(a) The Tohoku-Oki earthquake occurred where the Pacific Plate (oceanic crust) is subducting under the North American Plate (continental crust). (b) In the months prior to the earthquake, the Pacific Plate moved downward faster likely because the plate became denser. (c) In response, the subduction zone fault weakened, the shallow portion of the Pacific Plate resisted the downward motion, and GPS stations recorded plate wobbling. (d) The large earthquake was possibly triggered by the weakened fault. Credit: Felipe Gamo, University of Concepción, Chile