Tsunami (pronounced soo-nahm'-ee) is Japanese for "harbor wave" but is actually a series of waves usually generated in the deep ocean, causing massive amounts of damage upon landfall.
The most common cause of tsunamis is a 7.5+ magnitude earthquake under the seabed floor. Often these quakes occur at boundary lines where tectonic or continental plates meet. When the plates push against each other, pressure builds over time until a critical point is reached. The plates slip and thrust past each other lifting or dropping the seabed floor. Gravity forces the water column above to regain its equilibrium. In the process the displaced water rushes outward in a 360-degree circular pattern forming a series of radiating waves like enormous "ripples."
Though a tsunami in open ocean rarely reaches higher than a few feet (1+ meter) it is a very deep wave packing lots of power, making it significantly different from surface disturbances, like true ripples or wind-generated waves. It also differs in length from regular surface waves. A tsunami crest can be 620 miles long (1000 kilometers), but because the amplitude or height is minimal it usually cannot be detected in open ocean, even if passes beneath your boat! As it propagates outwards it can travel as fast as a passenger jet at 450 - 600 mph (724 - 965 kph) racing towards shores hundreds or even thousands of miles away.
As it approaches shore and depth decreases the tsunami will slow but the power it contains continues to roll forward increasing amplitude or height. Waves can rise 100 feet (30 meters) but more often its arrival is much more subtle. The ocean may draw back from the shore so far that it disappears from view before it starts flowing back in, not as a wave at first but more like a bathtub quickly rising. Within seconds the water level can rise 30, 60, even 100 feet (up to 30 meters) above sea level, becoming a rushing wall of water moving up to 40 mph (64 kph) overtaking and lifting everything in its path. The force can easily crush homes or other structures, carry off vehicles, uproot trees and flood low-laying coastal areas up to 1 mile (1.6 kilometers) inland.
The "ripples" that flow out from the epicenter of the disturbance hit the shore one by one with anywhere from 5 to 90 minutes between crests. People often assume once the water from the first wave withdraws the danger has passed and they will reenter the danger zone, only to have another wave hit. Unfortunately the first crest to make shore is usually not the most destructive.
Earthquakes are the most common cause of tsunamis, but underwater volcanoes, landslides or even an asteroid hitting a body of water can cause a tsunami by displacing a large amount of water.
A coalition of more than two dozen countries belongs to the International Coordination Group for the Tsunami Warning System (TWS). The group predicts where tsunamis will strike based on information gathered from tidal charts, seismic sensors, historical data, and oceanic buoys anchored to instruments that take pressure measurements at the sea floor. If TWS data indicates a tsunami might have been generated, potentially affected areas are immediately notified. Local authorities then order evacuations or other necessary measures.
One of the deadliest tsunamis in recorded history occurred on December 26, 2004 in an area unprotected by TWS. The surrounding countries were not members of TWS and so no buoys were anchored in the Indian Ocean when a magnitude 9.0 quake struck. The earthquate was centered 100 miles (161 km) off the coast of Sumatra, spawning a series of waves that devastated southeast Asia, killing many more than two hundred thousand people. The tragedy prompted India to commit to installing a warning system.
Tsunamis are often called 'tidal' or 'seismic waves' but these terms are misnomers. Other processes form tidal waves and not all tsunamis are initiated by seismic activity, as in the case of volcanic eruptions or landslides. A "rogue wave" is also a different phenomenon, little understood. This is a huge wave that forms out at sea, sometimes in calm waters. A rogue wave can reach 50 - 100 feet (15 - 30 meters) and sink a large ship in seconds, but these waves do not reach shore.
Tsunamis they can occur at any time of the day, in any season, and in any weather. If an earthquake strikes just offshore there may not be time for TWS to warn local populations. Therefore if you live within 1 mile (1.6 km) of a coastal area that is less than 25 feet (7.6 m) above sea level, it is recommended that when you feel a sizeable earthquake, once it has passed, you should immediately move inland until the location of the epicenter is known.