Cascadia's Fault - Jerry Thompson [13]
But those ruined hospitals and schools had convinced DeVall that we in North America ought to learn something from Mexico’s misery. “It might be prudent on our part to review these buildings to make sure that they will sustain an earthquake,” he said. “You definitely want that hospital not only to be standing up afterwards, but functioning afterwards.” He pointed out that New Zealand, another seismically active area, had already begun this kind of survey of essential infrastructure. The bad news was that, as far as he knew, no seismic inspection was planned for urban regions of the Pacific Northwest.
News reports at the time said the Mexico City rupture had been unexpected. What I wanted to know was why. Aren’t all earthquakes unexpected? “Well,” explained Dieter Weichert, the Mexico disaster “was a bit of a surprise because they had a historic record of two hundred years without a large earthquake. And there was reason for thinking there might not be one.” True, several smaller shocks had damaged Mexico City in the past and yes, there had been ruptures to the north and south of the plate that slipped this time. Through all the previous rumbling and lurching, however, this one segment of the ocean floor had remained quiet and nobody knew why. Could one subsection of the ocean floor be so different from the broken fragments on either side? To me this sounded like the missing clue at the heart of a great mystery.
The unexplained absence of large quakes over a long time seemed to be the soft underbelly of the “aseismic subduction” hypothesis—the conventional wisdom that the jagged edges between some tectonic plates are “not like the others” and don’t get stuck together. Weichert told me that some parts of the ocean floor were younger and hotter rock than the overlying continental crust and perhaps that was why this slab had been aseismic. The interesting twist was that in the aftermath of Mexico City, at least for the scientists gathered in this room, there was reason to believe something might be wrong with conventional wisdom—at least the aseismic part of it.
“People thought that the plate was subducting smoothly,” said Weichert. And yet, quite obviously, it had been stuck and building up strain for two centuries. Seismologist John Adams from the Geological Survey of Canada’s earth sciences lab in Ottawa told us that a tectonic plate very much like the one that wrecked Mexico City—only three times larger—lies just offshore west of Vancouver Island, extending down the outer coastline of Washington, Oregon, and northern California.
“It’s been demonstrated that the Juan de Fuca plate is subducting under North America. But we see no earthquakes along it. And we have a historic record of 150 or 200 years without large earthquakes. Therefore, there are two possibilities: either it is smoothly sliding under North America, doing it continually and without large earthquakes, or strain is building up for a large earthquake—of a type that would only happen every 200 to 500 years.”
Was there an echo in this story? I was hearing for the first time about a slab of Pacific Ocean floor called the Juan de Fuca plate, which runs from Vancouver Island to northern California and is sliding eastward underneath the continental plate of North America. This eight-hundred-mile (1,300 km) fault, the boundary line between two tectonic plates, will later be named the Cascadia Subduction Zone. As I listened to John Adams, it sounded exactly like Dieter Weichert’s description of the plate that got stuck sliding underneath Mexico. Until September 19, both fault systems had been thought of as aseismic.
I didn’t know it at the time, but John Adams had already been studying this mystery for five years. He had published three papers on faults that lie deceptively silent for hundreds of years, including an alpine fracture zone in his home country of New Zealand. Now, in one long breath, he had just warned us that what we saw in Mexico City might also happen in the Pacific Northwest. And Adams