Cascadia's Fault - Jerry Thompson [109]
Goldfinger had gone to sea in 1992 and discovered the underwater Elvis, a man-with-a-guitar–shaped mound of folded and fractured ocean sediment, along with eight other new faults in the upper plate along the continental shelf. He wrote that the rough edges of these fractures may limit the size of Cascadia’s big subduction quakes by inhibiting the build-up of strain energy and concluded that Cascadia may be the type of subduction zone at which magnitude 9 events “do not occur.” Without magnitude 9 ruptures, the Adams hypothesis had to be wrong. Smaller jolts just wouldn’t do what the hypothesis demanded.
But a subsequent research voyage in 1999 turned things around for Goldfinger and Nelson. The evidence in favor of big landslides was very obvious in the offshore mud when examined close up. Not only did they confirm the same thirteen turbidites along 375 miles (600 km) of coastline, but they ventured farther north to the Nootka fault, at the upper end of the Juan de Fuca plate off Vancouver Island, and farther south all the way down to Cape Mendocino in California. Along the way they collected nearly a hundred new cores and added several discoveries of their own, extending the count from thirteen to eighteen events—presumably large quakes—and extending the timeline back to the end of the last ice age, roughly ten thousand years ago. They saw these dark, sandy landslide scars on the ocean floor as “earthquake proxies,” the telltale markers of Cascadia’s long and violent past.
In the lounge aboard the Roger Revelle, Goldfinger explained the challenge of adding the five new turbidites to the series of thirteen already established. The problem with the new samples was that they came from offshore river channels that were not physically connected to the network of channels flowing primarily from the Columbia River and the Strait of Juan de Fuca up the coast. The new evidence was found in a completely separate, unrelated grid of outflow channels from Barclay Canyon, off Vancouver Island, and from the Rogue River Canyon, midway down the Oregon coast.
How would it be possible to know whether the five additional turbidite flows had happened all the way down the coast at roughly the same time, in the same kind of synchronous gushes that Adams noticed in the main Cascadia channel? The question could be answered using oilfield techniques well known to many of the faculty at OSU, where petroleum geology was a significant part of the academic program. As Chris Goldfinger likes to tell it, oil drillers have been doing this sort of thing for years.
Again he pointed to the overhead map, zeroing in on the offshore region near the Oregon–California border. “These channel systems don’t have the same sources and they’re even further apart [than the channels that Adams studied]. They don’t have anything in common,” he said. Oregon’s Rogue River, for example, flows directly from Crater Lake—the former volcano Mount Mazama—to the sea with no downstream connection to the Cascadia channel. There is no confluence of canyon heads and tributaries that would physically link the Rogue turbidites with the others farther north.
The stratigraphic patterns in all the samples, however, did look very nearly identical. The relative age, thickness, and spacing of the alternating bands of turbidite sand, silt, and gray-green ocean mud were the fingerprints of Cascadia’s history. Goldfinger and Nelson used a process known as wiggle-matching to make a detailed, layer-by-layer examination of all the minute gradations of muck that had been laid down on the ocean floor.
“Correlating the wiggles” in core samples from the entire length of the Cascadia Subduction Zone took quite a while, but the match-up was pretty convincing. “Even though this hadn’t been used before in paleoseismology,” Goldfinger said, “this is basic, subsurface oilfield geology. This is how oil deposits are