Just after 3.20 am on Friday 12 August the Japanese prefecture of Fukushima was shaken by a large earthquake. At its epicenter (the point on the ground directly above the quake) this measured 5- on the Japan Meteorological Agency's Seismic Intensity Scale, which measures the effects of an earthquake rather than the amount of energy released. A 5- indicates people running out of buildings in panic, things falling from shelves, damage to non-earthquake resistant buildings and automatic gas cutoff devices present in Japanese buildings activating.
The location of the 12 August earthquake, and where it was felt.
There are differences of opinion about how deep the quake was, and how large it was on the Richter Scale (which does register energy released). The Japan Meteorological Agency (which also monitors earthquakes, volcanoes, tsunamis etc.) registered this quake as a magnitude 6 quake at a depth of 50 km, whereas the United States Geological Survey recorded this as a magnitude 5.2 quake at a depth of 9.9 km. Since the Richter scale is logarithmic (a magnitude 6 quake is 10 times as powerful as a magnitude 5 quake) and rock absorbs a lot of energy, either of these proposed quakes could have produced the observed effects.
See also The Nagano Earthquake of 30 June 2011, Magnitude 7.1 Earthquake off the Sanriku Coast of Japan, Magnitude 6.4 Earthquake of the Coast of Honshu and Earthquakes on Sciency Thoughts YouTube.
