Cascadia's Fault - Jerry Thompson [103]
CHAPTER 18
Episodic Tremor and Slip: Tracking Cascadia with GPS
At four o’clock in the morning on August 31, 1983, the ramp crew at Anchorage International Airport finished refueling a Boeing 747 operated by Korean Air Lines. A few minutes later KAL flight 007 took off on the final leg of its journey from New York to Seoul. As the sun rose the big jet roared across the International Date Line and headed south and west toward its home port. Then, for reasons never verified, the plane allegedly strayed off course over the Sea of Japan west of Sakhalin Island and was shot down by fighter jets from the Soviet Union.
All 269 passengers and crew aboard KAL 007 were killed and for several weeks the Cold War threatened to escalate into something worse. When flight data recorders were recovered from the wreckage, the Soviets refused to release them to international aviation authorities investigating the crash, so it was impossible to prove or disprove the conflicting stories of why the Korean jet had drifted into prohibited airspace. But after the heat of the moment faded, one positive change did come about as a result of the tragedy.
To reduce the odds that a navigational error could ever cause something like this again, President Ronald Reagan ordered the U.S. military to make its NAVSTAR Global Positioning System (GPS) available for civilian use. GPS did not change the world of aviation overnight because there still weren’t enough satellites in orbit in 1983 to make the system fully functional. The precision of the new and still-evolving technology would eventually have a significant effect on everything from tracking animal herds in the wilderness to finding a freeway exit to the nearest pizza parlor—and measuring the drift of continents.
By figuring its distance from at least four of the NAVSTAR satellites, a civilian-made GPS receiver could calculate its position anywhere on the surface of the earth to within roughly a hundred feet (30 m). The system was based on the same principles of trilateration that Jim Savage and Herb Dragert and others had used to measure the shift of mountain peaks in Puget Sound and on Vancouver Island. Their Geodolites had calculated the distance between two brass survey markers by measuring how long it took a laser beam to travel from one peak to another, bounce off a set of reflectors, and return to the starting point. GPS positioning did pretty much the same thing.
By 1992, when Mike Schmidt of the Geological Survey of Canada climbed Mount Logan with one of the slightly clunky but then state-of-the-art units, he was able to measure latitude and longitude with an accuracy of a few millimeters. They could nail the altitude within about ten or fifteen millimeters (0.4–0.6 inches). As Schmidt and Dragert and their geodesy team from the Pacific Geoscience Centre built their array of permanent tracking stations to measure the movement of Vancouver Island, enough new GPS satellites were being launched to make measurements far more accurate than laser beams bounced between mountain tops had been. The enhanced precision would also create one of the most baffling new mysteries Cascadia watchers had ever seen—a mystery once solved that would stun even those who thought they knew what to expect from an active subduction zone.
“We have now established that this site,” said Dragert, pointing at the Albert Head GPS monument (near Victoria on Vancouver Island) behind him, “that particular point right there, is moving at a rate of about six millimeters per year towards Penticton, roughly in a sixtydegree azimuth direction.”
While six millimeters of horizontal creep didn’t sound like much—less than a quarter of an inch—a dose of context changed the picture. Given that the last major earthquake to relieve stress on Cascadia