Cascadia's Fault - Jerry Thompson [93]
Over the next weeks and months the OSU team discovered nine more strike-slip fractures off the Washington and Oregon coast: cracks that penetrated both the Juan de Fuca plate and the overriding continental plate. The ocean floor, at the point where it dives beneath the continent, was buckled, crushed, and deformed into cracks and folds very much like the mangled terrain Gary Carver and his colleagues had found onshore around Humboldt Bay, just down the coast in California.
This was literally and figuratively the cutting edge—the point of impact between two tectonic plates. For Chris Goldfinger the bottom line in this wealth of data was that the newfound fractures and deformations in the crust might be telling us something about the width of the locked zone and also about the kinds of rough spots—the asperities—that the down-going oceanic plate could get stuck on, preventing the entire subduction zone from rupturing all at once.
Goldfinger and another colleague, Robert McCaffrey, published their findings in Science on February 10, 1995, concluding that a series of “smaller” earthquakes—perhaps magnitude 8s along the subduction zone or even magnitude 7s in these newfound cracks in the upper plate—could account for pretty much all the Cascadia geologic evidence to date. Since nobody really knew how big a quake had to be to drown the tide marshes that Brian Atwater had found, since nobody really knew how big a shockwave had to be to trigger the offshore landslides that John Adams had written about, it was entirely possible that smaller ruptures could have done all the damage discovered on this coast.
With a fractured and buckled outer edge, the North America plate might be incapable of magnitude 9s simply because it couldn’t build up and store enough strain for a long enough period to generate a fullzone rupture. That was the “good news.” The decades of terror scenario seemed to Goldfinger and McCaffrey more likely than a magnitude 9 apocalypse. They suggested that the seismic hazard and public safety implications of Cascadia’s fault did not look quite as daunting as they had before.
In the fall of 1995, however, an international team of mud, marsh, and sand diggers thought enough evidence had accumulated to suggest quite the opposite—that a magnitude 9, full-length rupture had occurred along Cascadia’s fault. And they were willing to speculate that it happened roughly three hundred years ago. A dozen scientists from federal, state, provincial, and university research labs on both sides of the Canada–U.S. border got together and jointly published a summary of all their separate bits and pieces of evidence for Cascadia’s most recent quake.
From John Clague and Peter Bobrowsky’s samples of dead plants from sunken marshes that had been quickly covered by sheets of sand left behind by tsunami waves sweeping across the western beaches of Vancouver Island near Tofino and Ucluelet, to Gary Carver’s similar evidence of drowned trees in northern California, the picture looked remarkably consistent all the way down the coast. What Brian Atwater had found in estuaries along the Washington shore, Alan Nelson and his USGS colleagues had found in Oregon. The dozen scientists spent considerable effort—including eighty-five new radiocarbon-dated samples—to obtain the most accurate timeline possible. They found that all the ghost forests and marsh plants had been killed at roughly the same time as the land dropped down and was covered by tsunami sand—roughly three centuries ago.
Given the long distance between Tofino, British Columbia, and Humboldt County, California, the dozen “marsh jerks” (as they jokingly called themselves after the jerky spikes in a sawtooth graph denoting the quake-induced sinking of land) said it was all Cascadia’s fault. The plate boundary was the only fault common to all the far-flung sites. They reported their findings in a paper published