Cascadia's Fault - Jerry Thompson [58]
The instrument he hauled out of its foam-padded metal case was about twice the size of that old breadbox your grandmother used to have. It was a Rangemaster III, with a bright orange housing, knurled brass knobs, a black instrument panel, and a self-contained digital computer that flashed the distance calculation to an LED readout. State of the art in 1976, it still appeared in perfect working order after many years in a storage locker. The helicopter pilot then flew across to Mount Whymper, the next nearest mountain in the hazy distance, where he landed and set up a reflector box on a tripod directly above the survey marker on that peak.
Back on Mount Landalt, Schmidt peered through his viewfinder on the Rangemaster and used a portable radio to call the pilot, who then tilted the bank of mirrors on Mount Whymper until Schmidt got a return signal—a reflection of the laser beam—on Mount Landalt. Scintillating shafts of cherry-red light bounced off a dozen mirrored prisms in the reflector box. The Rangemaster then performed its magic: a quick calculation of the time it had taken the beam to shoot across the valley from one peak to the other and bounce back. With laser gear like this they could measure the distance between peaks up to twenty-five miles (40 km) apart and be accurate within fractions of an inch. A significant improvement, but there was still another problem to solve.
With nothing more sophisticated than pack horses, climbing gear, and transits (telescopic instruments mounted on tripods for measuring precise, horizontal angles between objects that are far away) the British Columbia survey crew back in 1937 could accurately plot the geometry between a series of peaks. But computing the exact distances between mountains was extremely difficult. In those days most surveyors still used sixty-six-foot chains (eighty chains to the mile, or about fifty to the kilometer) to establish a baseline measurement. If you know the angles and the length of one side of a triangle (the baseline), you can calculate the lengths of the other two sides. But because of jagged mountain terrain and dense bush, the distances calculated and printed on the old maps of Vancouver Island were too imprecise to work as reference points in a modern-day study of minute tectonic creep along a fault. The new laser equipment that became available in the 1970s changed everything.
Dragert and Schmidt and their team used the laser Rangemaster to redraw the original triangles between the peaks and see whether they had changed. With the Rangemaster’s new measurements they knew exactly how long each leg of the triangle was and could then calculate the precise angles between the peaks. Then they compared the new triangles to the old ones from 1937. When they did—bingo!—they saw that the angles had changed, which meant that at least some of the peaks had moved since 1937. “We proved that the margin was deforming,” said Dragert. “The mountains were indeed being squeezed landward,” ever closer to the continental mainland.
He pointed to a specific example on the old map, a triangle of dark lines drawn by the original surveyors. The new laser triangle clearly did not match the old one. It was bent out of shape because the mountain closest to the west coast—Mount Grey, a 4,570-foot (1,390 m) peak about halfway up the Alberni Inlet—had been shoved eastward nearly eight inches (20 cm) in less than forty years. Not a huge amount, by the sound of it, but imagine the entire island coastline, hundreds of miles’ worth of mountain rock, being pushed horizontally like that. When I thought of another few inches of horizontal movement each year—for 310 years since the last Cascadia earthquake—the total amount of accumulated strain built up along the fault was mind-boggling.
“And that was totally consistent with our expectation,” said Dragert, “that if the subduction zone is locked, we have to see deformation.” He poked his finger emphatically at the map again. “And indeed we saw the deformation which was the final nail saying, ‘Look—this is not slipping smoothly. This