What is Subduction; subduction zone?

Example 15.10 (The Tōhoku earthquake, 2011)

On 11 March 2011 an earthquake of magnitude 9.1 struck off the north-east coast of Honshu, Japan. It happened about $25\,\mathrm{km}$ down, on the plate boundary itself, where the Pacific plate, moving west at about $83\,\mathrm{mm}$ a year relative to the plate that carries northern Japan, sinks beneath it at the Japan Trench. The two sides of the fault slipped past each other by as much as 50 to $60\,\mathrm{m}$, over an area about $400\,\mathrm{km}$ long and $150\,\mathrm{km}$ wide. The sea floor above moved with it, and raised a tsunami that ran up to $38\,\mathrm{m}$ high on the coast; at least $15\,703$ people were killed and $4647$ were missing. The same coast had been struck by tsunamis from this subduction zone in 1611, 1896 and 1933, and perhaps by one as large as that of 2011 in the year 869.