A. What Is A Tsunami?

Tsunamis are not wind-generated waves. Rather, they are shallow-water waves, with long periods (time between two sucessional waves) and wave lengths (distance between two sucessional waves). The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in might have a period of about 10 seconds and a wave length of 150 m (492 ft.). A tsunami, on the other hand, can have a wavelength in excess of 100 km (62 mi.) and period on the order of one hour. As a result of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets very small. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s/s) and the water depth - let's see what this implies: In the Pacific Ocean, where the typical water depth is about 4000 m (13,123 ft.), a tsunami travels at about 200 m/s, or over 700 km/hr (447mph). However, when the ocean is 6100 m (20,000 ft) deep, unnoticed tsunami travel about 890 km/hr (550 mph), the speed of a jet airplane. And they can move from one side of the Pacific Ocean to the other side in less than one day. Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses.

B. Physics of Tsunami

The phenomenon we call a tsunami (soo-NAH-mee) is a series of waves of extremely long wave length and long period generated in a body of water by an impulsive disturbance that displaces the water. Tsunamis are primarily associated with earthquakes in oceanic and coastal regions. Landslides, volcanic eruptions, nuclear explosions, and even impacts of objects from outer space (such as meteorites, asteroids, and comets) can also generate tsunamis.

As the tsunami crosses the deep ocean, its length from crest to crest may be a hundred miles or more, and its height from crest to trough will only be a few feet or less. They can not be felt aboard ships nor can they be seen from the air in the open ocean. In the deepest oceans, the waves will reach speeds exceeding 970 km/hr (600 mph). When the tsunami enters the shoaling water of coastlines in its path, the velocity of its waves diminishes and the wave height increases. It is in these shallow waters that a large tsunami an crest to heights exceeding 30 m (100 ft.) and strike with devastating force.

The term tsunami was adopted for general use in 1963 by an international scientific conference. Tsunami is a Japanese word represented by two characters: "tsu" and "nami". The character "tsu" means harbor, while the character "nami" means wave. In the past, tsunamis were often referred to as "tidal waves" by many English speaking people. The term "tidal wave" is a misnomer. Tides are the result of gravitational influences of the moon, sun, and planets. Tsunamis are unrelated to the tides; although a tsunami striking a coastal area is influenced by the tide level at the time of impact. Also in the past, the scientific community referred to tsunamis as "seismic sea waves". "Seismic" implies an earthquake-related mechanism of generation. Although tsunamis are usually generated by earthquakes, tsunamis are less commonly caused by landslides, infrequently by volcanic eruptions, and very rarely by a large meteorite impact in the ocean.

As a tsunami leaves the deep water of the open sea and propagates into the more shallow waters near the coast, it undergoes a transformation. Since the speed of the tsunami is related to the water depth, as the depth of the water decreases, 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 several 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 (a step-like wave with a steep breaking front). Reefs, bays, entrances to rivers, undersea features and the slope of the beach all help to modify the tsunami as it approaches the shore. Tsunamis rarely become great, towering breaking waves. Sometimes the tsunami may break far offshore. Or it may form into a bore. 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 m (50 ft.) for tsunamis of distant origin and over 30 m (100 ft.) 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 m (1000 ft.) 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 run-up height, of 30 meters (98 ft). A notable exception is the landslide generated tsunami in Lituya Bay, Alaska in 1958 which produced a 525 meter (1722 ft) wave.

Since science cannot predict when earthquakes will occur, they cannot determine exactly when a tsunami will be generated. But, with the aid of historical records of tsunamis and numerical models, science can get an idea as to where they are most likely to be generated. Past tsunami height measurements and computer modeling help to forecast future tsunami impact and flooding limits at specific coastal areas. There is an average of two destructive tsunamis per year in the Pacific basin. Pacific wide tsunamis are a rare phenomenon, occurring every 10 - 12 years on the average.

Information from Washington University tsunami page