By noaa.gov.
A tsunami is a series of waves generated by a large and sudden displacement of the ocean. Large earthquakes below or near the ocean floor are the most common cause, but landslides, volcanic activity, certain types of weather and meteorites can also cause tsunamis.
Tsunamis radiate outward in all directions from their source and can move across entire ocean basins, around islands and into bays, sounds and up rivers. When they reach the coast, they can cause dangerous coastal flooding and powerful currents that impact marine operation and navigation, and can last for several hours or days.
Tsunamis are infrequent but can pose a serious threat to life and property when they occur. Tsunamis have claimed hundreds of thousands of lives and caused hundreds of billions of dollars in damage around the world.
In deep ocean water, tsunami waves may go unnoticed. But as the waves travel closer to the shore, they build in height as the water becomes more shallow. The speed of tsunami waves depends on ocean depth: The deeper the water, the faster the wave travels. Tsunami waves may travel as fast as jet planes through deep waters, only slowing down when reaching shallow waters. While tsunamis are often referred to as tidal waves, this name is discouraged by oceanographers because tides have little to do with these destructive waves — though a tsunami at high tide may cause more damage and flooding.
What causes a tsunami?
About 80% of all known tsunamis are triggered by earthquakes. These seismic events move Earth’s surface, displacing the water above and generating waves that rapidly travel in all directions across the ocean or body of water.
Not all earthquakes create tsunamis. An earthquake must be big enough and close enough to the ocean floor to cause the vertical movement of the ocean floor that typically sets a tsunami in motion.
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