Cascadia's Fault - Jerry Thompson [106]
Could two plates really slip without producing detectable seismic waves? Dragert and Wang noted that all the GPS tracking stations that moved backward were some distance to landward of the locked part of the zone. So whatever was causing the backward movement had to be happening way down deep, where the rocks were hotter, softer, and less likely to stick together for long periods. They calculated that if an area 30 by 190 miles (50 by 300 km) were to move backward less than an inch (2 cm), the fault would still be generating the energy equivalent of a magnitude 6.7 earthquake. But where was that energy going?
They concluded that these slip events were probably transferring stress “uphill” to the shallower part of the locked zone in “discrete pulses.” So even though nobody could feel them at the surface, each time one of these bizarre reversals happened, it was probably pushing the fault one notch closer to failure—a giant earthquake.
The mystery of “silent slip” took another unexpected turn when Herb Dragert traveled to a science conference in New Zealand, where he learned from Kazushige Obara that something very similar was happening in Japan. “He’s the one that discovered tremors,” said Dragert, “but he didn’t know what they were. He had no idea there was crustal displacement—crustal motion—involved with these. He just kind of said, ‘Hey, these are weird signals that aren’t earthquakes, but they’re not volcanic.’ So he called them deep, non-volcanic seismic tremors.”
After quizzing Obara about the details, Dragert returned to the Pacific Geoscience Centre and started hunting for a connection between tremor and slip. Garry Rogers, by now one of Canada’s top seismologists, had an office just upstairs and down the hall. Dragert provided Rogers with dates when the zigzag patterns had showed up on the GPS. Rogers then dug out boxes of seismograph records from the PGC archives and both were stunned to find a near-perfect match-up.
“I opened the box,” said Rogers, recalling the search, “and there was the tremor!” He showed me the seismogram and pointed to a squiggle of tremor noise that coincided with one of the backward jumps on the GPS. “Herb gave me the next date—I opened the next box—and there was the tremor event. And boy, the hairs just stood up on the backs of our necks.”
“Oh yeah,” Dragert said, beaming. “Yeah, that was exciting. Every time a slip event occurred, there was a huge increase in this background noise, a huge increase. And so it was at that point the eureka came through. We said, ‘Hey, these things are intimately related.’ We knew we had something.”
Condensing it all into a neat little sequence, Rogers explained that the deepest part of the fault—way down where it’s hot and gooey—fails, or slips loose, every fourteen months. When it slips—for a period of about ten days—the GPS antennas on the surface record a backward jump. The land actually recoils as the fault slips, and the seismographs record a silent tremor. Fourteen months’worth of deep tectonic stress is transferred upward into the colder, harder rocks of the part of the fault that has remained stuck. Then, with the stress transferred, the lower part of the fault locks up again and the cycle repeats itself. “It’s a very unique phenomenon,” Rogers said. “We called it episodic tremor and slip, or ETS for short.”
The most fascinating revelation in all of this was that the upper part of the fault—the part that’s been locked in place and building stress ever since the last great Cascadia earthquake more than three hundred years ago—gets another increment of stress added to its load on an incredibly and mysteriously regular basis. Almost like clockwork. In other words, the stress doesn’t just add up gradually until the