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Geologic Hazards on the Oregon Coast
The Science of Tsunamis
Tsunamis, the first cousins to earthquakes, can arrive with only slight warning. A tsunami, also called a seismic sea wave, or incorrectly called a tidal wave, travels across the deep ocean at speeds up to 500 miles per hour. A tsunami generated offshore from Japan or Alaska might not hit the Oregon coast for several hours. A tsunami following a Cascadia earthquake may hit in less than 30 minutes.

On the open ocean, a fast moving tsunami might be a wave only three to four feet high, with 100 miles separating wave crests. Approaching the coast, however, the tsunami begins to slow in shallow water, and successive waves bunch up, increasing in height. As the ocean bottom shallows even more, the wave rapidly rises and may break several tens of feet high with incredible destructive power. It has been conjectured that the configuration of the Oregon and Washington continental shelf could produce tsunami waves that would appear to rise slowly out of the ocean but build up to 30 feet or more in height as water is cast shoreward.

If you throw a pebble into standing water, a succession of ripples or waves moves across the water. Similarly, tsunamis almost never come as single waves but arrive as multiple crests that are sometimes hours apart. Often the first tsunami is not even the largest or most destructive, and wave four or five may be the largest of all.

Tsunamis are caused by undersea volcanic eruptions, landslides, or faulting as slabs of the sea floor are displaced vertically. Most commonly, rapid uplift or subsidence of the sea floor along faults is transmitted to the surface of the ocean, forming unusually large waves. Coastal slides from land under the water, also triggered by earth movement, can intensify the effects of tsunamis.

The undersea subduction zone, paralleling the Oregon coast at a distance of about 75 miles, is the junction between the Juan de Fuca and the North American tectonic plates. The two plates lock together, but periodically the stress is released suddenly with a snapping motion, and the resulting shock may trigger a tsunami. Distinctive, thin deposits of shallow marine sands along the coast are physical evidence of these ancient waves.