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A tsunami is produced by an undersea earthquake or, much less frequently, by volcanic eruptions, meteorite impacts, or underwater landslides. Tsunami is a Japanese word represented by two characters: "tsu" and "nami." The character "tsu" means harbor, and the character "nami" means wave. In the past, tsunamis were often referred to as "tidal waves." The term "tidal wave" is a misnomer. ( Also described in Panel text, pg. 9. )
When the sea floor moves, it pushes up a column of water that generates waves, which spread out from the disturbance in all directions, much like the ripples caused by throwing a rock into a pond. The waves are huge; time between wave crests may be from 5 to 90 minutes, and the wave speed in the open ocean will average 725 kilometers (450 miles) per hour. A person in a boat in the ocean would neither feel nor see a tsunami pass.
As a tsunami leaves the deep water of the open sea and progates into the more shallow waters near the coast, it undergoes a transformation. When the depth of the water decreases, the base of the waves hit the ocean floor and the speed of the tsunami diminishes. The change of total energy of the tsunami remains constant. Therefore, the speed of the tsunami decreases as it enters shallower water, and the height of the wave grows. Because of this "shoaling" effect, a tsunami that was imperceptible in deep water may grow to be over 15 meters (50 feet) or more in height.
When a tsunami finally reaches the shore, it may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore. Reefs, bays, entrances to rivers, undersea features and the slope of the beach all help to modify the tsunami as it approches the shore. Tsunamis rarely become great, towering breaking waves. Sometimes the tsunami may break far offshore. Or it may form into a bore: a step-like wave with a steep breaking front. A bore can happen if the tsunami moves from deep water into a shallow bay or river. The water level on shore can rise many feet. In extreme cases, water level can rise to more than 15 meters (50 feet) for tsunamis of distant origin and over 30 meters (100 feet) for tsunami generated near the earthquake's epicenter.
The first wave may not be the largest in the series of waves. One coastal area may see no damaging wave activity while in another area destructive waves can be large and violent. The flooding of an area can extend inland by 305 meters (1000 feet) or more, covering large expanses of land with water and debris. Flooding tsunami waves tend to carry loose objects and people out to sea when they retreat. Tsunamis may reach a maximum vertical height onshore above sea level, called a runup height, of 30 meters (98 feet). A notable exception is the landslide generated tsunami in Lituya Bay, Alaska in 1958 which produced a 525-meter (1722 feet) high wave.
Adapted from - West Coast and Alaska Tsunami Warning Center - Physics of Tsunamis
Pacific Northwest Tsunamis
A 100-year historical database indicates that an average of five tsunamis per year occur in the Pacific and that during this period tsunamis have killed more than 50,000 people. Since 1945, more people have been killed as a result of tsunamis than as a direct result of an earthquake's ground-shaking. The Worldwide Tsunami Database (from the National Geophysical Data Center) shows that largest tsunami wave recorded in the Northwest is a 30-meter (98 feet) wave on Vancouver Island; it was generated by an Alaska earthquake in 1946. The largest one listed for Washington, Oregon and California is a 6.3-meter (21 feet) wave at Crescent City, California; it was generated by the 1964 Alaska earthquake.
We get small tsunamis almost every year, but almost all of these are too small to notice at the beach. Large tsunamis that cause damage occur every few decades. The last major one was in 1964. Landslides into Puget Sound, rivers and lakes also cause tsunamis. We get one of these in the Northwest every few decades.
The Cascadia Subduction zone earthquake of January 26, 1700 produced a tsunami felt in Japan. When the Seattle Fault last had a major movement, roughly 1100 years ago, it generated a tsunami that deposited sands at West Point in Discovery Park, Seattle, as well as in Hood Canal.