In the wake of Japan’s disastrous tsunami, geologist Brian Atwater recently parked his car next to the Lucky Deer Trading Post at Discovery Bay, 10 miles south of Port Townsend. He pulled on rubber boots and raingear, and set off for the mudflats to review the evidence of our own tsunami history.
From a wooden footbridge a few hundred yards from Highway 101, Atwater pointed down to the muddy bank of Snow Creek, which meanders across the flats on its way to the bay.
“I don’t know of another place in the Northwest you can see this so clearly,” he said. “Every tsunami wants to leave its signature, and you’re looking at one.”
He climbed down into the creek and waded across, each step sinking six or eight inches into the muck, trying to swallow a boot. On the far side, he shoveled away wads of peat and seagrass to get a better look at the four-foot high bank. Using a 20-inch length of plastic gutter, he cut away a vertical core, then used a gardening scoop to clean off the edges. The result was a muddy cylinder, five inches round.
To the layman, it was so much mud. To Atwater and other geologists, it is a slice of geological history, a record of tsunamis past.
The tsunami “signatures” are narrow bands of gray-green sand, interspersed with the deep brown mud. Atwater pointed to an inch-thick band of gray and speculated: “That is probably the 1700 waves.”
A tsunami specialist at the U.S. Geological Survey in Seattle, Atwater is one of the scientists who believe that the Northwest was hit on January 26, 1700, by a major earthquake, caused by slippage in the offshore Cascadia subduction zone, where the North America plate and Juan de Fuca plate intersect. That quake in turn triggered a tidal wave of proportions similar to the one that hit Japan this month.
The January date is based on Japanese accounts of an “orphan” tsunami that washed away entire villages on that day — even though there was no earthquake. Scientists have linked those accounts to Pacific Northwest evidence that includes submerged “ghost forests” on the Washington coast and sandy sediments like those they have found at Discovery Bay.
Atwater and other scientists have been digging in the Discovery Bay mudflats for more than a decade. In one study, researchers dug multiple trenches, took core samples, and counted nine different layers of marine sediments. Carbon dating of those marine layers enabled them to estimate when they were deposited.
Researchers believe as many as nine tsunamis have raced up Discovery Bay over a span of about 2,500 years. That means the region tends to get a significant tsunami about every 300 years, Atwater explained. “It’s not like clockwork, because we don’t see regular intervals. But nine tsunamis in 2,500 years suggests an average interval of 300 years.”
That’s one of the reasons scientists believe the region is about due for another big one.
Discovery Bay seems like an odd place to find evidence of tsunamis. It is 100 miles from the open sea, and shielded from the strait by Protection Island.
But Juan de Fuca Strait is a natural conduit for tsunamis, and Atwater believes Discovery Bay’s depth and funnel shape serve to “amplify” incoming waves as they race southward down the bay. Then, when they reach the mudflats, they continue inland for some distance, depositing the sediments that show up in recent diggings.
The tsunami of January 1700 would have been created by displacement along the massive Cascadia fault, and the wave would have reached Neah Bay in about 25 minutes — roughly the same time it took for the Japanese wave to hit the shore on March 11, Atwater said. Beyond Neah Bay, the 1700 tsunami would have continued eastward down the strait, inundating shoreline areas along the way.
The theory is supported by Native American oral legends, including one recorded at Neah Bay by pioneer anthropologist James Swan in 1864. That story says that the ocean flooded the village of Neah Bay and other low-lying areas, carrying away people, canoes, and houses.
Destructive waves can also be generated locally — by movement in any of the faults that crisscross the region, including Discovery Bay. While these faults would not cause a quake comparable to Japan’s, their local effects could be every bit as severe, Atwater cautioned.
He believes Japan’s disaster holds important lessons for the Pacific Northwest. “The March 11 earthquake and tsunami in Japan provide a mirror,” he said, “in which we can view our own earthquake and tsunami hazards.”
This story originally ran in the Port Townsend Leader.
