Cascadia's Fault - Jerry Thompson [69]
What made the dead trees important was the possibility that they could help pinpoint the year and season of the earthquake that presumably had killed them. The first scientist to try this tactic was David Yamaguchi, who had earned a PhD in forestry from the University of Washington and was working on a project for the USGS to use tree-ring dating as a way of figuring out when Mount St. Helens had erupted prior to 1980. He offered to help Atwater by trying this same technique to date the coastal earthquakes.
In May 1987 they took their first trip together to Willapa Bay. Atwater showed Yamaguchi the stumps of Sitka spruce, the main arboreal victims of great Cascadia ruptures. Yamaguchi chainsawed a few samples, but they didn’t look very promising because tree-ring scientists usually prefer to sample from tree trunks—not stumps. Unfortunately, the spruce trunks had all but rotted away.
The great moment of good fortune came a few months later when they worked their way through the mist and saw for the first time the weather-beaten and moss-draped trunks of western red cedar—what would become known as the Ghost Forest of the Copalis River. “When Dave and I first started working together, we didn’t know that big forests of dead cedar trees existed,” Atwater told me. “Red cedar is more durable. The trunks are still here, standing dead three hundred years after they were killed.”
They figured similar trunks could probably be found along other tidal streams as well, and the more evidence, the better. Yamaguchi came up with the clever idea of placing ads in coastal newspapers, asking local residents if they knew about any more of these ancient beauties. And they did. Cards and letters arrived pointing them toward ghost forests near Grays Harbor, Willapa Bay, and along the Columbia River, a stretch of the Washington coast nearly sixty miles (100 km) long.
Did they all die during the same year and season? They should have if that entire segment of the coast had broken all at once in a single earthquake. Or did they instead die in different years at difference places as a result of a series of smaller earthquakes? Timing was everything.
Yamaguchi’s first effort to establish a time of death for the spruce stumps had failed because, with all the rot, there were not enough rings left to count. But working with red cedars would be different. Step one of the ring-matching process involved finding a group of same-age trees that were at least as old as the ghost forest—and still alive—to establish a baseline growth pattern up to the current date. Wide rings that grow during good years with plenty of rain, for example, should be found in all the trees in the area. The same with narrow rings that grow in years of drought or fires or other kinds of trauma. The patterns should all match year by year, almost like fingerprints of the local climate.
Once this ring pattern was established, Yamaguchi would be able to work backward from the current year’s growth ring and assign specific dates to individual rings in the past to determine in which year the ghost forest cedars died. Later in the summer of 1987 he and Atwater found the live trees they needed for comparison. At the time Weyerhaeuser was harvesting the fringes of a stand of old-growth red cedars that had witnessed—and survived—the great earthquake by inhabiting a hillside above tides, on an island in the middle of Willapa Bay.
“It’s a shame that these trees were being cut,” Yamaguchi commented. “They’re beautiful trees. But we recognized that that was a place where we could gather modern reference samples.”
Each day after the loggers went home, Yamaguchi cut radial slices from the stumps they had left behind. Back at the lab, he drew diagrams with paper and pencil to confirm that they all shared a similar “bar code” of wide and narrow rings. Then he looked at the samples from the various ghost forests. “A number of them had more than two hundred rings in a series,” said