Earthquakes are a daily phenomenon in Japan as about 1,500 tremors are recorded every year. The reason for so much seismic activity is that the portion of the Earth’s crust (or the tectonic plate) on which Japan is situated has fractures at many places. Geologists have observed fractures in the Earth‘s crust wherever one tectonic plate presses against the other and have labeled them as ‘faults’. Speaking about Japan, four tectonic plates converge beneath its landmass and the surrounding seas. They are: (1) The Philippines Plate, (2) the Pacific Plate, (3) The North American Plate, and (4) the Eurasian Plate. These plates are continuously exerting pressure against one another. When any tectonic plate gives way under tremendous pressure, it releases a lot of energy, which is known as the earthquake.
Graphics given below explain what happened on March 11, 2011. According to the graphic on the top, the North American Plate overlaps the Pacific Plate deep beneath Japan and the surrounding seas. On one hand, the North American plate is exerting tremendous pressure on the Pacific Plate as it gradually advances toward it; while the Pacific Plate, on the other hand, is slowly sinking into the softer interior layer known as mantle situated beneath the Earth’s solid crust. These movements have been going on steadily for millions of years. On the fateful day of March 11, a portion of the North American Plate resting on the Pacific Plate suddenly snapped under tremendous force of accumulated pressure and its forward portion sank lower. This incident took place under the Pacific Ocean floor about 130 kilometers away from Honshu Island. The snapping of the North American Plate there caused the sea floor to rise. As a result, tons of sea water also raised upward sending tsunami waves in all the direction.