Some facts about tsunamis

Japan’s biggest earthquake since records began 140 years ago hit its northeast coast today, causing a 10-m 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 70km. The quake that caused the 2004 tsunami was 3km 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.

— A tsunami is not a single wave, but a series of waves. They can travel across the ocean at speeds of up to 1,000 km (620 miles) an hour, the speed of a jet aircraft.

— The vast size of the Pacific Ocean and the large earthquakes associated with the “Ring of Fire” combine to produce deadly tsunamis in the Asia-Pacfic. A tsunami can travel across the Pacific Ocean in less than a day.

— As the waves approach land, the ocean recedes dramatically exposing reefs, as the waves draw the water out.

— As the trough of the wave drags along the sea floor, slowing it down, the crest rises up dramatically and sends a giant wall of whitewater onto land. The first wave may not be the biggest.

— The destructive force of a tsunami comes not from the height of the wave, but from the volume of water moving. It is as if the ocean floods the coast, smashing everything in its path, and then just as quickly recedes. Many people who survive the initial wave impact are washed out to sea as the tsunami recedes.

SOME OF THE WORLD’S WORST TSUNAMIS:

— The 2004 Indian Ocean tsunami was the world’s most deadly, killing 226,000 people, with a maximum wave height of about 50 metres.

— The world’s biggest tsunami, caused by a magnitude 8 quake which caused a massive landslide, hit the remote Lituya Bay in Alaska on July 9, 1958. As the wave swept through Lituya Bay, it was forced to rise up, reaching an estimated height of 1,720 feet on the other side of the bay, becoming a mega-tsunami. The sparsely populated bay was devastated, but damage was localised.

The Krakatau island volcanic eruption of 1883 generated waves reaching heights of 125 feet, killing some 36,000 people. It was the most violent volcanic eruption in modern history.

In Japan in June 1896 a tsunami struck Sankiru killing more than 27,100 people following a 7.6 magnitude quake.

In 2010 many people who survived the 8.8-magnitude quake on Feb. 27 in Chile were killed hours later by the massive tsunami, outraging Chileans who said there was no warning the waves were coming.

— Tsunami waves of up to 1.5 metres (5 feet) went on to hit far-flung Pacific regions from the Russian far east and Japan to New Zealand’s Chatham Islands.