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March 30, 2000

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Eyes on Mauna Loa

With the east rift zone eruption of Kilauea continuing into its 18th year, much of our focus in volcano and seismic monitoring at the Hawaiian Volcano Observatory is on Kilauea. At the same time, however, Mauna Loa receives a lot of our attention, and it should never be overlooked in terms of the hazards that it poses to us.

In the earlier part of the 20th century, Mauna Loa was the more active of the two volcanoes in southeast Hawai`i. HVO scientists during that period suggested that its eruptions followed a pattern of shifting from active rift zone to summit caldera to active rift zone, such that they actively tried to forecast eruptions. It was felt that, sometimes, large or significant earthquakes beneath its flanks foreshadowed rift zone eruptions.

A magnitude-6.2 earthquake in the upper southwest rift zone preceded the largest historic eruption of Mauna Loa in 1950. Eruptions in 1942, 1975, and 1984 were preceded by magnitude-6.0, -5.5, and -6.7 earthquakes, respectively, in the Ka`oiki fault system.

After the large southwest rift zone eruption in 1950, Mauna Loa went into a 25-year-long period of repose that ended with the July 1975 eruption. That eruption was followed by the March 1984 eruption. Since 1984, of course, Mauna Loa has not erupted.

Before volcanoes erupt, typically their first signs of unrest are increases in the number of very small, or micro-, earthquakes occurring within them. This is a principal reason why HVO established and maintains its permanent seismographic network on the island of Hawai`i, and why our network is most concentrated on Kilauea and Mauna Loa.

Prior to the 1975 eruption, HVO scientists recognized and prepared a report on a clear increase in microearthquake activity beneath Mauna Loa. The increase began nearly a year before the eruption. Because of the historical patterns of Mauna Loa eruptions noted by earlier HVO scientists, then-HVO staff offered similar forecasts of an eruption to follow the 1975 eruption.

In late 1983, microseismicity beneath Mauna Loa increased sharply, and a damaging earthquake occurred beneath its southeast flank in November 1983, in the Ka`oiki fault system, which lies between Mauna Loa and Kilauea. These events are viewed as clear precursors to the 1984 eruption.

Since the 1984 eruption, the rates of microearthquake occurrence beneath Mauna Loa appear to be gradually decreasing. Our conventional wisdom, in combination with the historical observations made by early HVO scientists and our detailed tracking of the 1975 and 1984 eruptive sequences, suggests that the next eruption of Mauna Loa is not imminent. In addition, we would interpret a pattern of microearthquakes, resembling those before the last two eruptions, as indicating a likely future eruption. The answers and proof clearly lie in the future.

While we await eruptions on Mauna Loa and continue to monitor the ongoing Kilauea eruption, we are working to improve our resources for studying and monitoring our volcanoes. We have added new seismographic stations along Mauna Loa's southwest rift zone. In partnerships with other government agencies, we have also added state-of-the-art seismic instrumentation at a few locations on the island, and three new down-hole seismic stations will be installed within the next three months. These will all help our studies of Mauna Loa, in particular, and volcanic and seismic processes in Hawai`I, in general.

On Saturday, April 8, from 9 a.m. to 12 noon at the Kona Surf Hotel, the Center for the Study of Active Volcanoes at the University of Hawai`i at Hilo is sponsoring the second of two public symposia entitled "What's New on Mauna Loa Volcano?" These symposia bring together scientists and emergency managers to share their insights and concerns with the public. To find out more, please attend this CSAV symposium on April 8.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes toward the coast. Lava stopped entering the ocean at Lae`apuki, and breakouts on Pulama pali above Lae`apuki indicate a blockage of that tube system. A second flow, located to the east of the first flow, is active in the area near the boundary of Hawai`i Volcanoes National Park and is intermittently entering the ocean between Waha`ula and Kamokuna. Breakouts on Pulama pali of this eastern flow provide spectacular viewing at night. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

There were no earthquakes reported felt during the week ending on March 30.

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Updated: 5 Apr 2000