FACTBOX-Some facts about tsunamis

Fri Mar 11, 2011 11:09am GMT
 

March 11 (Reuters) - Japan's biggest earthquake since records began 140 years ago hit its northeast coast on Friday, causing a 10-metre tsunami that swept away all in its path.

The 8.9 magnitude quake caused many injuries, sparked fires, unleashed a rolling wall of water and prompted warnings to people to move to higher ground in coastal areas.

Here are some facts about tsunamis:

* HOW TSUNAMIS OCCUR:

-- During a strong earthquake oceanic plates can lurch by many metres, rupturing the ocean floor. This in turn suddenly moves a massive amount of water. This is what happened in the earthquake that caused the deadly Indian Ocean tsunamis of December 2004.

-- The water displaced by the 2004 Aceh earthquake was like tipping out the volume of Sydney Harbour within a few minutes.

-- Major quakes that rupture the ocean floor are usually shallow quakes occurring at a depth of less than 70 km (44 miles). The quake that caused the 2004 tsunami was 30 km below the seafloor.

* TSUNAMIS RISE UP:

-- On the ocean surface, tsunamis start as an insignificant ripple capable of passing under a ship unnoticed, but they become giants as they approach land and the ocean becomes shallow.   Continued...

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