# Volcano Watch — Next Mauna Loa eruption likely at summit

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Over the years, scientists at the Hawaiian Volcano Observatory have developed good techniques for short-term eruption predictions (a few hours to a few days' warning) for Kīlauea and Mauna Loa Volcanoes. However, our capabilities for long-term predictions consist predominantly of educated guesswork.

Over the years, scientists at the Hawaiian Volcano Observatory have developed good techniques for short-term eruption predictions (a few hours to a few days' warning) for Kīlauea and Mauna Loa Volcanoes. However, our capabilities for long-term predictions consist predominantly of educated guesswork. The short-term predictions are mainly based on observations of current seismic activity and surface deformation, whereas long-term predictions rely mainly on long-term trends and interpretation of the eruptive history of the volcano.

A major problem in interpreting the eruptive history of any volcano is that the period of activity recorded by man is very brief and may be unrepresentative of the longer-term eruptive history. On Mauna Loa, the oldest historic lava that can be positively identified was erupted in 1843. Any older flows must be mapped and their ages determined by radiocarbon dating of charcoal preserved beneath the flow as it covered and burned the forest.

Mauna Loa has shown a definitive eruption pattern during the short period since 1843. This pattern allows us to predict where Mauna Loa is most likely to erupt, but not when it is likely to erupt. The pattern is identified from the 48 Mauna Loa eruptions that have occurred since 1843. Eighteen of these have occurred below the 12,000-foot elevation and are termed flank eruptions, whereas the remaining 31 eruptions have occurred above the 12,000-foot elevation and are considered to be summit eruptions. The pattern that has emerged from study of this brief part of Mauna Loa's eruptive past is that no flank eruption has ever been followed by another flank eruption. Flank eruptions have invariably been followed by summit activity. About half of those summit outbreaks have been followed by flank outbreaks within a few days, whereas the other half have been followed by a repose period as short as six months (in 1880) to as long as nine years (1975-1984).

The last eruption on Mauna Loa, in March and April 1984, was three weeks long and occurred along the northeast rift zone above Hilo. Because this was a flank eruption, we can expect the next outbreak at Mauna Loa to occur at the summit. That could be quickly followed by a more hazardous flank eruption, but the odds favor a repose period of a few months to several years between the summit and flank eruptions. Where the next flank eruption might occur is even more difficult to predict. Of the 17 flank eruptions since 1843, eight have occurred on the northeast rift zone, seven on the southwest rift zone, and two on the north flank of the volcano.

Our field crews continue to monitor the gradual swelling of Mauna Loa as magma accumulates prior to its next eruption. Before the last two eruptive outbreaks in 1975 and 1984, the summit region and the Kaoiki fault zone on the south flank of Mauna Loa became increasingly active seismically over a period of at least one year. To date, the fact that we are not seeing any significant increase in seismic activity in these two areas, suggests that we are still at least one year, and perhaps as long as three years, from renewed activity on Mauna Loa. When that activity does occur, it will most likely be at the summit.

### Volcano Activity Update

As we wait for the next eruption on Mauna Loa, the nearly 10-year-long eruption of Kīlauea continues along the middle east rift zone from the episode 51 vents on the west flank of the Puu Oo cone. The eruptive activity shut off once again this past Monday and remained unchanged through Friday morning, when this column was written. The longest eruptive hiatus since episode 51 began last March has been about five days long. If the activity follows the same pattern, we expect the eruption to restart before midday Saturday. During the eruptive break, the summit has been slowly inflating as magma accumulates in the magma reservoirlocated several kilometers beneath the summit. As the surface uplifts and extends, numerous small earthquakes occur beneath the summit. Nearly all are too small to be felt, although one earthquake occurred at 10:28 p.m. on October 1 that had a magnitude of 3.0. This earthquake was located beneath the south flank of Kīlauea just south of Makaopuhi Crater.