Geologist comments on Oregon’s tsunami hazard

Records show an equivalent event on the Oregon coast occurred in 1700

Earthquake-caused tsunamis as severe as those that swept southeast Asia on Sunday have happened in the past off the Oregon coast, according to a University of Oregon geoscientist.

In fact, a tsunami caused by a magnitude 9.0 earthquake occurred on Jan. 26, 1700, wiping out Oregon tribal villages in low-lying coastal estuaries and causing damage as far away as Japan. Ray Weldon, who researches and teaches about of geologic hazards, says he hopes coastal residents will be motivated to learn about Oregon’s tsunami potential in light of the devastation along the coasts of nine Asian nations on the Indian Ocean.

Weldon is leading a team of scientists studying uplift along the coast and how it informs the size of future earthquakes and tsunamis. He says a comparison of modern uplift rates in the Pacific Northwest to that predicted by models of past earthquakes–like that big one of 1700–reveals many similarities. “This tells us that the subduction zone is accumulating strain for the next great earthquake,” he says. Historically, such events tend to occur, on the average, every 300 to 500 years.

Damage from an undersea earthquake as large as the one that shook the Indian Ocean could span from Northern California to Vancouver, B.C. Weldon and his colleagues estimate that the resulting tsunami would add to the damage in the low-lying coastal part of the Pacific Northwest and extend beyond the region to as far away as Japan.

Their data corresponds with written records in Japan and Native American legends about the 1700 quake, which are consistent with other geological evidence. “From northern California to Vancouver, B.C., the Native American stories tell of battles between gods along the coast, whales carried over the land and dropped, rivers becoming salty during the flood, and canoes thrown into trees,” Weldon says.

These stories, related by survivors who lived on or escaped to higher ground, describe the destruction of villages in tidal estuaries. The geologic record now includes widespread evidence of submerged coastal estuaries, marine fossils and sand deposits carried by the tsunami far up coastal rivers, and drowned coastal forests. Radiocarbon dating and tree ring analysis established that most of these forests were drowned in the winter of 1699–1700.

Weldon, who has lived in Indonesia and has relatives in Thailand, says last weekend’s tragedy brings home the need for coastal residents and tourists to learn about and take precautions against tsunamis. “For an earthquake as strong as the one that hit southeast Asia, the shaking at the Oregon coast would last for up to 90 seconds and be great enough to cause significant damage and loss of life,” he says. “Most significantly, a tsunami will arrive at the coast as soon as minutes following the shaking to within a half-hour.”

If a major earthquake occurs off the coast it is crucial to move immediately to higher ground, preferably at least 100 feet above sea level. Weldon warns against becoming “lulled into complacency” by small initial waves or by the sight of the water withdrawing as this can rapidly reverse – a feature of the deceptive behavior of the ocean during subduction earthquakes.

To view maps of Oregon communities at risk for tsunamis, go to http://sarvis.dogami.state.or.us/earthquakes/Coastal/Tsumaps.HTM. Links to more information about Oregon’s tsunami warning system, mitigation efforts and details about historic earthquakes in the Pacific Northwest are available at the website for the Oregon Department of Geology and Mineral Industries, http://www.oregongeology.com/earthquakes/Coastal/TsunamiIntro.htm.