The Big-Shake, Big-Wave Theory
See how a megaquake would shake out in the Pacific Northwest
Coincidence? Or significant cluster? Some geologists, including Tom Parsons, a U.S. Geological Survey (USGS) geophysicist at the Pacific Coastal and Marine Sciences Center in Menlo Park, California, say it was chance. “Based on the evidence we’ve seen,” he says, “we don’t think that large, global earthquake clusters are anything more than coincidence.” Parsons and his colleague, University of Texas at El Paso seismologist Aaron Velasco, studied 30 years of major quakes (7.0 and larger) to see if they triggered subsequent 5.0-plus quakes. They found none.
Parsons’s study didn’t settle the question. Far from it, in fact. “Make no doubt about it: we’re in the middle of a global cluster of megaquakes,” says Chris Goldfinger, director of the Active Tectonics and Seafloor Mapping Lab at Oregon State University. “Everybody’s noticed it. There are seismologists who say it’s not statistically significant. But it’s happening. The reason it’s downplayed is that nobody’s figured out a mechanism—how and why they’re happening now.”
Goldfinger is no fringe scientist, and what’s especially troubling is that this sort of clustering has been seen before. Six of the world’s 16 largest recorded2 megaquakes happened between 1952 and 1964. More worrying, all six of the ’52–’64 cluster megaquakes occurred around the infamous Ring of Fire, the volcano-dotted arc that traces the edge of the Pacific plate. Of the remaining ten largest megaquakes, five have occurred since 2004. All five were along the Ring of Fire.
“Places that were previously considered safe, well, they’re now being reconsidered,” Goldfinger says.
The Pacific Northwest is at the very top of that list.
THE PROBLEM IS the Cascadia subduction zone, or CSZ. This is an enormous fault that parallels the West Coast for about 740 miles, from the Brooks Peninsula on Vancouver Island to Cape Mendocino in Northern California. It sits about 50 miles off the coast, marking the line where the North American plate meets the Juan de Fuca plate. The CSZ ends where the San Andreas Fault begins, about 100 miles north of San Francisco.
The San Andreas you’re familiar with. It’s a transform fault—one characterized by lateral movement—where the Pacific plate grinds north past the North American plate. The creeping section of the San Andreas, south of the Bay Area, sheds its built-up strain in frequent small earthquakes, like a forest that burns so often it never has the chance to stockpile fuel. The northern and southern ends of the fault aren’t moving, which leads geologists to believe they eventually will lurch, resulting in a quake as large as 8.1.