Worst of Eruption Is Probably Over, Scientist Says : Volcanoes: The leader of the American team that successfully predicted that Mt. Pinatubo would blow warns that mudslides now present a greater danger.

MENLO PARK, Calif. —  The geophysicist who led a team of American scientists that successfully predicted the eruption of Mt. Pinatubo in the Philippines said Monday he believes the worst of the eruption is probably past, but damage from mudslides could be far more devastating than the eruption itself.

“The mudflow problem is just beginning,” David H. Harlow of the U.S. Geological Survey said after returning from a monthlong expedition to Mt. Pinatubo.

Harlow said he and other scientists expect the volcano to continue to erupt off and on for as long as two years, but the evidence suggests that future eruptions will not match the June 15 blast that left vast areas “looking like a gray, wasted desert.”

The signs that Mt. Pinatubo was about to erupt were so clear that thousands of lives were saved by mass evacuations, Harlow said. But the huge eruption dumped millions of tons of debris onto the slopes of the mountain, and heavy rains will undoubtedly send much of that crashing down on villages below.

And the monsoon rains, he added, are just beginning.

Harlow said he arrived in the Philippines on May 25 after scientists there asked for help in analyzing data from the volcano. The following days brought harrowing experiences as the mountain rumbled just 16 miles from his command post.

“It was clear the volcano was going to erupt,” he said. “When and how violently was not so clear.”

The challenging task of deciding when to tell people to flee for their lives fell to Harlow and his colleagues, and he admitted Monday that they had some anxious moments. They did not want to wait too long, thus jeopardizing lives, nor did they want to issue a false alert, thus undermining the people’s confidence in what they had to say about the volcano.

“Your emotional swings are tremendous,” he said. “One of the most difficult times was the time between the evacuation and the eruption. Mercifully, that only lasted two days.”

A marked increase in small earthquakes, plus data that indicated the mountain was bulging, compelled the scientists to call for the evacuation on June 13. In all, about 89,000 people, including 15,000 Americans, fled from the area.

The scientists also backed farther away from the mountain, but on the morning of June 15 “we decided we could handle it, so we went back” to a windowless bunker at Clark Air Base.

They had reason to doubt the wisdom of that move almost immediately.

“At 2 p.m., all the (unmanned scientific) stations between us and the mountain went out,” indicating that something serious was happening deep beneath the cloud of ash that hid the top of Mt. Pinatubo.

Harlow would learn later that of the nine U.S. scientific instruments on the mountain, only one survived the blast.

Meanwhile, the sky grew darker and darker until “it was dark as midnight, with ash falling all over the place,” Harlow said.

That left the scientists with only one prudent course: “We left the base gladly.”

As they were fleeing the base in the face of what he described as an enormous blast, Harlow feared for his life for the first time.

During the following days the mountain belched out more than two cubic kilometers of rock and ash in violent explosions that have marked it as the most powerful volcanic eruption of this century. It will take the mountain centuries to recharge its volcanic reservoir of that much material, which is why the scientists believe the eruptions will subside significantly.

But on June 15, the volcano did just what the scientists had predicted.

“It was an enormously successful forecast,” Harlow said. “One could say the volcano behaved very nicely. It performed on schedule.”

Harlow credited his success in part to something that has become ubiquitous in modern society: the personal computer.

Small computers enabled scientists to assimilate enormous amounts of data in time to predict the eruption.

“We could not have done it with the old computers,” said volcanologist Robert Tilling of the Geological Survey, who helped organize the team. Bulky old computers like those used only 10 years ago when Mt. St. Helens erupted in western Washington could not be moved quickly to such a remote location as Mt. Pinatubo, he said.

But new, lightweight computers could.

“We were able to set something up in days that in the past would have taken us weeks or months,” Tilling said.

The end result, both men said, can be measured in terms of lives saved.

About 300 have died, although many more could die in the mudslides that are yet to come.

“It was not an effort that produced a lot of heroes,” Harlow said. “A lot of people just worked very hard to make this happen.”