Cascadia's Fault - Jerry Thompson [63]
The key study by Jim Savage and his colleagues was hotly debated. Did the data really prove that mountain peaks on opposite sides of Puget Sound were being squeezed closer together? The increments of movement measured were quite small and the Geodolite—the best available technology at the time—was being pushed to the limits of its accuracy. Was the signal Savage detected “robust,” or was it just a few spikes in the white noise?
“Savage concluded that the mountaintops were probably moving closer together in the direction of plate convergence,” Atwater said, “and he took that as evidence that the subduction zone really is locked. Some people dismissed it. And others said, ‘No, it’s a real signal.’”
Atwater told me he decided to focus personally on the vertical motion predicted by Ando. “When they said the Pacific Coast was rising three millimeters a year relative to Puget Sound, I said, ‘Ah ha! Three meters per thousand!’ I think: You know, those are large amounts—a thirty-foot difference in the level of the shoreline.” That was the kind of motion he thought he’d be able to see in the geology. He would go out to the coast and find out whether a three-thousand-year-old shoreline was now thirty feet (9 m) above sea level, simple as that.
Making the debate more interesting, however, were the seismic data that the University of Washington’s own Robert Crosson had published more than a decade earlier, in 1972. His fault plane solutions for small to moderate temblors occurring in the upper plate near Seattle suggested a north–south compression—directly contradicting Savage’s east–west motion. Crosson had concluded that the eastward subduction of the Juan de Fuca plate had stopped.
Although not as widely circulated, there was also the January 1983 PhD thesis of Garry Rogers, who would go on to become a leading seismologist at the Pacific Geoscience Centre in Sidney, British Columbia. Rogers had constructed a theoretical model to explain how there could be north–south ruptures (roughly parallel to the Washington coastline) and east–west compression at the same time. He noted that the underlying oceanic plate seemed to be deforming as it turned a corner beneath Puget Sound. Instead of continuing toward the east as it had for millions of years, the Juan de Fuca plate had apparently started rotating at an oblique angle to the coast, shifting to a northeasterly movement. As it turned, it created a bulge in the overlying plate that became the Olympic Mountains in Washington State. Stress in the lower plate increased until the rock began to collapse in on itself. This partial collapse of the oceanic slab did not, however, relieve all of the stress caused by making the turn. Resistance to movement along the fault plane caused rocks to shear in some places. Imagine a deck of cards spread horizontally by the force of your hands. Once the stress exceeded the strength of the rock, it fractured in cracks that ran parallel to the coast: the north–south fault planes that Crosson had detected.
Meantime, the continued movement of the lower plate perpendicular to the coast would still cause northeasterly compression, and either the strain would be stored in the rocks until they failed in very large earthquakes or the plates would creep past each other aseismically. The trouble for Rogers and the others who wanted to see this question answered once and for all was that the seismic data seemed to suggest the aseismic option. No earthquakes in recorded history.
The only evidence