“You can always find something that shows whether the area went up or down,” he said.

They were there for about a week on that first trip, and what they could not map themselves they learned by asking around. “Places where you don’t have any info, the next best thing is to ask the fishermen, and especially the clammers,” Dr. Plafker said. “They know where the tide is.”

Over all, an extensive stretch of the coast, including islands in Prince William Sound, had been lifted as much as 38 feet in some places, while along much of the Kenai Peninsula and Kodiak Island, a large area had subsided up to eight feet.

“We were trying to figure out whether those ups and downs had anything to do with how it happened,” said Dr. Plafker, who returned to the state for more field work that summer. “No one had ever seen this kind of deformation before.”

At the time, tectonic theory was being vigorously debated, as was evidence that the seafloor was spreading as new crust formed in the middle of the oceans. The question was what happened to this new crust; one theory was that the whole planet was growing slightly.

Many ideas, including plate movement, about what caused the quake were floated. One of the most prevalent explanations suggested that the quake had occurred where one plate was rotating past another. But if this were the case, Dr. Plafker said, there would have been evidence of a large vertical fault somewhere in the vast expanse of land that was deformed by the earthquake.

He knew from his fieldwork that such evidence did not exist. “I had the advantage of seeing the rocks,” he said.