Plate tectonics
The earth is divided up by different parts of the Earth's crust. The seperate parts of the Earth's crust are called tectonic plates. These tectonic plates move in different directions due to convection in the Earth's inner layers. An earthquake is caused when these tectonic plates moving against each other, stress builds up and is released in powerful waves of energy. The Sendai Earthquake was caused by movement in the Continental and pacific tectonic plates along the Japan Trench. The Japan Trench is where the Pacific Ocean plate sliding under the continental place. The movement of the tectonic plates slipping under another tectonic plate is called subduction. In the event of the Sendai earthquake tension built up from these plates moving against each other built up and was released violently. This release of energy was so huge that the earthquake lasted for 6 minutes. The earthquake was so strong it shifted the Earth on it axis of rotation and shortened the length of day by about a microsecond. The movement of the tectonic plates also caused Japan's northern Honshu coastline to drop by 0.6 metres and move eastward by 2.4 metres. The movement in the plate tectonics also displaced a lot of water which caused a huge tsunami to sweep the east coast of Japan.