Cascadia's Fault - Jerry Thompson [53]
They used heavy piston rigs to gouge core samples that showed a long series of landslides of silt and sand (those turbidity currents that Goldfinger explained years later aboard the Roger Revelle) that had flowed down the canyons and been deposited on top of the older Missoula mud. One of those layers was the infamous Mazama ash, which allowed them to radiocarbon date all the other turbidites and calculate the average amount of time between landslides—which turned out to be roughly 550 years. The timing between slides seemed unusually consistent. Another coincidence? Not likely, yet nobody knew how to explain what appeared to be a recurring cycle. Thirteen turbidite landslides—roughly 500 to 600 years apart. Why?
The core samples also painted a vivid picture of what happened once these debris flows began to move downhill. Griggs and Kulm had calculated that each year about one million cubic feet (28,000 m3) of muck was being carried across the continental shelf by the Columbia and dumped at the head of Willapa Canyon. So a million cubic feet of this stuff piles up every year for five centuries—and then something makes it tumble.
The core samples traced the downhill flow of these currents and showed that some were so high and fast they had splashed over the walls of the main deep-sea channel and spread out sideways as much as 10 miles (17 km). The biggest flows were more than 325 feet (100 m) high and ran more than 400 miles (650 km) down the channel. At approximately 30 feet or more (10 m) per second, one of these swirling plumes would take nearly two full days to run its course—to run out of “downhillness,” as Chris Goldfinger had put it. All of which was amazing enough, although the key question—what had triggered the landslides so regularly for thousands of years—remained unanswered.
Griggs and Kulm offered two possible causes, “periodic earthquakes or severe storms.” They drew no conclusion of their own. As Goldfinger told their story years later to a newer generation of graduate students off the coast of Sumatra, nobody in 1970 would have believed the earthquake hypothesis because there had never been a big subduction shake in the Pacific Northwest in all of recorded history.
The logic sounded straightforward: if these kinds of megathrust events were possible, we would have seen one by now. Surely in 150 years of recorded history one of these monsters would have attacked. There was a well-accepted principle in geology called uniformitarianism, which held that “the present is the key to the past.” Geologic processes that we see happening now are the same processes that happened long ago. Therefore, if we see no great earthquakes in Cascadia now, this subduction zone has probably always been quiet.
Without more data, it was simply easier to believe that some howling great winter storms had triggered all those offshore landslides. But every 550 years? How could anything in nature be so apparently punctual? That part still rankled for those who were suspicious of coincidence. And there was one doubter in particular who just wouldn’t let it go.
Seismologist John Adams, whom I’d met at the Pacific Geoscience Centre on Vancouver Island in 1985 while filming my first earthquake documentary, already knew that plate boundaries could take several centuries to build up enough strain to rupture. Before moving to Cornell University in New York in the late 1970s to work as a postdoctoral research associate, he had completed a study of the Alpine fault, along the southwest coast in his home country of New Zealand. There, instead of subducting, or diving underneath, the Pacific plate was obducting—being forced