Tsunami waves are generated by events that displace of water. This displacement can be caused by movement of the ocean floor, by underwater (or at the shoreline) landslides or land slumps, volcanic activity, large releases of gases from the ocean floor, atmospheric pressure waves, by a large meteorite or asteroid plunging into the ocean.
Displacements of the ocean floor...
Tsunami waves are often generated along fault lines in the earth's crust, typically in areas where the continental and oceanic plates are in compression. As a general rule, the continental plate rides over top of the oceanic plate. While the edges of both plates are engaged (not moving with respect to each other), the compressive forces that result from the overall plate movement causes tension to build up between the plates in the subduction zone. Eventually the increasing pressure causes the edges of the plates disengage, allowing the plates to shift. The earthquake that results from the fracturing and subsequent movement of the plates may produce a tsunami. Earthquakes can also generate a tsunami by causing subterranean landslides and land slumps, especially near the edge of the continental shelf.
Movement of tsunami waves...
Tsunami waves move rapidly across oceans one generated. The speed and height of the tsunami wave depends on the depth of the ocean floor. In areas of the Pacific where the ocean depth is 20,000 feet, tsunami waves are less than a foot high and move at speeds of about 550 mph - about the speed of a jet. The tsunami wave length can stretch nearly 100 miles and this large wavelength allows the tsunami to travel great distances while losing little energy. As the wave encounters shallower water the speed of a tsunami wave slows and the height increases. In about 300 feet of water, a tsunami wave will slow to about 60 mph and in 30 feet of water the wave will slow to 20 mph.
When tsunami waves reach the coast...
As a tsunami wave approaches the shallower depths near the coast, the speed of the wave slows and the height of the wave increases. The initial movement of water along the coast may either recede out to the sea or increase toward and onto the land. The recession of water toward the sea is strong indication that a powerful wave is approaching. If the tide is behaving in a way that it is not supposed to, or is otherwise unpredictable, a deadly tsunami wave may be imminent. If this is the case, leave the beach. It is also important to remember that tsunamis typically generate a series of waves and the first wave may not be the largest
When a tsunami reaches shore, it may appear as a rapidly rising or falling tide, a series of breaking waves, or even a tidal 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. While considerable attention is directed toward the flooding effects of the tsunami, much of the damage is due to the force generated by the rapid movement of water. Even in cases where no flooding of land occurs, the rapid movement of water in channels, bays, and harbors can cause considerable damage to boats, docks and marinas. In addition, the strong currents can become deadly and wash people out to sea.