A 8.1-magnitude earthquake struck the South Pacific island nation of Tonga Wednesday, prompting a tsunami warning to be issued.

— A tsunami is a series of waves of extremely long wave length and long period.

— It is generated in a body of water by an impulsive disturbance that vertically displaces the water.

— The term tsunami was adopted for general use in 1963 by an international scientific conference.

— Tsunami is a Japanese word represented by two characters: "tsu" and "nami."

— Tsunamis are not caused by the tides and are unrelated to the tides.

— A tsunami striking a coastal area is influenced by the tide level at the time of impact

— Tsunamis are most often caused by earthquakes and landslides.

— On the average, two tsunamis occur per year throughout the world which inflict damage near the source.

— Approximately every 15 years a destructive, ocean-wide tsunami occurs.

— Tsunami velocity depends on the depth of water through which it travels

— Tsunamis travel approximately 475 mph in 15,000 feet of water. In 100 feet of water the velocity drops to about 40 mph.

— In deep water (greater than 600 feet), tsunamis are rarely over 3 feet and will not be noticed by ships due to their long period

— As tsunamis propagate into shallow water, the wave height can increase by over 10 times.

Source: National Oceanic & Atmospheric Administration