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, 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 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 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 4½ 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 is involved in research.
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,” he said. “No one had ever seen this kind of deformation before.”
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, he 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.