FACTBOX-Some facts about a tsunami

Sept 30 (Reuters) - A huge sub-sea quake generated a tsunami in the Pacific Ocean on Wednesday, killing an unknown number of people in American and Western Samoa and sowing fear as far as New Zealand.

Following are some facts about a tsunami.

* A tsunami -- from the Japanese "tsu" (harbour) and "nami" (wave) -- is a fast travelling wave typically generated by vertical movements from seismic activity such as a powerful, shallow sub-sea earthquake.

* Tsunami waves can travel at speeds of 800-1,000 km (500-600 miles) per hour. The height of normal waves and tsunami waves is similar in deep ocean water, but near shore, tsunami waves slow and swell, reaching heights of 10 metres (32 feet) or more.

* Normal ocean waves only involve motion of the uppermost layer of the water, but tsunami waves involve movement of the entire water column from surface to sea floor. This means a normal wave is like a small ripple on top of the ocean, but tsunamis are like the entire ocean getting deeper all at once.

* A tsunami is often wrongly described as a tidal wave. Although a tsunami's impact can be influenced by tidal levels, tsunamis are unrelated to tides.

* Major tsunamis occur about once a decade. Based on historical data, about 59 percent of the world's tsunamis have occurred in the Pacific Ocean, 25 percent in the Mediterranean Sea, 12 percent in the Atlantic Ocean and 4 percent in the Indian Ocean.

* The largest recorded earthquake, of 9.5 magnitude in Chile in May 1960, generated a tsunami that swept across the Pacific, killing scores of people in Hawaii, Japan and elsewhere.

* The quake that triggered the 2004 Indian Ocean tsunami was magnitude 9.15. That quake and tsunami killed around 230,000 people in Indonesia, Sri Lanka, Thailand, Malaysia and other countries as far afield as Somalia.

(Sources: Pacific Tsunami Warning Center www.prh.noaa.gov/ptwc/, Factiva, Reuters)