Sky News
During a tsunami the ocean suddenly floods the coast, smashing everything in its path, and then just as quickly recedes.
Large quakes are the main cause of tsunamis, but they can also be sparked by other cataclysmic events, such as volcanic eruptions and even landslides.
During a strong quake, oceanic plates can lurch many metres and rupture the ocean floor.
This movement can suddenly move a massive amount of water.
:: Read about today's quakes in Indonesia
Major quakes that rupture the ocean floor are usually shallow quakes occurring at a depth of less than 44 miles (70km).
The 9.1 magnitude quake that caused the devastating 2004 Boxing Day tsunami was 18 miles (30km) below the seafloor.
Recent Deadly Quakes and Tsunamis
:: Japan, March 2011 - A 9.0-magnitude earthquake hits underwater, causing a tsunami that kills more than 19,000 people
:: Haiti, January 2010 - A 7.0-magnitude earthquake kills between 250,000 and 300,000 people
:: Sichuan Province, China, May 2008 - Around 87,000 people are lost or killed after an 8.0-magnitude earthquake
:: Kashmir, October 2005 - At least 75,000 are killed by a 7.6-magnitude earthquake
:: Indian Ocean, December 2004 - More than 226,000 die when a tsunami sparked by an undersea earthquake, magnitude 9.1, strikes off Indonesia
Japan's police agency has said nearly 7,200 died in the disaster.
The movement of the quake can also affect the likelihood of a tsunami occuring.
The vertical movement of some earthquakes can cause the seabed to heave and displace water vertically, sending towering waves racing toward shores.
The 2004 tsunami, which killed nearly a quarter of a million people, and the 2011 disaster in Japan were both caused by these "mega thrust" quakes.
Strike-slip quakes cause a horizontal - instead of vertical - movement, with the tectonic plates sliding against each other, creating more of a vibration in the water.
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 620mph (1,000km), the speed of a jet aircraft.
A tsunami can travel across the Pacific Ocean in less than a day.
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.
And 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 inside it.
The word tsunami is derived from the Japanese words for "harbour" and "wave".
During a tsunami the ocean suddenly floods the coast, smashing everything in its path, and then just as quickly recedes.
Large quakes are the main cause of tsunamis, but they can also be sparked by other cataclysmic events, such as volcanic eruptions and even landslides.
During a strong quake, oceanic plates can lurch many metres and rupture the ocean floor.
This movement can suddenly move a massive amount of water.
:: Read about today's quakes in Indonesia
Major quakes that rupture the ocean floor are usually shallow quakes occurring at a depth of less than 44 miles (70km).
The 9.1 magnitude quake that caused the devastating 2004 Boxing Day tsunami was 18 miles (30km) below the seafloor.
Recent Deadly Quakes and Tsunamis
:: Japan, March 2011 - A 9.0-magnitude earthquake hits underwater, causing a tsunami that kills more than 19,000 people
:: Haiti, January 2010 - A 7.0-magnitude earthquake kills between 250,000 and 300,000 people
:: Sichuan Province, China, May 2008 - Around 87,000 people are lost or killed after an 8.0-magnitude earthquake
:: Kashmir, October 2005 - At least 75,000 are killed by a 7.6-magnitude earthquake
:: Indian Ocean, December 2004 - More than 226,000 die when a tsunami sparked by an undersea earthquake, magnitude 9.1, strikes off Indonesia
Japan's police agency has said nearly 7,200 died in the disaster.
The movement of the quake can also affect the likelihood of a tsunami occuring.
The vertical movement of some earthquakes can cause the seabed to heave and displace water vertically, sending towering waves racing toward shores.
The 2004 tsunami, which killed nearly a quarter of a million people, and the 2011 disaster in Japan were both caused by these "mega thrust" quakes.
Strike-slip quakes cause a horizontal - instead of vertical - movement, with the tectonic plates sliding against each other, creating more of a vibration in the water.
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 620mph (1,000km), the speed of a jet aircraft.
A tsunami can travel across the Pacific Ocean in less than a day.
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.
And 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 inside it.
The word tsunami is derived from the Japanese words for "harbour" and "wave".
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