The source of the earthquake was the giant thrust fault that marks the surface of the Pacifi c plate where it moves beneath northern Honshu ( Figure 4.3 ). Subduction earthquakes , which result from the slip of two plates at a subduction zone, are fairly common in this region, but the 2011 event was unusual because of its size. Few subduction earthquakes in Japan have magnitudes greater than 8.5, yet the 2011 earthquake released nearly twice the energy of a magnitude 8.5 quake. Portions of northeastern Japan moved 2.4 m closer to North America during the earthquake. Remarkably, the seafl oor east of the epicentre, about 200 km east of the Japanese coast, moved 50 m toward the east and rose about 7 m.