A tsunami is a wave train, or series of waves, generated in a body of water by an impulsive disturbance that vertically displaces the water column. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis. Tsunamis can savagely attack coastlines, causing devastating property damage and loss of life. Tsunami is a Japanese word with the English translation, "harbor wave." Represented by two characters, the top character, "tsu," means harbor, while the bottom character, "nami," means "wave." In the past, tsunamis were sometimes referred to as "tidal waves" by the general public, and as "seismic sea waves" by the scientific community. The term "tidal wave" is a misnomer; although a tsunami's impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. The term "seismic sea wave" is also misleading. "Seismic" implies an earthquake-related generation mechanism, but a tsunami can also be caused by a nonseismic event, such as a landslide or meteorite impact.
Historically, undersea earthquakes trigger most tsunamis, but volcanic eruptions and landslides have produced the highest. Such quakes are most likely to occur along the subduction zones of the Pacific rim, where the ocean floor is being shoved beneath the edge of continents and island chains. Alaska, Japan and the western coasts of Central and South America are located on such subduction zones, which is why some of the world's most powerful tsunamis occur there.
Only certain types of undersea earthquakes typically cause tsunamis, for example, the horizontal shifts that occur along strike-slip faults. The San Andreas, which runs offshore in places along the California coast, leave the ocean floor relatively undisturbed. But if the seafloor is abruptly dropped or raised, water is displaced and the possibility of a tsunami Is high. This is what happened during a major earthquake on March 27, 1964, in Anchorage, Alaska. The quake hoisted 35,000 square miles of seafloor and Islands (equal in area to the state of Indiana) as much an 50 feet. The sudden upward, movement also raised the ocean layer above the seafloor, and the water poured away in a series of long waves.
How undersea volcanic eruptions produce tsunamis is still debated. The 1883 eruption of the volcanic Island of Krakatau, located In the Sunda Straits between Sumatra and Java, Indonesia, generated huge tsunamis whose waves reached as high as 150 feet. Though Krakatau was uninhabited, the waves drowned about 36,000 people on neighboring islands; more people have died as a result tsunamis than any others In recorded history.
Physics of Tsuanmi: 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 600 miles per hour (970 km/hr). 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 can crest to heights exceeding 100 feet (30 m) and strike with devastating force.
Tsunamis are typically shallow-water waves. Shallow-water waves are different from wind-generated waves, such as the waves observed on a the beach. Wind-generated waves usually have period (time between two sucessional waves) of five to twenty seconds and a wavelength (distance between two sucessional waves) of about 100 to 200 meters. A tsunami can have a period in the range of ten minutes to two hours and a wavelength in excess of 500 km. It is because of their long wavelengths that tsunamis behave as shallow-water waves. A wave is characterized as a shallow-water wave when the ratio between the water depth and its wavelength gets very small. The speed of a shallow-water wave is equal to the square root of the product of the acceleration of gravity (32ft/sec/sec or 980cm/sec/sec) and the depth of the water. The rate at which a wave loses its energy is inversely related to its wavelength. Since a tsunami has a very large wave length, it will lose little energy as it propagates. Hence in very deep water, a tsunami will travel at high speeds and travel great transoceanic distances with limited energy loss. For example, when the ocean is 6100 m deep, unnoticed tsunami travel about 890 km/hr, 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.
As a tsunami move from the deep water of the open sea and propagates into the more shallow waters near the coast, it undertakes 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 meters or more in height.
When a tsunami
finally hits 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 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 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 m for tsunamis of distant origin and over 30 m 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 or more, covering large
areas 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. A notable exception is the landslide generated tsunami in
Lituya Bay, Alaska in 1958 which produced a 525 meter wave.