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Speaking Of: : Volcanoes : When Will a Volcano Blow? Quakes, Ground Tilt Are Clues

March 21, 1995|KENNETH REICH | TIMES STAFF WRITER

While Californians are aware of the frustrating inability of seismologists to predict earthquakes, fewer know of the progress of scientists in predicting volcanic eruptions, another of nature's threats to the Pacific Coast.

The crucial precursory volcanic events have become known, volcanologists say.

Randy White of the U.S. Geological Survey explained: "The scenario for really big explosive eruptions is, first, basalt (magma) coming into the bottom of the magma chamber, and that gives itself away as deep, long-period earthquakes. Then that stirs the pot and starts producing shallow, long-period earthquakes and probably sulfur-dioxide emissions. Then, as the pressure builds, you would start seeing the ground deformation, the tilt."

In 1991, by conservative estimates, at least 10,000 lives were saved in the Philippines because a two-day warning that Mt. Pinatubo would undergo a major eruption allowed timely evacuations of American-operated Clark Air Base and nearby Philippine cities.

And last September, 30,000 people living in and around the port of Rabaul in the South Pacific nation of Papua New Guinea were evacuated the night before two vents on opposite sides of the caldera, the broad crater-like basin of the Rabaul volcano, erupted with great fury. Two people were killed, compared to about 500 fatalities in the last major eruptions at Rabaul in 1937.

These successes, however, do not mean that volcanic predictions are always precise or reliable.

If prediction, as defined by White, includes time, place and size, then scientists have been much more successful with time and place than size. The Geological Survey scientist said this amounts to "a forecast, not a prediction."

At Mt. St. Helens in Washington state in 1980, scientists, noting earthquake and, particularly, harmonic tremors--the long-period quakes indicating the upward movement of molten material--warned of a serious eruption, and the authorities established an evacuation zone.

But the huge eventual eruption of May 18, 1980, exceeded their expectations.

A Geological Survey scientist, David Johnston, was killed at his observation station, six miles from the eruption site. His supervisors believed that he would be safe, but the blast extended more than three times farther than they expected.

More recently, it now appears that some scientists may have initially overestimated the dangers of a huge eruption at Popocatepetl, the 17,887-foot volcano 39 miles southeast of Mexico City, when it spewed out ash last Dec. 21.

Michael Sheridan, chairman of the Geology Department at the State University of New York's Buffalo campus, declared at the time that "the chamber is loaded," and, at another point, said: "That volcano is getting ready to do something pretty big."

Dan Miller, a research geologist at the Cascade Volcanic Observatory in Vancouver, Wash., said, "It's a volcano that has frequently erupted violently in the past, and it's to be considered very dangerous now."

Sheridan, Miller and others cautioned, however, that careful observations had to be made before the situation was clear, and the record since Dec. 21 indicates that, for now, no big eruption is in the offing.

On the other hand, Miller said, there are continuing high emissions of sulfur dioxide gas from the volcano, ranging from 1,000 to 12,000 metric tons per day. Only Sicily's Mt. Etna "has those kinds of numbers during non-eruptive periods," Miller noted.

"I think it's a system that bears watching," he concluded.

Bill Rose, professor of geology at Michigan Technological University in Houghton, observed that "people are very conscious of false alarms, so there is a sensitivity (among volcanologists) about the way predictions are made."

In the longer term, cautioned Stanley Williams, a volcanologist at Arizona State University, different mountains seem to have different patterns, and for a general evaluation of the potential for an eruption, scientists need to understand a volcano's history, if it is available.

"St. Helens is an example of the best we could hope for," Williams said. "Don Mullineaux and Dwight Crandall (two Geological Survey scientists) had selected it (in 1978) as the most likely next eruption of the 20th Century, and they were right.

"With another 40 volcanoes, it is very, very uncertain. All kinds of attention is being paid to Rainier (in Washington state), because of the hazard to metropolitan centers, yet we have a very poor sense of the likelihood of an eruption there within the next 10 or 100 years."

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Speaking Of: Vocanoes

* WHERE THEY ARE: The world has about 1,500 active or potentially active volcanoes. There are thousands more cinder cones in volcanic vicinities. About 75% of the world's volcanoes are in the so-called Ring of Fire around the Pacific ocean.

* ANATOMY OF AN ERUPTION

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