A great earthquake with a moment magnitude of 9.0 occurred on March 11, 2011, rupturing the plate boundary off the Pacific coast of the northeastern Japan. Large displacements induced by this great earthquake were observed by the GPS Earth Observation Network system (GEONET) over the entire Japanese Islands, and were also detected by the seafloor geodetic observation (SGO) along the Japan Trench. The maximum horizontal displacement observed by the GEONET reaches 5.4 m at the tip of the Oshika Peninsula, and exceeds 30 m at ocean bottom detected by SGO. The subsidence up to 1.1 m was observed by the land GPS along the Pacific coast, and the uplift up to 5 m was detected by the SGO near the epicenter. Based on these geodetic data, the area of coseismic slip is estimated to be ∼450 km long and ∼200 km wide along the Japan Trench with a maximum slip of larger than 50 m near the trench. Because no M≥8.5 earthquake was identified there during the historical period, how strains are accumulated and released is the most essential problem to be solved. The shallower portion of the plate interface has been considered to be decoupled, but such an idea needs to be revised after the Tohoku earthquake. The postseismic deformation exceeds 70 cm for 180 days after the main shock. The coseismic subsidence areas begin to uplift except for the area in the Iwate prefecture. Estimated afterslip exceeds 2.0 m and its area extends to the west, south and north of the coseismic slip area with a moment magnitude of 8.5 for 180 days. The area of the afterslip was extended westward and reaches a depth of approximately 90 km of the subducting plate. Northern and southern edges of the area of afterslip seem to be limited by the source region of the 1994 Sanriku-Haruka-oki earthquake and the north limit of the overriding Philippine Sea plates, respectively. Although the afterslip decayed rapidly with time, the area of afterslip is estimated to be ∼400 km long and ∼180 km wide with a maximum slip of larger than 0.15 m during the period from August 8 to September 7, 2011 that corresponds to a moment magnitude of 7.6. It is definitely important that we continue the observation of the postseismic deformation and carefully monitor the temporal and spatial change of the afterslip for the purpose of the mitigation of hazard due to incoming huge interplate earthquake and understanding the earthquake generation cycle.
