Cascadia's Fault - Jerry Thompson [104]
In other words, if Cascadia had ruptured right then and there—after three hundred years of stress accumulation—the entire city of Victoria would have rattled, rumbled, and slid sideways nearly six feet (1.8 m) back toward the west as the strain came out of the rocks. The longer it takes the zone to fail, the more stress there will be—ready to snap in the next great rupture. “We proved that the margin was deforming,” said Dragert emphatically. “The mountains were indeed being squeezed landward. So it’s not a hypothetical problem; it’s a real problem. We are gaining strain energy all to be ultimately released by a large megathrust earthquake.”
The switch from lasers to GPS technology made it possible to track tectonic movement 24/7 without having to wait for budget approval to hire another helicopter. “You didn’t have to fly to the mountaintop—you just set up the instrument,” Dragert enthused, “a totally automated instrument that told you what its position was day after day after day. It worked under any weather conditions. Much cheaper. I mean we basically had to give up [laser] trilateration because it turned out to be too expensive,” he explained. “With this new technology we could measure even longer baseline distances—not just fifty kilometers, hundreds of kilometers—to a precision of one or two millimeters.”
The Pentagon’s new technology gave scientists a close-up view of tectonic motion that had been inconceivable thirty years earlier, when the debate about continental drift began in earnest. GPS proved not only that plates were shifting but how fast, in which directions, and how high the outer coastlines were rising, bending, and buckling. Before another year had passed, the satellites would also reveal to Dragert and his colleagues the next big secret of subduction.
“Without GPS technology we would never, never have observed ‘silent slip,’” he said, with fingers putting quotation marks around the term. The first time he spotted a tiny “backward movement” of Vancouver Island, it was so subtle, so apparently insignificant, it just had to be a glitch. At least that’s what Dragert thought at first.
He remembered the bafflement as if it were yesterday. On a desktop computer screen he opened a file that showed the data from the station nearest Victoria. “We were kind of looking back over the last four years of data in ’96. And we said, ‘Gee, there’s this funny offset,’ right around October ’94. So, what’s going on?” His finger traced the steady, almost straight-line movement of the GPS antenna at Albert Head.
As expected, it had been creeping relentlessly east day by day for months, when all of a sudden there was a sawtooth zigzag in the data. The Albert Head antenna had apparently switched direction and doubled back in the opposite direction for about ten days. A concrete tower built into the solid bedrock of Vancouver Island had inexplicably stopped moving toward Penticton and was temporarily sliding back west. After the ten days, it started moving east again.
Because only one of the four existing stations had displayed this maverick behavior, Dragert convinced himself the data had to be wrong. “So I said, ‘Okay, most likely our monument is unstable.’ For some reason, even though this is in concrete—we have rebar drilled into the bedrock, so there’s a very good coupling to the bedrock, and it’s very competent bedrock, it’s not fractured or weathered—so, strange as it may seem, maybe we just didn’t see something. Maybe there’s a fracture zone somewhere that we were unaware of and it’s tilted our monument.”
To find out what had gone wrong, Dragert and his