When a strong earthquake rocked northern Chile on April 1, scientists were quick with an explanation: It had occurred along a fault where stresses had been building as one of the Earth's crustal plates slowly dipped beneath another. A classic low-angle megathrust event, they called it.
Such an explanation may seem straightforward now, but until well into the 20th century, scientists knew relatively little about the mechanism behind these large seismic events. But that all changed when a devastating quake struck south-central Alaska on March 27, 1964, nearly 50 years to the day before the Chilean quake.
Studies of the great Alaskan quake - undertaken largely by a geologist who, when he began, knew little about seismology - revealed the mechanism by linking the observed changes in the landscape to what was then a theory, plate tectonics.
That theory, that the Earth's top layer consists of large tectonic plates that are moving and colliding, helps explain the formation of mountains, volcanoes and other land features, as well as the occurrence of earthquakes. The Chilean quake, which was measured at magnitude 8.2 and killed at least six people, happened where an oceanic plate, the Nazca, slides beneath a continental one, the South American, at a shallow angle.
But in 1964 plate tectonics had many doubters, and until the Alaska event and the work of the geologist, George Plafker of the U.S. Geological Survey, no one had made the connection between these plate movements and earthquakes.
"Plate tectonics was originally proposed as a kinematic theory - it was about displacements, movements and velocities," said Arthur Lerner-Lam, deputy director of the Lamont-Doherty Earth Observatory, part of Columbia University. "The great accomplishment was to link earthquakes to those movements."
The Alaskan quake, which struck the south-central part of the state late in the afternoon of Good Friday, was of magnitude 9.2, making it still the most powerful earthquake ever recorded in North America, and the second-most powerful in the world after a 1960 earthquake in Chile. The ground shook violently over a huge area for about 4 1/2 minutes. More than 125 people died, Anchorage was heavily damaged, and much of the young state's infrastructure was destroyed.
The quake spawned a tsunami that spread across the Pacific. But most of the deaths occurred in Alaskan coastal towns and villages that were hit by local tsunamis, which were generated by slumping or landslides underwater near the shore. Some areas were inundated even before the shaking stopped, and water heights reached 150 feet or more in some cases. In the port of Valdez, much of the waterfront quickly disappeared as the sediments it was built on turned to jelly and collapsed.
"This was during the Cold War," said Peter J. Haeussler, a geologist with the geological survey in Anchorage. "There were an awful lot of people who thought a nuclear bomb had gone off."
Plafker had previously done geological mapping in Alaska - to better understand the state's resource potential, not its earthquake risk - and was in Seattle at a scientific meeting when the quake occurred. "They needed somebody to get up there and appraise what really happened," said Plafker, who at 85 still goes to the survey's office in Menlo Park, Calif., regularly and is involved in research. The agency sent him there a day later with two other scientists.
"It was the usual kind of thing that happens," he said. "Whoever is nearest and knows something about the area is sent, and is just expected to know everything."
By some estimates, an area two-thirds the size of California had been affected by the quake, and the scientists set about studying the changes. What they found was astounding: barnacle-covered rocks that had risen and were now high and dry.
"At first it wasn't really clear," Plafker said. "But these ones that are uplifted, the barnacles will desiccate and turn white - it's almost like painting a line on the shoreline."
Elsewhere, they saw forests that had dropped so much that the trees were below the high-tide line and were being killed by salt water.
"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," Plafker said. "They know where the tide is."
Overall, 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 8 feet.
"We were trying to figure out whether those ups and downs had anything to do with how it happened," said 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, 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.
Plafker explained the quake by proposing that plates were colliding, as tectonic theory would have it, and at a low angle. One plate was sliding gradually beneath another, creating a long shallow fault zone that rippled a huge area when it slipped. His idea not only accounted for all the uplift and subsidence, it also explained what was happening to the new crust that was being formed in the oceans. Rather than adding to the circumference of the Earth, Plafker said, "it was being stuffed under the continental margins."
Plafker's great contribution was to recognize that the pattern of deformation and tectonic theory went together, Haeussler said. "It became very clear that the only thing that fit the data was this low-angle thrust idea," he said.
Plafker was aided, Haeussler said, by the geography of southern Alaska. The area where the two plates meet - the Pacific plate is sliding, or subducting, beneath the North American - has islands and other landforms where the deformation could be observed. In most areas around the Pacific Rim, where most of the world's megathrust quakes occur, the junction of the two plates is offshore. Usually all of the uplift, and most of the subsidence, occurs on the seafloor, out of view.
That was presumably the case with the April 1 Chilean quake, which was centered in the Pacific about 55 miles northwest of the port of Iquique. Mapping the uplift would probably require a seagoing expedition - something scientists may want to undertake, since the quake did not appear to be strong enough to relieve all the strain in that part of the plate junction, making another large quake likely.
Plafker agreed that the Alaskan geography worked in his favor. So did his initial lack of a full understanding of seismology.
"Basically, I worked in Alaska doing my same kind of thing - regional geology," he said. "All the earthquake stuff just got kind of worked in as a little diversion.
"It was probably to my advantage, my ignorance of the whole game," he added. "And my trust in the rocks and barnacles."
By HENRY FOUNTAIN
The New York Times