Cascadia's Fault - Jerry Thompson [65]
This was no gentle or gradual transition zone from one geologic era to another. The peat had a sharp upper boundary caused by an almost instantaneous and probably cataclysmic change in the level of the land and sea. Was it physical proof that the ground had slumped during an earthquake, that the plants of a marsh or forest meadow had been drowned quite suddenly by the incoming tides and possibly buried under the sands of a huge tsunami? Could this finally be Cascadia’s real smoking gun?
Geologists would need to ask this question at many more places than Neah Bay to know for sure. A few samples at a single bay would not be enough to prove the case. But Atwater remained there long enough to jot a note.
“I went over to the post office and sent Tom Heaton a postcard,” he laughed. “I thought, ah—he’s the one person in the universe who’d be especially interested in this result.” Instead of the uplifted beach terraces Atwater had expected to find, instead of confirming the Ando and Balazs hypothesis, here he was, mud spattered and dripping, with evidence that Heaton was right.
In April and May 1986, Atwater took day trips from Seattle to other spots on Washington’s outer coast. “I visited each of the four big estuaries in southern Washington,” he explained. “Copalis, the next one to the south, Grays Harbor, Willapa Bay, and finally the Columbia River down at the Oregon border. And each of these streams had the same signature of abrupt lowering of land and marshes and forests that had been at or above high-tide level, then got abruptly dropped down.”
During the summer of 1986 Atwater and two co-workers uncovered evidence of at least six different events—presumably six different earthquakes—that had each caused about three feet or so of down-drop. The distant geographic spacing along the Washington shore could be evidence that the quakes were big. If the coastline had slumped in river mouths and bays that were many miles apart, the quakes must have been big. But it would take further digging along the Oregon coast and up the west side of Vancouver Island to say just how big.
Were they magnitude 8s? Or magnitude 9s? It was too early to tell. Judging by what he’d found thus far in the four widely separated river estuaries, Atwater was pretty sure they were bigger than anything recorded in Washington’s written history. To be on the safe side, he still exercised the normal scientific caution with careful wording in what would soon be considered a breakthrough research paper, published in Science on May 22, 1987.
“Intertidal mud has buried extensive, well-vegetated lowlands in westernmost Washington at least six times in the past 7,000 years,” he declared in his opening line. “Anomalous sheets of sand atop at least three of the buried lowlands suggest that tsunamis resulted from the same events that caused the subsidence. These events may have been great earthquakes from the subduction zone between the Juan de Fuca and North America plates.”
May have been ... Until other scientists read the paper, studied the data, and agreed that Atwater’s conclusions were valid, these new discoveries were still not established or accepted as facts beyond a reasonable doubt. The paper would nevertheless create quite a stir. With the possible exception of those turbidite samples recovered by Griggs and Kulm from offshore landslides that also may have been triggered by temblors (a hypothesis still doubted at the time), Atwater’s sunken peat layers were considered the first direct evidence of large subduction earthquakes in the Pacific Northwest. What happened in Alaska and Chile had happened here too. And would probably happen again.
CHAPTER 12
Cedars, Peat, and Turbidites: A Tipping Point at Monmouth
After more than ten years