Cascadia's Fault - Jerry Thompson [62]
As we got closer I guessed the marsh grass might be waist deep if the ground were solid enough to hold a human’s weight. The tan thatch might have been the river’s opposite bank in some previous lifetime. Now it was a saltwater maze with a Medusa-like braid of channels meandering through and around it half a dozen different ways to the sea.
The tide marsh looked like a big unmown pasture with these hulking dead cedars scattered across the flat ground, standing in defiance of time, weather, and gravity. How could fully grown cedars several centuries old be standing in knee-deep salt water, their storm-battered trunks naked of bark, bleached gray by the sun and draped in moss and lichens, in the middle of a tidal marsh on Washington’s west coast? Western red cedars don’t grow in salt water.
One of two things could explain the wrongness of this picture. Either the level of the sea had risen far enough to drown the trees or the land had dropped, slumping far enough below mean high tide to turn a forest meadow into a salt marsh. The two men in the canoe up ahead were about to show us how they had bored and scraped and dug the answer from the damp coastal muck.
Decked out in gumboots, faded orange rain gear, and a green life jacket that was never zipped shut and wearing a bright red tuque against the cool ocean mist, paleogeologist Brian Atwater had packed a folding army shovel and his trusty old Grumman canoe to give us a first-hand look at the Washington coast evidence of monster quakes. The dead cedars had become a vital clue in the ongoing mystery of Cascadia’s fault. Atwater’s colleague David Yamaguchi, from the University of Washington in Seattle, led the investigation that established the time of death. Together they were a forensic team worthy of their own CSI spinoff.
Atwater’s discovery of this ghost forest on the Copalis River in March 1986 did not come about by accident. He’d been driving to the coast at every opportunity for months, specifically in search of proof that subduction earthquakes had left their marks on the Washington shoreline. His journey through the tentacles of saltchuck, sand, and river mud had begun in Seattle in October 1985 when his employer, the U.S. Geological Survey, organized a seminar on seismic hazards in the Pacific Northwest and invited all those doing active research in the region to attend.
If the two plates were sliding past each other smoothly, at a constant rate, and without getting stuck together, then according to Ando and Balazs there should be a slow, continuous, and irreversible rise in land levels along the outer coast. And that was something Atwater figured he could probably measure and verify—or disprove. It sounded like an interesting research project.
On the other hand, if the two plates were stuck together by friction, strain would build up in the rocks and the upper plate would bend down along the outer edge and thicken inland, humping upward until the rocks along the fault failed. In the violent, shuddering release of strain during an earthquake, the upper plate would snap back to the west, toward its original shape.
But the clear signal—the geodetic fingerprint—of each individual earthquake would be the abrupt lowering of land behind the beaches when the upper plate got stretched like taffy and then sank below the tide line as the upper plate snapped back to the west. That’s precisely what George Plafker had found in Alaska and Chile. It’s also what John Adams thought the mountain-tilting data predicted for Cascadia.
Although journalists and members of the public who attended the Seattle meeting saw little of the ferment and discord behind the scenes, significant doubt and strong disagreement had