The Pacific Plate is the tectonic plate subducting beneath Japan and responsible for the Tohoku earthquake. Along the Japan Trench, the dense oceanic Pacific Plate moves westward and descends beneath the overriding Okhotsk microplate, which is often grouped with the North American Plate in simplified maps.
On 11 March 2011, friction locked part of this convergent boundary for centuries. When the fault suddenly ruptured, the seafloor shifted dramatically, generating the magnitude 9.0–9.1 undersea megathrust earthquake and the devastating tsunami. The rupture extended for hundreds of kilometres along the trench.
A common mix-up is choosing the North American or Eurasian Plate because Japan sits near several complicated plate-boundary systems. However, the plate being forced downward at the Japan Trench is the Pacific Plate. The Philippine Sea Plate also subducts beneath southwestern Japan, but it was not the plate involved in the main Tohoku rupture. The earthquake occurred east of the Tohoku region, where the Pacific Plate meets the overriding plate.