Slightly before 7.45 am on 28 January 2012 local time (slightly before 10.45 pm on 27 January, GMT) an Earthquake shook the Yamanashi Prefecture in Japan, and was felt in surrounding areas. The quake was recorded as measuring 5.5 on the Richter Scale by the Japan Meteorological Agency, and as occurring at a depth of about 20 km. The quake was followed after a few minutes by an aftershock measuring 4.1 on the Richter Scale, also at a depth of 20 km. There are no immediate reports of any casualties or serious damage.
Map of the effected area from the Japan Meteorological Agency (red cross). The colours refer to the intensity of shaking, higher numbers indicate more severe shaking.
Japan lies on the boundary between four tectonic plates, the Eurasian, Philippine Sea, Pacific and North American. Yamanashi Prefecture is in the Yatsugatake Mountains, which lie over the border between the Eurasian and Philippine Sea Plates. The Philippine Sea Plate is being subducted beneath the Eurasian Plate in the Nankai Trench to the south of Honshu Island. As it passes under the island it sticks to the underside of the Eurasian Plate, causing the overlying plate to crumple, and forming fold mountains. This causes periodic build ups of pressure, which are released in Earthquakes. In addition as the Philippine Sea Plate sinks into the Earth, it is melted by the heat of the planet's interior. When this happens some of the melted rock rises up through the overlying Eurasian Plate, leading to the formation of volcanoes in the Yatsugatake Mountains.
Map of Japan showing the plate boundaries. The Yatsugatake Mountains are part of the SW Japan Arc.
See also Explosion on Suwanosejima, Earthquake in the Babuyan Islands, Massive deep Earthquake south of Japan, Manila shaken by mild earthquake and Earthquakes on Sciency Thoughts YouTube.