Cascadia's Fault - Jerry Thompson [14]
Garry Rogers was the young PGC seismologist whose job it was to monitor the two hundred or so temblors that rattle through southwestern British Columbia every year (only two or three of which are strong enough to be felt). He told us the historical lack of huge megathrust events off the west coast of Vancouver Island could be very misleading.
“The implication,” said Rogers with focused intensity, “is that the possibility for very large earthquakes—the kind that occurred in Mexico just recently—does exist on the west coast of Canada. The problem is that in the 150 short years that we’ve been here, we have not seen any examples of earthquakes on our subduction zone. Not even small ones.”
He explained that those two hundred rumbles occur because of stress within the overlying crustal plate, relatively close to the surface. The much larger shock—if it does happen—would occur almost forty miles (65 km) below ground along the length of the subduction zone. Like Adams, Garry Rogers thought the absence of deep Juan de Fuca quakes put seismologists in a quandary.
“At the moment, we just don’t know,” he said. “It’s a subject of scientific debate. But if we compare other areas around the world that are very similar to our subduction zone, we find that we are the only one that has not had large earthquakes.”
For seismologists in 1985 it was hard to imagine why the Juan de Fuca plate (or the Cocos plate in Mexico) would be special—the only place on the planet where two plates glide past each other trouble free. How could this not be like the dangerous and deadly subduction faults off the coasts of Alaska, Chile, and Japan? Although Rogers didn’t seem like a gambler, he was willing to speculate.
“A more likely scenario, comparing it with other zones, is that we are capable of large earthquakes but with very long intervals in between them,” he said. The long quiet history of Juan de Fuca could mean “it’s stuck and one of these days we’re gonna have a monster earthquake like Mexico had.”
If the fault were “stuck,” I wondered, could the build-up be measured and—if you could see the stress increasing—would it be possible to predict the next quake? “It may be,” answered Rogers. “And, in fact, one rather suspects it should be, because before such a large earthquake a tremendous amount of strain is stored up. We might be able to detect a deformation like that. In fact, they can see this kind of thing in Japan since their last big earthquake—deformation going on.”
Evidently rocks bend and tilt under stress and there are changes in electrical signals coming from the earth, all of which could be monitored. Rogers described prediction as a dark art that was still many years away from success, but his point was that there are things we could and should be doing to confirm or deny the possibility of large subduction earthquakes off the Pacific Northwest coast.
It turned out that John Adams was already doing exactly that kind of research. Less than a year earlier, while working at Cornell University in New York State, he had published the first in a series of papers with new data showing that the coastal mountains of Washington and Oregon were in fact being bent and tilted landward, probably by the force of plate tectonics.
A magnitude 8 or higher, here on my West Coast—really? I’d been living in Vancouver nearly ten years at that point and had never heard anything about a monster shockwave. Not a word of it. How could I, a working journalist covering British Columbia for most of a decade, have missed a blockbuster story like that? Well, it turns out the banner headline was being written in the present tense at that very moment. This news had not escaped the confines of laboratory walls until now.
With a quickening pulse, I turned back to Dieter Weichert and asked for context. He recited what sounded like a well-rehearsed list of the most recent moderate-size temblors in the Pacific Northwest: “For ten years, we’ve always warned people that there are earthquakes—in Seattle-Tacoma [in 1965], under Pender