Volcano Watch — What's up with Mauna Loa?

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Of the more than 30 eruptions of Mauna Loa in the last hundred years, the 1975 and 1984 eruptions are the first events for which we have detailed seismic and deformation data. Thus, we consider the 1975 and 1984 data sets important standards against which future unrest can be compared and interpreted.

The July 1975 eruption was preceded by at least a year of increased seismic activity and inflation of the summit magma chamber. Civil authorities were briefed on the activity in the fall of 1974, and Big Island residents were alerted to the possibility of an eruption through extensive news media reports. The eruption lasted only one day, and the summit magma chamber began to inflate immediately thereafter.

The inflation continued steadily, and the rate of intermediate-depth earthquakes began to increase in the early 1980s. In late 1983, microseismicity beneath Mauna Loa increased sharply, prompting HVO to forecast an eruption within the next two years. In early March 1984, shallow earthquake activity beneath the summit caldera dramatically increased, and the eruption started several weeks later.

Our capability to detect unrest at Mauna Loa has increased significantly since its last eruption. To monitor seismicity, we have added new seismic stations along the southwest rift zone, as well as state-of-the-art seismic instrumentation at several other locations on the island that will aid in the monitoring of Mauna Loa. To track the deformation of the ground surface, we have installed three electronic tiltmeters and three extremely sensitive strainmeters into 130 m (425 ft) deep boreholes and positioned four continuously-recording GPS instruments on Mauna Loa. All the instruments relay data to HVO via radio modems and satellite links, so that we can look at changes occurring on the volcano within one to ten minutes. In addition, we survey over 50 benchmarks on the volcano yearly with GPS to determine the overall pattern of deformation, and we do additional tilt and leveling surveys at the summit to measure vertical changes.

When magma is accumulating beneath Mauna Loa, we observe swelling of the ground surface. Monitoring stations move away from the area above the summit magma chamber, and we measure increases in distance between benchmarks, increases in their elevation, and a radial pattern of tilt away from the locus of inflation. Conversely, during an eruption, the deformation patterns are reversed as magma leaves the reservoir to move to the surface. Modeling of data gathered over the last two eruption cycles of Mauna Loa reveals that the top of the magma reservoir is located about 3 km (1.9 miles) below the southeastern part of Moku`aweoweo, the summit caldera.

The reservoir started re-inflating immediately after the 1984 eruption and, by 1993, had filled with as much magma as was accumulated between the 1975 and 1984 eruptions. However, the rate of inflation slowed considerably after 1993, and results from both continuously recording instruments and recent surveys confirm that magma is not currently accumulating beneath the summit. Furthermore, we have not observed any dramatic increase in the level of seismic activity since 1984.

If the pre-eruptive patterns of 1975 and 1984 are any indication of what we can expect, we should observe an increase in the number and size of earthquakes, and renewed inflation of the summit at least a year prior to the next eruption. While those of us living on the slopes of Mauna Loa might be relieved that the next eruption does not appear to be right around the corner, we should realize this is a temporary reprieve. Let's all take advantage of it to plan and prepare for the inevitable.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lavamoves away from the vent toward the ocean in a network of tubes and descends Pulama pali in several separate tubes. Breakouts from the tube system feed surface flows above and on the pali. Many surface flows, mainly from breakouts of the ocean entry tubes, are also observed in the coastal flats. Lava continues to enter the ocean at Kamoamoa and the area east of Kupapa`u.

The public is reminded that the benches of the two ocean entries are very hazardous, with possible collapses of the new land. The steam clouds are extremely hot, highly acidic, and laced with glass particles. Swimming at the black sand beaches of the benches can be a blistering or even deadly venture.

There were no earthquakes reported felt during the week ending on October 18.