What? An earthquake measuring 9.0 on the Richter Scale struck off Japan's north-east coast, about 250 miles (400km) from Tokyo at a depth of 20 miles.
When? The magnitude 9.0 earthquake happened at 2:46pm (local time) on Friday March 11, 2011.
Where? The earthquake occurred 250 miles off the North East Coast of Japan's main island Honshu.
Why? Japan is located on the eastern edge of the Eurasian Plate. The Pacific Plate, which is an oceanic plate, subducts (sinks under) the Eurasian Plate, which is a continental plate, to the east of Japan. This type of plate margin is known as a destructive plate margin. The process of subduction is not smooth. Friction causes the
Pacific Plate to stick. Pressure builds and is released as an earthquake.
The earthquake occured at a relatively shallow depth at 20 miles below the surface of the Pacific Ocean. This, combined with the high magnitude, caused a tsunami (find out more about how a tsunami is formed on the BBC website).