BISHOP MUSEUMThe State Museum of Natural and Cultural History
For Museum Information, call (808) 847-3511
Tsunami, also known as seismic sea waves, are caused by sudden changes in the seafloor, generally earthquakes and more rarely large landslides. Tsunami are sometimes mistakenly called "tidal waves", but they are not caused by tidal action. Not all earthquakes are tsunami-genic (generate tsunami); to generate a tsunami, the earthquake must occur under or near the ocean, be large, and create vertical movements of the seafloor. It is thought that tsunami-genic earthquakes release their energy over a couple of minutes, much more slowly than the sudden lurching earthquakes, which release their energy in seconds. In fact, some tsunami-genic earthquakes can not be felt by people, so gradual is their energy release. Much of the earthquake's energy, which can be equivalent to many atomic bombs, is transfered to the water column above it, producing a tsunami. All oceanic regions of the world can experience tsunami, but the Pacific Ocean is especially vulnerable because of the many large earthquakes associated with the "Ring of Fire" along its margins.
In the deep ocean, tsunami have very small amplitudes (wave heights are only a few inches), wavelengths of up to 1000 kilometers, and speeds of more than 800 kilometers per hour (500 miles per hour), the speed of a jetliner. The slope of a tsunami surface at sea is only about a centimeter per kilometer (an inch per mile). A tsunami may take 4-6 hours to reach Hawai‘i from the Aleutian Islands, 7-8 hours from Japan, and 14-15 hours from Chile, but its energy will only dissipate slightly as it crosses the entire ocean. In fact, once the tsunami reaches the other side of the ocean thousands of kilometers from its source, it can bounce off the land and return in the direction it came, although its energy will decrease from the reflection. It is easy to see that at these scales the Pacific Ocean becomes like a pond to the tsunami.
A tsunami carries an enormous amount of energy that is spread over a large volume of water in the deep sea. However, when a tsunami reaches shallow water, such as a coastline, the energy is concentrated into a smaller volume and the wave's power overwhelms whatever is in its path. In shallow water, its speed decreases and its amplitude increases to dangerous heights, sometimes 50 feet or higher, and it spreads inland many hundreds of feet (in some cases a mile or more). A tsunami is not a single wave, but a set that may last for several hours, and the first wave is not always the largest.
The wave tank on display here attempts to illustrate the difference between normal wind-driven waves, the kind we experience most of the time in Hawai‘i , and tsunami. Unlike wind-driven waves, which wrinkle only the upper few meters of the ocean, a tsunami extends for thousands of meters, all the way to the ocean floor. Although it is difficult to simulate a tsunami at this scale, you can see that waves produced by large movements of the seafloor can mobilize much larger water volumes than wind-driven waves, and this immense volume of water is the source of the awesome power of a tsunami.
BISHOP MUSEUM
The State Museum of Natural and Cultural History
For Museum Information, call (808) 847-3511