| Geologic Hazards on the Oregon Coast |
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An Introduction to Tsunamis
Most inhabitants of the Pacific Northwest have never experienced a tsunami—in contrast to experiences of flooding, landslides, earthquakes, and even volcanic eruptions.
The past occurrence of seismic sea waves in the Pacific Northwest has come to light with recent research that matches records from Japan with carbon-isotope-14 data from wood buried in tsunami sands on the West Coast. Because the Japanese data are so accurate, exact dates can be given for sea wave occurrences. The date of the last large tsunami, recorded in the sands and correlated with Japanese records, was January 26, 1700. The earthquake that generated the wave registered M 9, and the ensuing tsunami destroyed coastal villages in Japan.
In Oregon, prehistoric runups (i.e., how high a tsunami wave reaches above mean sea level) can be deduced with numerical methods. From such models, it was concluded that a tsunami that struck Salishan Spit in Lincoln County between 300 and 800 years ago had a runup of up to 40 feet above sea level. It is likely that the same wave probably overtopped a 16-foot-high barrier ridge at Cannon Beach and breached a 20-foot ridge at Seaside.
One of the largest subduction zone earthquakes ever recorded was the M 9.2 quake on March 27, 1964, centered in Prince William Sound, Alaska. This generated a tsunami that struck the Oregon coast at 11:30 p.m. with waves as high as 10 feet, swamping houses, destroying bridges and sea walls, and tragically killing four children. A family was camping at Beverly Beach and sleeping in a small driftwood shelter at the time. The second wave reached them, throwing the mother out onto the beach and the father up against a cliff, while the children were washed out to sea. In March 1999, a plaque remembering the children and providing information about tsunamis was dedicated at Beverly Beach State Park.
A predicted tsunami on the Oregon coast in 1995 turned out to be a barely recognizable small wave, but the effect on local residents was revealing. Hysterical television and radio warnings led to panic as food and bottled water were emptied from grocery shelves and families were hastily thrown into automobiles that sped to higher ground. Other people, driving to the coast to view the event, caused traffic jams that were worse than the natural disaster which never materialized.
The recently heightened awareness of the potential for a seismic sea wave to inundate the western coastline has caused the National Oceanic and Atmospheric Administration, the U.S. Geological Survey, and the Federal Emergency Management Administration to initiate a program to upgrade their seismic system in order to predict tsunamis more accurately. As a tsunami traverses the ocean, a network of sensitive recorders on the sea floor measures pressure changes in the overhead water, sending the information to sensors on buoys, which, in turn, relay the data to satellites for immediate transmission to warning centers.
Tsunami maps of the Oregon coast were produced by DOGAMI in response to a bill passed by the 1995 Legislature, limiting construction of new hospitals, schools, and other similar public-service buildings in tsunami flood zones. Additional mapping was begun in 1997 with the establishment of the Center for the Tsunami Inundation Mapping Effort at the Hatfield Marine Science Center in Newport. The maps, as well as offshore detection systems, public education, and evacuation planning, are part of a strategy to save lives and reduce loss from tsunamis. In order to educate and alert coastal residents and visitors to potential risks, interpretive signs have been installed to explain hazards. Blue-and-white reflective signs depicting high waves and a person running uphill are being placed at a variety of locations in coastal communities.
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