Cascadia's Fault - Jerry Thompson [105]
How could that happen? Dragert laughed heartily. “We couldn’t explain it. We simply said, ‘That’s life,’ and we went on.”
But Dragert and his colleagues kept watching the incoming data, determined to solve the mystery. Then one day—there it was again. The same apparent glitch, the same displacement. But now they had fourteen continuously monitored GPS stations in the Pacific Northwest, giving them much more precise data.
So Dragert started calling around to find out if any of the other research teams had seen anything this weird. Sure enough, when asked to take a closer look at their data plots, several of them saw a similar kind of reversed movement. Six other GPS antennas in both Canada and the United States had “jumped backward.” In all a cluster of seven adjacent sites strung out across southern Vancouver Island from Ucluelet to Nanaimo and to Victoria and down Puget Sound as far south as Seattle had suddenly slipped backward in what looked like a slow, silent earthquake that took anywhere from six to fifteen days to happen.
And it was definitely a geographic cluster rather than a random scatter. The antenna up at Holberg, on the north end of the island, didn’t move at all. Neither did the towers at Williams Lake or Chilliwack, British Columbia, or Linden, Washington. Only the stations in the middle moved. The stations at the extreme north and south ends of the GPS array didn’t flinch.
Not only that but the slippage had started a few days earlier down in Washington State and then moved gradually toward Vancouver Island—almost like the slow-motion unzipping of a fault. Dragert beamed. “We were saying, ‘Holy crap, this is great! This is absolutely great!’ This not only told us that the signal was real, it told us the signal was constrained to a given area. And it took time to travel from the south to the north.” Basically the signal looked like a ripple moving through rock.
“It was like something migrating underneath our feet,” he said. There was still no logical explanation, however, for the odd, backwardjumping movement of a handful of GPS antennas, so a professional skeptic’s first response was to say it still was probably some kind of mistake. If the GPS monuments are locked in solid rock, then something else must be wrong. Before publishing their data, Dragert and his colleague Kelin Wang had to rule out every conceivable analytical glitch and recalculate all the GPS orbits, just to be absolutely sure what they were seeing was real. Eventually they arrived at the conclusion that the silent backward slip was not a fantasy.
On his computer screen Dragert again traced the upward-slanting line of data with his finger. His hunch was that somehow, way down in the lower part of the subduction zone, a small measure of tectonic stress was being temporarily relieved. The deepest part of the zone had—in his words—come unsprung. It had slipped.
When he expanded the timeline to display several years of continuous movement, the zigzag pattern became even more obvious. The reversals put spikes in a straight-line graph that made it look like a saw blade with evenly spaced, sharp teeth. The thing that struck me about it was the regularity. How could anything in nature be that punctual? Why would it keep coming back every fourteen months?
“Interesting question,” Dragert replied. “We have no idea.”
In a paper published in Science on May 25, 2001, Dragert and Wang released their