Cascadia's Fault (5 page)

Two other factors added to the toll of damage and destruction as well. Most of the Mexico City seismic regulations were written and passed into law as a result of the 1957 event and did not apply retroactively to thousands of city blocks constructed before that. Tragically, this included many schools and hospitals built decades earlier. “Seeing a lot of hospitals and schools damaged, which are traditionally thought to be areas of refuge in a disaster,” said DeVall, “to find them gone is, to my mind, a serious problem.”

If there was any encouraging news from Mexico, it was that the damage was not evenly spread across the entire urban area, according to Robert Lo, a civil engineer who specializes in soil conditions. Lo even joked that on the day they had arrived in Mexico, roughly three weeks after the temblor, they hadn't seen any significant damage. “We thought we were in the wrong city,” he smiled.

It soon became obvious that the majority of the urban area had been
built on solid ground and that well-engineered buildings were able to ride out the shockwaves with relatively little damage. Ironically, a lot of older buildings not expected to survive—built of masonry and other materials considered quite brittle and susceptible to shockwaves—were still standing because they weren't tall and hadn't resonated with the vibrations coming from this particular quake. The airport remained open throughout the disaster, the metro subway kept running, nearby dams did not collapse, and most of the main waterlines survived the shaking as well. Even the electricity remained on in many neighborhoods.

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 wasn't the only one to say it.

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 Island in 1976, central Vancouver Island in 1946.” It was true, he conceded, “We have never talked about this big subduction earthquake. We knew about the possibility, but certainly with a fifty–fifty chance, you're not going to say there is a big earthquake waiting for us.”

First, I asked myself, who else knew there was even a fifty–fifty chance of a magnitude 8 rupture? Probably nobody except the scientists. Then it occurred to me—okay, so the senior seismologist at the Canadian government's West Coast geoscience laboratory is a cautious man who doesn't want to alarm the public without reasonable and probable cause. I understood that. Yet now, in the aftermath of Mexico City, he was apparently ready to raise the biggest, reddest warning flag I'd ever seen.

“Now you're saying it?” I prompted.

Weichert took the plunge: “We're saying yes, we have to come to grips with this problem. The chance has increased, in our minds, from a fifty–fifty chance to something like a seventy–thirty chance
for
the earthquake to happen within, say, the next two hundred years.”

As a scientist, he really couldn't say for sure when the megathrust might happen—two hundred years from now, or
tonight
—so Weichert had erred on the side of caution. That's what responsible government scientists do. Kaufman and I, however, figured Weichert, Rogers, and Adams had given us a clear signal that the risk level was sufficiently high to justify front-page treatment of the issue.

 

On Sunday, November 3, 1985, I flew from Vancouver to San Francisco en route to the U.S. Geological Survey laboratory at Menlo Park, California. First thing Monday morning we shot an interview with
USGS seismologist William Bakun, who not only reinforced what the Canadian team had told us the previous week but made an even more ominous prediction. He said the Juan de Fuca plate could generate a disaster even larger than the one in Mexico.

“We have to take seriously the possibility that a great earthquake—a very great earthquake, such as the 1960 Chilean earthquake—might occur along the Washington, Oregon, and British Columbia coast,” said Bakun. “We're talking about as big an earthquake as has occurred in historic time—in the world.”

Knowing almost nothing about what happened in Chile, twenty-five years earlier, I again asked for clarification. “Where would that be on the Richter scale?”

“Off it,” he laughed weakly, and then quickly followed with an explanation. A moderate earthquake is defined as magnitude 5.0 to 5.9; strong is 6.0 to 6.9; major is 7.0 to 7.9; and a
great
earthquake registers 8.0 or higher on the Richter scale.

Because the scale is logarithmic, there is a tenfold increase in the amplitude of the shockwaves with each higher whole number on the scale. If a magnitude 4 caused rocks to vibrate and move less than half an inch (1 cm), a magnitude 5 would cause them to move four inches (10 cm). Some studies have estimated that this tenfold increase in the amplitude of the shockwaves would require thirty-two times more energy. So a magnitude 9 would generate thirty-two times more energy than a magnitude 8.

The Mexico City quake was an 8.1 and the 1960 Chilean disaster was a 9.5, the largest temblor ever recorded by modern instruments. That means the Chilean rupture generated more than thirty-two times the energy of the Mexico City event. And here was William Bakun of the USGS telling us to expect the same in the Pacific Northwest.

We had come to Menlo Park primarily because Bakun and his colleague Allan Lindh had recently launched the first high-profile earthquake
prediction
experiment on U.S. soil. The Chinese and Japanese had
both been running prediction studies for several years already, but given their spotty results and the controversial nature of spending money to forecast disaster, this was a bold leap for the USGS. As a journalist I figured the first thing people living in any hazard zone would want to know was:
when
will the Big One finally happen? Now some of America's top scientists were trying to provide an answer.

“We can predict earthquakes, in one sense,” Bakun said, cautiously. “We can identify sections of plate boundaries that will eventually fail in large, damaging earthquakes.” Figuring out
where
the San Andreas fault might break again, or being pretty sure that the Juan de Fuca plate will rip loose from the North America plate
some day,
sounded like important science to me, although I'm pretty sure that's not what most people think of as prediction. Bakun agreed. “We still do not know how to predict earthquakes on a short-term basis. That has turned out to be a very difficult problem, and it's a focus of our ongoing research.”