The cause
The 2011 Tohoku earthquake struck offshore of Japan, along a subduction zone where two of Earth's tectonic plates collide. In a subduction zone, one plate slides beneath another into the mantle, the hotter layer beneath the crust. The great plates are rough and stick together, building up energy that is released as earthquakes. East of Japan, the Pacific plate dives beneath the overriding Eurasian plate. The temblor completely released centuries of built up stress between the two tectonic plates, a recent study found.
The March 11 earthquake started on a Friday at 2:46 p.m. local time (5:46 a.m. UTC). It was centered on the seafloor 45 miles (72 kilometers) east of Tohoku, at a depth of 15 miles (24 km) below the surface. The shaking lasted about six minutes.