Cascadia's Fault - Jerry Thompson [102]
But there was still one more question to answer. “Kenji’s story seemed to hang together very well,” David Yamaguchi agreed, “but it was still very much a story that was hypothetical. He could not prove conclusively that that wave came from here,” the Pacific Northwest coast of America.
To do that something a little more tangible was needed. And somewhere along the way another eureka moment happened. If radiocarbon would never be precise enough by itself, and if the outer wood of those trees in the ghost forest was too chewed up to give the exact year of death, what about digging deeper? Why not dig up the roots of those standing dead cedars and look for that final year’s growth ring? Would the roots be preserved well enough underground to show a ring for 1699? If so, wouldn’t that suggest the trees did not live long enough to generate a growth ring for 1700? That’s the question Brian Atwater asked himself, and he called on David Yamaguchi’s expertise one more time.
“Brian asked me for leads,” Yamaguchi recalled. “He asked me to choose my top-ten list of trees that I thought gave me the best tree-ring dates—that I was most confident in. And then he went out with a team—a young team—and excavated these trees down to roots where they found that they had intact bark.”
At the ghost forest, Atwater packed his pickup truck full of muddy roots and a gang of exhausted diggers with aching muscles and drove home. Back in Seattle he sanded the samples in his backyard, checked them with a hand lens, and then packed them off to the Lamont-Dougherty lab in New York. “Trees only put on rings between about May and September,” Yamaguchi explained, “and so if they were killed by an earthquake in January of 1700, their outermost rings should be 1699.”
And that’s exactly what they found. Yamaguchi matched the root rings against “the barcode” of rings from the witness trees and confirmed—beyond any reasonable doubt—that the cedars of the ghost forest were killed in the winter of 1700. “Radiocarbon dating can’t do it. Chemical dating can’t do it. It was up to tree rings,” Yamaguchi proclaimed with a wide smile.
All that wet and dirty shovel work had finally paid off. “This finding here at this estuary,” said Atwater, standing on the bank of the Copalis River, “and at the other three estuaries in Washington to the south of us—all having trees dying between August of 1699 and May of 1700—that kind of long reach of coast all dying together at the time of the solitary tsunami in Japan makes a very big earthquake at our Cascadia Subduction Zone more plausible than it was before.”
For Atwater it was pretty clear from the beginning that a rupture had happened here and that a big tsunami had been generated. The evidence had always been there in the geology along the west coast. Cascadia’s fault had indeed written its own history in rock and mud. “It just becomes reinforced by having a human record in Japan that says there’s something corresponding over there—that people observed, and wrote about and that got preserved in those wonderful records.”
Together Atwater, Yamaguchi, Satake, and their colleagues had carried the investigation to what seemed like a logical conclusion. They tracked down the parent earthquake of Japan’s orphan wave of 1700. It had been an exciting, frustrating, and tedious journey that ended with deadly implications.
How did it feel to learn something so dire? “It’s mixed feelings,” admitted Yamaguchi. “As a scientist, it was sort of a eureka feeling—wow, we’ve worked this out! As a resident of Seattle, it’s also sort of a humbling and scary kind of feeling because you realize this is historical evidence for an earthquake of a size this region—and the world—has scarcely seen.”
“Once we admit this earthquake happened in the past,” Satake told me, “we also have to