The particularity of Japan
The archipelago is located at the junction of three tectonic plates , the Eurasian, Philippine and Pacific, with an additional small plates, like that of Okhotsk. The Pacific plate is sinking beneath the Eurasian plate along the Japan Trench at a rate of 8.5 cm / year. A small region of Eurasia provides a buffer and deforms intensely: the Okhotsk microplate.
Map plates around Japan (Institute of Earth Physics of Paris)
A little further south, the Pacific Plate also slides under the Philippine plate along the Izu Bonin pit, but its movement is only 5.5 cm / year. While a little further west, closer to shore, it is the Philippines plate that sinks under the Eurasian along the Nankai Trough at a speed of 4 cm / year.
These disparate movements cause stresses that build up during non-earthquake and relax when suddenly the earthquake occurs. An outburst of energy that causes a rupture of the crust.
March 11, 2011, the Japan Meteorological Agency provided an estimate of the earthquake magnitude of 7.9 within 3 minutes after the earthquake. The agency at the same time issued a tsunami warning for waves with a height of 6 meters along the coast of Miyagi Prefecture and 3 meters for Iwate and Fukushima. The warning system worked well, says Hiroo Kanamori, a seismologist at Caltech (California Institute of Technnology, United Kingdom), but the calculations have underestimated the wave height.