At about 6.15 pm Japan Standard Time (9.15 am GMT) northeastern Japan was shaken by a quake 235 south of the coast of Hokkaido and 293 km east of Honshu. This was recorded as measuring 6.8 on the Richter Scale at a depth of 10 km by the Japan Meteorological Agency and 6.9 on the Richter Scale at a depth of 26.6 km by the United States Geological Survey. This was sufficient to provoke a tsunami warning, though no hazardous wave was forthcoming; a 20 cm wave was recorded an hour after the quake in the port of Hachinohe in Aomori, with smaller waves recorded at other points on the coast; technically a tsunami, but far from hazardous.
The location of the 6.14 pm quake, and the areas that felt shaking. Higher numbers on key refer to more severe shaking. Japan Meteorological Agency.
At about 9.10 pm JST (12.10 pm GMT) a second large quake hit the same area, this one roughly 244 km south of Hokkaido and 279 km east of Honshu. This was measured as 6.1 on the Richter Scale by both the Japan Meteorological Agency and the United States Geological Survey, with the JMA placing it at a depth of 10 km and the USGS at 9.5 km. This quake did not cause a tsunami, but was felt across a much larger part of the country and caused Narita International Airport to close briefly.
The location of the 9.10 pm quake. United States Geological Survey.
Both these quakes occurred on the Japan Trench system, where the Pacific Plate is being subducted beneath the Othotsk Plate (an extension of the North American Plate) to the east of northern Japan. This was the same subduction zone that caused the devastating Tōhoku Earthquake and tsunami of 11 March 2011.
Japan has a complex tectonic environment with four plates underlying parts of the Islands; in addition to the Pacific in the east and the Othorsk in the North, there are the Philipine Sea Plate to the south and the Eurasian Plate to the West. This makes Japan extremely prone to Earthquakes, typically suffering dozens a week, and one-or-two larger quakes a month. This has made the country one of the world's best prepared for quakes, able to deal with quakes that would cause devastation in other parts of the world, though as the Tōhoku Earthquake demonstrated, truly large quakes can still cause Japan major problems.
The tectonic plates underlying Japan. Volcano Lovers.
See also Earthquake shakes Yamanashi Prefecture, Explosion on Suwanosejima, Massive, deep Earthquake, south of Japan, Hokkaido Earthquake and Earthquakes on Sciency Thoughts YouTube.