Fifty years ago, on March 27, 1964, the Pacific plate scraped under the North American plate beneath Alaska's Prince William Sound, triggering the second-largest earthquake ever recorded. In Anchorage, the Turnagain Heights neighborhood slid into the salt water of Knik Arm. A great wave triggered by landslides destroyed the town of Valdez — after the ground on which the town stood had already liquified — and a tsunami triggered by the quake itself killed 122 people, streteching as far south as Crescent City, California.
But the 9.2 magnitude quake itself was in Alaska; in most people's minds, tsunamis and big quakes were things that happened somewhere else, not the Pacific Northwest. Plate tectonics wasn't something that happened at all.
Obviously, a lot has changed. Valdez rebuilt across the harbor on higher ground that now holds the oil port from which the ill-fated Exxon Valdez set sail just 25 years ago in 1989. Low-lying coastal communities now sport tsunami escape route signs, although that is less reassuring than it might be if you're somewhere with no high ground. And it has become commonplace to explain earthquakes as the result of tectonic plates grinding together.
Even then, the idea wasn't new. A German scientist named Alfred Wegener had introduced it before World War One. But his theory and subsequent elaborations of it, attracted relatively little attention until the 1960s. University of Washington research professor of earth and space sciences emeritus Steven Malone explains that the Alaska quake was the first to be seen in the intellectual context of plate tectonics. It took place "sort of at the dawn of plate tectonics theory." And a lot of people didn't want to see it in that context.
"There was tremendous resistance to having that recognized as a subduction earthquake," says William Steele, director of outreach and information services at the UW's Pacific Northwest Seismic Network.
The year 1964 wasn't exactly the stone age. NASA had shot John Glenn into earth orbit two years before. And yet modern seismic monitoring was very new. Malone says that the Alaska quake was "the largest eqarthquake at that time that was recorded by [relatively] modern seismographs." There are better instruments now, but "the difference between [what existed in 1964] and not having any records for big earthquakes was huge." Malone says the resulting data "pointed us in a lot of new directions."
People had a lot to think about. "For the first time, the United States [outside Hawaii or the Aleutians] got clobbered," Steel says. It got people to reevaluate their assumptions. Until then, "I don't think they really thought that it could happen here."
In places, the Alaskan coastline sank 6 feet. The trees that had grown there above the high tide line were flooded. Eventually, they died.
By the late 1980s, researchers had found similar signs of subsidence in estuaries along the Washington coast. Gray stumps made it clear that whole forests of cedar had expired there at some point. Did their ghostly presence signal a quake and tsunami somewhere back in Washington/s history? And if so, when?
The trees didn't die in the tsunami. They died because the quake had fundamentally altered the land on which they grew. "What killed the trees in Alaska, and at Cascadia as well," explains U.S. Geological Survey research scientist and UW affiliate professor Brian Atwater, "was the lowering of land — the subsidence that allowed post-earthquake tides to cover the forest floors too often for the trees to survive."
The evidence of a great Cascadia earthquake and wave had been there all along, but until recently, no one had known how to interpret it. “The oral traditions of Cascadia’s native peoples . . . tell of flooding from the sea,” Atwater and colleagues write in "The Orphan Tsunami of 1700." People who lived along the coast “faced horrific tsunamis, like the one implied by the story of a sea flood that swept canoes into trees. Survivors then watched tides relentlessly cover their subsided, bayside fishing camps. Several archaeological sites tell wordlessly of the waves and tides that overran them. Each lies buried beneath tidal mud.”
Like what you just read? Support high quality local journalism. Become a member of Crosscut today!