Cascadia's Fault - Jerry Thompson [107]
“It’s like adding straws to a camel’s back,” Rogers suggested. “Probably when one of these straws are added it will break the camel’s back.” And we’ll have the megathrust earthquake we’ve all been waiting for. But maybe—just maybe—with this bizarrely regular timing of the ETS events, there might be a way to anticipate that quake.
CHAPTER 19
Turbidite Timeline: Cascadia’s Long and Violent History
“Initially, my colleague Hans Nelson and I didn’t believe it,” said Chris Goldfinger with a smile. “We thought it was probably wrong. It was way too simple. It can’t be right. So we wrote a proposal to the NSF [National Science Foundation] to go prove John Adams wrong.”
This was Goldfinger holding court in the ship’s lounge off the coast of Thailand en route to Sumatra to collect piston cores of ocean mud from landslides triggered by the catastrophic quake and tsunami of Boxing Day 2004. He was setting the scene for those who had not yet experienced the intrigue and frustration of trying to study deadly subduction zones that lie hidden beneath the sea. For Goldfinger and his research partner at Oregon State University, Hans Nelson, the story had begun back in 1985 when John Adams, at the Geological Survey of Canada, wrote a controversial paper based on some old OSU core samples.
The original work in question had been done in 1968 by graduate student Gary Griggs and his thesis advisor, LaVerne Kulm, who (along with Bob Yeats) later became Goldfinger’s thesis advisor as well. The Griggs and Kulm piston cores revealed a series of undersea landslides that had traveled hundreds of miles downhill from the edge of the continental shelf into deeper ocean water along a network of seafloor canyons and channels off the coasts of Washington, Oregon, and northern California. The question in 1970, when the original data were published, was what had caused the landslides.
The core samples had been gouged with steel tubes and plastic pipes from a web of deep-sea channels many miles apart, each showing evidence of thirteen or more landslides. Griggs and Kulm over beers after work one night came up with several possibilities. Either the sediment flows “self-triggered” once the seafloor mud had piled up deep enough to collapse under its own weight. Or big storms with very deep waves might have done it. Or perhaps it was big earthquakes.
Griggs and Kulm knew from looking at the core samples and measuring how thick the layers were and how far they were from the Mazama ash layer that each of the thirteen slides had happened within minutes of each other—all up and down the coast. How could so many turbid flows happen in so many canyons so far apart all at once? If it were mere coincidence, the same coincidence had happened thirteen times, once every six hundred years. In one of a series of papers on the subject, Chris Goldfinger would later refer to this as “coincidence beyond credibility.”
The self-triggering idea seemed the least plausible because it was unlikely that sediment would accumulate at exactly the same rate along hundreds of miles of the continental shelf. Rivers of different size and volume dump differing amounts of debris in different places along the coast at different rates—so how would piles of sand so far apart all collapse into turbidity currents at exactly the same moment? Thirteen times?
Storms with waves big enough to trigger deep-sea landslides also seemed a tad improbable, especially on such a wide geographic scale. They were pretty sure that turbulence from waves generated by winter’s worst storms generally did not reach that far below the surface, so how could they disturb heaps of sand at the heads of steep canyons (where most of these landslides began) that were anywhere from 500 to 1,300 feet (150–400 m) deep? If storm-triggered landslides had happened, the odds were higher that they would have occurred at different times in different places—not all at once, as the core samples showed.
Seismic shockwaves also seemed unlikely culprits