A tsunami is a series of ocean waves generated by any rapid large-scale disturbance of the sea water. Most tsunamis are generated by earthquakes, but they may also be caused by volcanic eruptions, landslides, undersea slumps, or meteor impacts.

The waves radiate outward in all directions from the disturbance, and can propagate across entire ocean basins. For example, an earthquake in Chile caused a tsunami across the Pacific in Japan. Tsunami waves are distinguished from ordinary surf waves by their great length, often exceeding 100 miles in the deep ocean, and by the long amount of time between successive peaks five minutes to an hour. The speed at which tsunamis travel depends on the ocean depth. A tsunami can exceed 500 mph in the deep ocean but slows to 20 or 30 mph in the shallow water near land. In less than 24 hours, a tsunami can cross the entire Pacific Ocean.

In the deep ocean, a tsunami is barely noticeable and will only cause a small and slow rising and falling of the sea surface as it passes. Only as it approaches land does a tsunami become a hazard. As the tsunami approaches land and shallow water, the waves slow down and become compressed, causing them to grow in height. In the best of cases, the tsunami comes onshore like a quickly rising tide and causes a gentle flooding of low-lying coastal areas.

Updated: Saturday, October 4, 2014 8:59