They are typically generated by seismic or volcanic activity or by underwater landslides. A tsunami consists of a series of high-energy waves that radiate outward like pond ripples from the area in which the generating event occurred. Typically, the size of the waves increases over time. Tsunamis are commonly 60 or more miles from crest to crest and travel at remarkable speeds, often more than 600 miles per hour in the open ocean. They can traverse the entire Pacific Ocean in 20 to 25 hours.
Tsunamis are extremely destructive to life and property.
The tsunami caused by the 1883 eruption of Krakatau, caused more than 30,000 fatalities and the 1886 tsunami on the Sunriku coast of Japan killed about 26,000 people. Tsunamis affecting Washington state may be induced by an earthquake of local origin, or they may be caused by earthquakes at a considerable distance, such as from Alaska or Japan. The large waves are often preceded by coastal flooding and a quick recession of the water. In general, scientists believe it requires an earthquake of at least a magnitude 7 to produce a tsunami. Ninety-four tsunamis have killed over 51,000 coastal residents during the past 100 years.