Causes of tsunamis

Tsunamis are ocean waves produced by earthquakes. The word comes from Japanese and means "harbor wave," because of the devastating effects these waves have had on low-lying Japanese coastal communities. Tsunamis are often incorrectly referred to as "tidal waves." Not all earthquakes produce tsunamis, but when they do, the waves may sweep ashore causing damage locally and at places thousands of miles from the earthquake epicenter. More than 90 percent of the deaths from the 1964 earthquake were a result of tsunamis. One hundred six Alaskans died from these waves and an additional 16 people died from tsunamis in California and Oregon.

There are several ways tsunamis are produced. One way is by regional uplift or subsidence (as discussed in the previous section) of the seafloor during an earthquake. Tsunamis started this way can travel long distances and cause destruction thousands of miles from where the wave was generated. Underwater landslides are another cause of tsunamis. Destruction in Seward, Whittier, and Valdez, and other places in 1964 was from these underwater landslides. These tsunamis are localized. Above water landslides can also cause local tsunamis if they enter a body of water. On July 9, 1958, in Lituya Bay, Alaska, a large earthquake started a giant landslide that ran into the head of the bay and generated a tsunami. The wave ran up a mountainside on the opposite side of the bay to a height of more than 1,720 feet. Two fishing vessels anchored in the bay were sunk and two people died.

Volcanos can also cause tsunamis. During an eruption of Augustine Volcano in 1883, a tsunami almost 30 feet tall entered English Bay near Seldovia. Tsunamis started by this process are uncommon, but present a real threat to residents of the lower Cook Inlet region, the Alaska Peninsula, and the Aleutian Islands.