Cascadia's Fault - Jerry Thompson [97]
When they compared the rings of “witness trees” with rings from the ghost forest, they knew the forest floor had sunk beneath the tides some time in the late seventeenth century. “The closest I could get was 1691 as a limiting date for the earthquake,” Yamaguchi told me. “The earthquake had happened some time shortly after then, but I didn’t know how many years afterward.” Insects, rot, and roughly three hundred North Pacific winters had pitted and peeled away too much of the outer bark of these standing dead cedars. The final few years of growth rings—the crucial last clues—were missing.
A dating technology was being tested at the University of Washington, however, and it might pick up where the tree rings left off. Atwater was keen to try it. “Brian realized that he could use the preliminary 1691 date as a tool to help him do something called high-precision radiocarbon dating,” Yamaguchi continued. Wood samples were locked for weeks inside shielded atom counters and the resulting timeline was accurate to “the 95 percent confidence interval.”
Like the weather-beaten tree trunks, though, radiocarbon technology fell slightly short of the mark. It moved the goalposts closer together without truly scoring a victory. The window had shrunk a little more, putting Cascadia’s most recent event some time between 1690 and 1720, but there was still no certainty about whether a single, giant rupture or a “swift series of merely great” earthquakes had done the job. Nevertheless, when news of this tighter timeline became public, it created a buzz. And a scientist from Japan had one of those eureka moments that researchers everywhere dream of.
CHAPTER 17
The Orphan Tsunami: Final Proof of Cascadia’s Last Rupture
At a science conference in Marshall, California, in September 1994, Kenji Satake was having lunch with Alan Nelson of the USGS when Nelson mentioned the frustration he and others were having with radiocarbon dating. Nelson and eleven colleagues were putting the final touches on a paper that was meant to collect and focus all the physical evidence for past quakes on Cascadia’s fault in a single volume. Satake’s specialty was subduction zones and tsunamis. He had studied the effects of the Okushiri disaster in Japan the previous year and was very interested to hear the latest news about Cascadia. Especially Atwater and Yamaguchi’s work with tree rings.
“I did not know much about the calibration technique of radiocarbon dating using tree-ring records,” Satake told me. “I met Alan. He explained to me the basics of the technique and I was very much impressed by the small uncertainty—only a few decades.” Something about those dates—a time span from 1690 to 1720 for Cascadia’s last rupture—caused Satake to pay special attention to this latest study.
He was working at the University of Michigan as an assistant professor. Before that he’d completed a two-year term as postdoctoral researcher at Caltech, where he had studied Cascadia’s quake and tsunami potential with Tom Heaton. Heaton, Hiroo Kanamori, and Stephen Hartzell were then banging the drum about similarities between Cascadia and Chile and other big subduction zones. But in 1994 there was still not enough unequivocal evidence of past ruptures in the Pacific Northwest for the case to be ironclad.
Later that same year Satake made a point of meeting Brian Atwater on a field trip to the coast as part of the Geological Society of America’s fall meeting. Satake would soon depart the United States for a job with the Geological Survey of Japan and was keen to see the Cascadia evidence first-hand before he left. Curiosity piqued by Atwater, Satake wanted to see more.
“He spent some time with me, down in Humboldt County,” confirmed Gary Carver. His recollection