On 11 March 2011 an earthquake categorised as 9.0 MW on the moment magnitude scaleoccurred at 14:46 Japan Standard Time (JST) off the northeast coast of Japan. Reactors 4, 5 and 6 had been shut down prior to the earthquake for planned maintenance.[19][20] The remaining reactors were shut down automatically after the earthquake, but the subsequent tsunami flooded the plant, knocking out emergency generators needed to run pumps which cool and control the reactors. The flooding and earthquake damage prevented assistance being brought from elsewhere. Over the following days there was evidence of partial nuclear meltdowns in reactors 1, 2 and 3; hydrogen explosions destroyed the upper cladding of the building housing reactors 1 and 3; an explosion damaged reactor 2's containment; and severe fires broke out at reactor 4. Due to high radiation levels, most workers at the facility had to evacuate the area, leaving only a small group to manage the entire crisis.
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