A month later , a major aftershock struck offshore on 7 April with a magnitude of 7.1 Mw . Its epicenter was underwater , off the coast of Sendai . The Japan Meteorological Agency assigned a magnitude of 7.4 MJMA , while the U.S. Geological Survey lowered it to 7.1 Mw . At least four people were killed , and electricity was cut off across much of northern Japan including the loss of external power to Higashidōri Nuclear Power Plant and Rokkasho Reprocessing Plant
major aftershock
April 2011 Miyagi earthquake
2011 Tōhoku earthquake and tsunami