Cascadia's Fault - Jerry Thompson [64]
Then, to complicate the contradictions, new data were released later in 1983 from Craig Weaver and Stewart Smith’s new seismic array, installed after the Mount St. Helens eruption. It revealed a fracture zone in which “maximum compression is northeast, approximately parallel with the direction of plate convergence.” Like the Savage data, this was interpreted as “evidence for locked subduction.” It seemed a tipping point might soon be reached.
But it was Tom Heaton’s talk at the USGS conference in Seattle about the bigger picture—Cascadia’s similarities with other dangerous subduction zones—that drew sparks and generated most of the attention. “He was out there voicing some views that were somewhat unpopular,” said Atwater. “It did serve as something of a lightning rod.” Thus the quest for proof, for convincing evidence, for resolution, became all the more enticing.
The conference made news all over the Pacific Northwest. At the Pacific Geoscience Centre about a week later, when the CBC camera crew and I showed up to shoot our very first Cascadia documentary, Dieter Weichert, who was running the seismology lab at the time, took the opportunity to go public with his own bold statement. He stuck his neck out and told us on camera that Canada’s team of experts, just returned from Mexico City, had decided the possibility of a great subduction earthquake was real.
The Geological Survey of Canada was revising the risk assessment for Vancouver and Victoria and the west side of Vancouver Island from fifty–fifty odds of a magnitude 9 disaster, to seventy–thirty in favor of a Mexico City- or Alaska- or Chile-type quake.
Brian Atwater drove west from Seattle in March 1986 toward Neah Bay and Cape Flattery, on the northwestern tip of Washington State, and started searching the beaches, tide marshes, and river estuaries for clues about whether the outer coast had risen or dropped. “I went to Neah Bay with Ando and Balazs firmly in mind,” he admitted. “I really went out there looking for elevated shorelines.” That’s not at all what he found.
Neah Bay was as good a place as any to start because the land all around it is so close to sea level it was highly likely he would be able to spot even slight changes in shoreline elevation, no matter which way they went—up or down. To the north, the bay opens onto the Strait of Juan de Fuca. Across the Strait is Canada’s Vancouver Island. To the south a narrow green valley runs from the back of the bay all the way down to the Pacific Ocean beaches. Standing there in the cold mist and rain it was easy to imagine how seawater might have filled the entire valley at some point in the past, connecting the Pacific with the Strait, cutting off Cape Flattery from the rest of the Olympic Peninsula and making it an island.
Atwater spent a few rainy days on the marshy floor of this valley. At first he poked holes with a core barrel and came up with nothing unusual, just signs that sand and silt had built the marsh by filling a former bay. No need for an earthquake or even for the chronic uplift of Ando and Balazs to explain this stuff. But late one afternoon, with the tide down, he tried his luck digging into the muddy bank of a stream that emptied into the marsh. Several swipes of his army shovel exposed something odd a few feet below the top of the bank, beneath a layer of sand from the bay.
It was a marsh soil, marked by the remains of a plant he had studied in San Francisco Bay: seaside arrowgrass. Pretty quickly he recognized what he was looking at—evidence that land formerly high enough above the highest tides for plants to be living on it had suddenly dropped down far enough for the plants to be killed by salt