Cascadia's Fault - Jerry Thompson [76]
The decay of carbon atoms gave them dates that were accurate to within a few decades, but that wasn’t good enough to tell whether all the marshes and estuaries had been buried at the same time by the same great earthquake and wave. Tree-ring dating would eventually help narrow the timeline, but now there was another wrinkle. The build-up of stress deformation along the coast did not seem to be occurring at the same rate everywhere.
A newer, more accurate releveling survey had been done along U.S. Highway 101 from Crescent City, California, all the way up the coast to Neah Bay, Washington. When Clifton Mitchell and Ray Weldon of the University of Oregon took a closer look at the numbers, they noticed that over the past fifty years the southern end of Oregon and the northwestern tip of Washington had been rising about an inch or more every decade.
The complication was that the area around Newport, about halfway up the Oregon coast, and around Grays Harbor, halfway up the Washington shore, did not appear to be rising at all. To Mitchell and Weldon this suggested that some points along the subduction zone might have asperities—rough spots where the two plates tended to hang up or get temporarily locked together. Some parts of the zone—like Newport and Grays Harbor—could be moving without seismic strain build-up while others were locked and loaded for a big rupture. Which sounded like evidence in favor of Carver’s “decades of terror” scenario.
Atwater still wasn’t sure. He looked at proof of sunken marshes from Vancouver Island all the way down to southern Oregon and possibly including Carver’s own tsunami sands near Humboldt Bay in California and thought the magnitude 9 scenario was still viable. He knew he’d have to find some way with tree rings—or whatever—to narrow down the dates. If all those places sank at the same time, it must have been a magnitude 9 or larger and exact dates would prove it.
Carver, on the other hand, could point to the big Nankai Subduction Zone off Japan, which had broken in two magnitude 8 events (1944 and 1946) during and after World War II. If they had been dated with radiocarbon there would have been no way to tell they were separate quakes. Radiocarbon might get you within a decade, but certainly not within two years. Dating the ghost forest’s time of death became all the more important.
Here along the west coast, Carver could imagine a sequence similar to the one in Japan. The Gorda plate could break loose first, then the main Juan de Fuca segment a few years later, with the Explorer and Winona segments up north of Vancouver Island after that. Whole decades of terror instead of one big event.
Garry Rogers at the Pacific Geoscience Centre had already written about the possibility of separate ruptures with differing magnitudes. Roy Hyndman and Kelin Wang, also at PGC, were working on a new paper that used temperature variations in the crust to calculate how much of the subduction zone might be stuck and which parts were sliding smoothly due to higher temperatures (near the melting point of rock) deep underground. Their preliminary conclusion was in favor of the magnitude 9 scenario. But soon a new line of offshore evidence from the team at Oregon State seemed to tilt again in favor of the segmented rupture pattern.
With data and new ideas coming together from several directions at once, it was exactly the kind of movable feast that scientists love to sink their teeth into.
In the summer of 1989 a new batch of data from Canada was released, adding weight to what Jim Savage had said earlier about the big squeeze in Puget Sound. Herb Dragert and Mike Lisowski had finally amassed enough evidence from laser beams bouncing off mirrors on the mountain peaks of Vancouver Island to release the numbers at a meeting of the International Association of Geodesy held in Edinburgh, Scotland, on August 3, 1989.
A dozen years’worth of repeated geodetic surveys, along with long-term tidal monitoring along the island coast, had revealed exactly the same kind of squeezed