Experts Tout Predictions of Eruptions : Volcanoes: Satellites and ground-based instruments make forecasts more reliable, scientists say. More such monitoring is urged.

Measurements by satellites and ground-based instruments are giving experts increasingly reliable predictions of volcanic eruptions, scientists said Tuesday.

Based on encouraging new measurements, researchers at the meeting of the American Geophysical Union called for much more pervasive monitoring of the world’s 600 volcanoes that could erupt at any time.

John J. Dvorak of the U.S. Geological Survey’s Cascade Volcano Observatory in Vancouver, Wash., said that only about 50 volcanoes in the world are regularly monitored.

The new instruments can measure ground deformation--a key component in forecasting many eruptions--down to mere millimeters of movement, Dvorak said.

Sometimes in the periods leading up to an eruption the deformation is so dramatic that it is apparent to the naked eye. At the city of Rabaul in Papua New Guinea, for example, residents waking up the morning of Sept. 19 could see that beaches next to two volcanic cones had risen six feet above the sea overnight. Eruptions began within an hour.

But at Rabaul too, instruments played a significant role, showing less visible deformation the night before, which--combined with 27 hours of an unusual number of earthquakes--persuaded authorities to begin an evacuation of the first of 50,000 residents, which they did successfully, with six hours to spare.

Dvorak said that at volcanoes such as Kilauea and Mauna Loa in Hawaii, and others in Indonesia and Japan, it has proved possible to make reliable calculations of minute-by-minute pressure increases within shallow reservoirs of magma, or molten rock, as the material begins a quick rise to the surface.

Tim Dixon of the University of Miami said the Global Positioning System operated by Pasadena’s Jet Propulsion Laboratory has proved invaluable in showing gradual horizontal movements in volcanic domes.

But, he added, for precise measurements on a continuous basis, and particularly measurements of vertical movement, instruments on the ground are important as well. When a volcano is about to erupt, strain may be accumulating so rapidly that it is vital to have instruments on site ready to transmit data instantaneously, he said.

In California, the most comprehensive monitoring is taking place in the Long Valley Caldera near Mammoth Lakes. Measurements there indicate no pending eruptions, researchers said.

The instruments installed near Mammoth Lakes showed an immediate increase in strain there after the 1992 Landers earthquake hundreds of miles away in the Southern California desert, he noted. But that had no perceptible volcanic effects.

Other scientists cautioned that before a fully reliable picture of deformation and strain at Mammoth is obtained, additional instruments, spaced out from various vantage points, are essential.

Alan T. Linde of the Carnegie Institution in Washington said strain meters in Iceland not only indicated to scientists when the Hekla volcano there was about to erupt in 1991, but also when eruptions were about to stop.

“There was a clear sign of an impending eruption 15 to 20 minutes before it occurred,” he said. “And there were indications how powerful it would be.”

One advantage of using the Mammoth area for extensive monitoring is that it can serve as a model for measuring activity at other, more distant volcanoes with instruments, California scientists said, even though the last volcanic activity at Mammoth was about 500 years ago.

New monitoring methods are not perfect, the scientists said. Experience in various parts of the world suggests that in some cases measuring instruments may provide only hours or minutes of warning before an eruption.

Even a few minutes warning, however, can provide a big advantage over no warning at all to those who may be victimized, they said.