December 28, 2004
Reprinted with permission from the Eugene RegisterGuard
Guest Viewpoint: Big tsunami hit Oregon in 1700 and will again
By Ray Weldon
At 9 p.m. Jan. 26, 1700, a magnitude 9.0 earthquake and accompanying tsunami hit the coast of Oregon. The size of the event and the kind of damage it caused, both near and far, were similar to that caused by Sunday's earthquake off Indonesia's Sumatra coast.
The tragic results provide us with an opportunity to educate ourselves about the hazard off our own coast, so that we may prepare for it and respond appropriately when it occurs.
Knowledge about the 1700 earthquake and tsunami has grown since its existence first was proposed in the early 1980s. The proposal first arose from a comparison of the Cascadia region's tectonic plate boundary to those that have produced historic earthquakes around the globe. Faced with initial skepticism, scientists have uncovered an ever-growing record of the events in 1700 that allow us to characterize the earthquake and tsunami and thus make forecasts about the impact of the next one.
The record now includes widespread evidence of submerged coastal estuaries, drowned coastal forests, marine fossils and sand deposits carried by the tsunami far up coastal rivers, ongoing accumulation of strain between tectonic plates along the coast, American Indian oral records and written tsunami accounts from as far away as Japan.
Perhaps most insightful are traditional Native American oral records. For example: "They had practically no way or time to try to save themselves. ... It was at nighttime that the land shook. ... I think a big wave smashed into the beach. The Pachena Bay people were lost, ... but they who lived at `House-Up-Against-Hill' the wave did not reach because they were on high ground. ... Because of that, they came out alive. They did not drift out to sea with the others."
The quotations are from ``The Tsunami at Anaqtl'a or `Pachena Bay,' '' related in 1964 by Louis Clamhouse and cited by Ian Hutchinson and Alan McMillan (1997).
Stories like this - from Northern California to Vancouver, B.C. - include battles between gods along the coast, whales carried over the land and dropped, rivers becoming salty during the flood, canoes thrown into trees, extreme cold, nighttime falling before many had gone to sleep and many dead in low-lying areas.
The final piece of evidence, which allowed scientists to calculate the exact time and size of the earthquake, came from tsunami records in Japan. It takes about 18 hours for a tsunami to cross the Pacific basin, and beginning on Jan. 27, the tsunami struck Japan.
Records that include the arrival time and height of the tsunami (on temple steps, government buildings and rice paddies) were used to calculate the initiation time and size of the water wave at the Oregon Coast.
As recorded by the American Indi- ans (a winter night not long before white men came) and drowned trees (in the winter of 1699-1700, based on carbon-14 dating and tree ring counts), it occurred at about 9 p.m. Jan. 26. The size of the waves in Japan tell us that it was a magnitude 9.0 earthquake, consistent with the less precise local geological evidence.
As knowledge of the earthquake hazard has grown, so have mitigation efforts. Building codes have been upgraded, tsunami inundation maps have been prepared, warning signs have been posted and warning systems have been implemented.
The shaking at the coast will last for up to 90 seconds and will be great enough to cause significant damage and loss of life. Following the main shock there will be aftershocks that will affect already damaged structures and weakened hill slopes; the aftershocks will diminish in frequency and strength over a time period of days.
Most significantly, a tsunami will hit. After the initial shaking, it may arrive within a few minutes, or perhaps not for a half an hour. The initial waves may wash in or out. They could arrive as a cresting wave or grow over tens of minutes. It is important not to be lulled into complacency by small initial waves or by the water withdrawing, because it could grow or rapidly return.
The most important way to mitigate the consequences of a large magnitude subduction zone earthquake and tsunami is to be prepared. Individuals, businesses and government agencies need to have a plan; inventing a response while the roof is falling or water is rushing at you is not likely to help much. Understanding what will happen and preparing for it is critical.
There are many good sources of information on the Internet for developing a plan; a good starting place is the U.S. Geological Survey (http:// earthquake.usgs.gov), which includes links to other resources such as the Red Cross and the Federal Emergency Management Agency. An Oregon resource, especially for coastal hazards, is the state Department of Geology and Mineral Industries (http://sarvis .dogami.state.or.us/).
It is important to remember that the extent of damage of such a large event makes it unlikely that emergency help will be available for you. There could be injured people from as far north as Vancouver, B.C., to as far south as Northern California, and from the coast to the edge of the Willamette Valley. Bridges and roads may be out. As the tsunami ripples across the Pacific, response will be needed in Hawaii and as far away as Japan. Having food, water, shelter and first aid resources for a few days may be necessary while lifelines are re-established and help arrives to your area.
Finally, one must keep perspective. Earthquakes of the magnitude of Sunday's or the 1700 event occur on average along the Oregon Coast about 300 to 500 years apart. The coast is a beautiful and dynamic place, and properly prepared, one can survive the rare subduction zone earthquake while continuing to enjoy all the benefits of our wonderful shore lands.
Ray Weldon is a professor of geological science at the University of Oregon.