time that the fissures on the southwest rift died out. By 7:00 a.m., the fountains were restricted to a zone on the northeast rift between 12,400 and 12,125 feet elevation. At 9:10 a.m., a new fissure opened up 4.3 miles farther east, at an elevation of 11,185 feet. This fissure then migrated downrift so that by 9:30 a.m., the curtain of fire was 1.25 miles long. A narrow flow moved about 3 miles down the southeast flank. At 10:30, intense steam emissions began along a fracture between 10,690 and 10,400 feet elevation, but no eruptive fissure formed in this area.

At 4:41 p.m., a new fissure opened up at the 9,350-foot elevation. This fissure rapidly migrated both uprift and downrift, so that by 6:30 p.m., a curtain of fire slightly longer than 1 mile was active. All the vents farther uprift quickly became inactive, and eruptive activity was confined to these lowest vents for the next three weeks. Six large vent structures eventually formed in this area around localized vents. Lava fountains never exceeded 160 feet in height. By daybreak on March 26, a fast-moving flow from these vents had advanced 5.5 miles to the northeast, and three shorter, less active flows advanced towards the east. These flows stopped advancing within 48 hours and never crossed the Powerline Road.

The principal flow advanced as a narrow, channelized 'a'a flow. The rate of advance slowed as the flow moved downslope, but by March 29, the flow had extended 15.5 miles to an elevation of 3,000 feet above Hilo. At this time, the flow front was less than 4 miles away from the outskirts of the city. The smoke from burning vegetation, the loud explosions of methane gas along the advancing flow, and the glow at night all contributed to the growing concern of Hilo residents.

On the morning of March 29, a channel levee broke at the 5,700 foot level, about 8 miles upslope from the flow front. The lava was diverted into a new subparallel flow, and the initial flow stagnated. This scenario was repeated on April 5, forming a third subparallel flow. By April 14, no active flows extended more than 1.25 miles from the vents, and on April 15, the eruption ended.

The natural breakdown of the levees along the flows produced the same result as intentional lava-diversion efforts applied elsewhere in the world. By diverting the flow of lava into a subparallel flow, the advance of the flows was arrested. The experience gained during the 1984 eruption will help us evaluate the likelihood that flows will continue to advance or whether they will naturally divert themselves. Many factors facilitated the natural lava diversion in 1984, including the relatively gentle slope above Hilo, the heavy vegetation through which the flows advanced, the low temperature of the erupted lava (making it relatively pasty), and the termination of the eruption after a relatively short period, before the flows could reach Hilo.

Mauna Loa has erupted 39 times in historic time (the last 162 years for observations of Mauna Loa), for an average recurrence interval of only four years. However, between 1832 and 1950, there were 37 eruptions for an average recurrence interval of only 3.2 years, whereas since 1950, there have been only the 1975 and the 1984 eruptions. In other words, Mauna Loa has been far less active since 1950 than in the preceding century. We suspect that the pattern of less frequent eruptions, evident since 1950, will continue and that we will have eruptions of Mauna Loa only every 10-25 years. At about the same time that Mauna Loa became less active in 1950, Kīlauea become more active (starting in 1952). It is possible that Mauna Loa will continue to experience infrequent eruptions until activity at Kīlauea declines.

Mauna Loa is currently continuing to inflate with magma, as it has since 1984. The level of seismic activity has not yet begun to increase as it did preceding both the 1975 and 1984 eruptions. The low level of seismic activity suggests that the next eruption is unlikely to occur within the next year. On the other hand, the rate and amount of magma accumulation beneath the summit suggests that the next eruption is still likely within the next five years.