For tsunami researchers, witnessing an event as wide-reaching and destructive as last winter’s Indian Ocean tsunami is exceedingly rare. This makes Atwater’s soggy forays into geologic history quite valuable. By unearthing sediment deposits tsunamis leave behind, scientists can study the waves’ origins, extent, and frequency. Such work helps avert surprises from locations that have the geological apparatus to produce a tsunami, but haven’t—in written history, at least.
History in Layers
Science Bulletins met up with Atwater at the Copalis River estuary on the Pacific coast of Washington State. The estuary is one of dozens that sit above an enormous fault plane that slants beneath the Pacific coast. Called the Cascadia subduction zone, the fault stretches 1,100 km from Vancouver Island, B.C., to northern California. One tectonic plate descends beneath another here. As they abrade, the overriding continental plate sticks and warps atop the subducting one. Strain builds over time. When the plates suddenly slip free, an earthquake occurs.
At this moment, the continental plate springs upward, and can launch a massive volume of water: a tsunami. Thus “unstuck,” the plate settles, now lower in elevation than it was prior to the earthquake.
Despite the fault’s existence, written records from the Pacific Northwest coast, which began about 200 years ago, are silent on the subject of large earthquakes and tsunamis generated from them. “I first went to the Copalis River in the spring of 1986,” says Atwater. “At that time, very few scientists believed that large earthquakes and tsunamis could happen here, and nobody had demonstrated that they had.” But Atwater became one of the first researchers to find geologic proof.
Atwater explains as he hacks into a marshy bank of the Copalis River with a World War II folding shovel. “What I’m unearthing is a record of a catastrophe from 300 years ago,” he says. Atwater points to the lowest of three distinct bands of sediment stacked in the bank. “Around 1700, this salt marsh, represented by the soil here, was up at the level of the present marsh above us. But then the land abruptly dropped a meter or two during an earthquake.” He traces the 10 cm thick layer of sand above the ancient marsh. “Then comes the tsunami, and lays down a sheet of sand. The sea was free to come in because the continental plate had dropped, so the ocean then laid down this top layer of mud.”
In 1986, Atwater surveyed a sand sheet that he suspected a tsunami washed into Willapa Bay, Washington. Sand deposits had been associated with only one tsunami previously, the 1960 event in the southeastern Pacific that affected Chile and Japan. Jody Bourgeois, a sedimentologist at the University of Washington, investigated further. “We started with little to go on,” she recalls. “We had to show that the sand layer was from a surge of a tsunami wave, and not from a high tide or a storm.” A team of colleagues mapped the sand and land-level changes along coastal Washington State and compared the results with deposits and eyewitness accounts of the 1960 tsunami. The picture began to come together.
Atwater and other researchers later found key clues in eerie stands of Western red cedars bordering the Copalis River and three other estuaries in the state. “We call it a ghost forest because the trees have been standing dead for centuries,” Atwater says. It’s a sign not of a tsunami, but of an earthquake capable of causing one. “After the earthquake drops the continental plate, saltwater can come in at high tide and routinely cover the forest floor, killing the trees,” he explains.
A Restless Record
At the start of World War II, a Japanese geographer looking at municipal documents from Japan’s Pacific coast noted a mention of a destructive tsunami on the evening of January 26, 1700. In 1996, Japanese researchers proposed that this event and the one that had affected the Pacific Northwest were one and the same. Tree-ring dating of the Western red cedars corroborated this date: the trees had all died at once, somewhere between August 1699 and May 1700. Recent analysis of flooding and other damage from the Japanese documents show that the tsunami’s parent earthquake was a gargantuan magnitude 9.
The mounting evidence since 1986 has convinced Earth scientists that the Pacific Northwest’s 1700 earthquake was just the most recent tsunami-generating quake of a surprisingly fitful fault. Additional deposit data have disclosed seven great Cascadia earthquakes over the past 3,500 years, with an average interval of 500 years.
As the geologic record unfolds, other researchers such as Ruth Ludwin, a seismologist at the Pacific Northwest Seismograph Network, are digging up oral traditions of Northwest Coast native communities that existed previous to written records. “It turns out there are stories amongst those tribes that are consistent with historical earthquakes and tsunamis on the coast of Cascadia,” says Bourgeois.
As the continental plate sluggishly gathers strain at Cascadia, there is no doubt that another massive earthquake and tsunami will roil the Pacific Northwest. When is unclear. To find out how science is reducing the risk of surprise and widespread damage, follow the essays about tsunami computer modeling and measurement in real time.
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