- On July 12, 1993 a magnitude 7.8 earthquake in the Sea of Japan produced tsunami that totally destroyed the southern half of Okushiri Island. Waves were greater than 30 feet and some could have been 100 feet. The earthquake was about 50 miles offshore and the tsunami arrived within minutes. 120 people died.
- The states most at risk for Tsunami are California, Hawaii, Oregon and Washington. Hawaii is at greatest risk and they have about 1 tsunami a year and a dangerous tsunami about every 7 years.
- On March 28, 1964 an extremely large earthquake (magnitude 8.4) struck Alaska. It caused tsunami waves that were very destructive in southeastern Alaska, in Vancouver Island, Canada, and in the States of Washington, California and Hawaii. Waves ranged in size from 6 to 21 feet. The tsunami killed more than 120 people and damages costing more than $106 million. It was the costliest tsunami ever to strike the Western United States and Canada.
- Although a large asteroid impact is highly unlikely, scientists studying the possibility have decided that a moderately large asteroid or about 5-6 km in diameter falling in the middle of the Atlantic Ocean, would generate a tsunami that would travel all the way to the Appalachian Mountains in the upper two-thirds of the United States. Coastal cities would be wiped out by such a tsunami.
- Nuclear explosions could possible create tsunami but none have ever been generated from testing yet. Furthermore, such testing is currently banned by international treaty.
When an undersea earthquake or other major disturbance causes a section of the ocean floor to suddenly rise or sink, the mass of water above the affected area also rises or sinks. This unexpected movement of the water creates a series of powerful waves.
Undersea earthquakes that cause massive changes to the ocean floor and the displacement of a large volume of water are the most common cause of a tsunami.
Tsunami can also be caused by other undersea events such as volcanoes or landslides
A tsunami can also be caused from events above the ocean floor. These events could include a meteorite crashing into the ocean, major landslides near a coastline or material from an erupting volcano forming a landslide. The impacts of tsunami triggered this way tend to be localised.
More than 75 per cent of tsunami are caused by undersea earthquakes.