At 09.57 local time on the morning of 10 July 2011 a magnitude 7.1 Earthquake occurred 210 km off the Sanriku Coast of Japan, at a depth of 10 km. This triggered an automatic tsunami warning by the Japan Meteorological Agency, leading to the evacuation of coastal residents and workers at the stricken Fukushima Nuclear Plant. In the event a wave was witnessed but only reached 30 cm in height.
Map showing the location of the earthquake, and areas where the shock was felt.
Sanriku is a historical province corresponding to the modern Aomori and Iwate Prefectures, as well as parts of Miyagi Prefecture.
The earthquake was felt by residents across a large area of northeast Honshu Island, though there were no reports of any casualties. It was followed by a smaller, magnitude 5 earthquake at the same location at 10.04 am, though this was not felt at all from land. Both these earthquakes were thought to be aftershocks of the devastating March 11 earthquake.
Japan is highly prone to earthquakes, as it is located on the margin of the Philippine, Pacific and Eurasian plates.
See also The Nagano Earthquake of 30 June 2011, Rethinking Energy and Earthquakes on Sciency Thoughts YouTube.