This 6.6 Magnitude quake hit at 10:13 AM local time (01:13 GMT), just off the west coast of the island of Honshu in central Japan. The epicenter was 45 miles west of the city of Nigata and 150 miles to the northwest of Tokyo. It was followed a couple of hours later by a 6.8 M quake nearby. Although the second quake was stronger, it was focused depper and thus cause less damage. According to the US Geological Service account of the quake, these quakes were different enough that the second one is not considered an aftershock of the first.
The earthquake caused damage to 875 buildings, especially older ones, 1,088 injuries and 11 deaths. The shaking was so strong that people said they couldn't stand during the quake. A nuclear power plant remains shut down two months after the quake due to damage that resulted in the release of radiation. This accident follows several other incidents at nuclear power plants in Japan that have called into question the safety of Japan's extensive nuclear power program.
Japan is located at the western edge of the Pacific Plate, where it interacts with the Eurasian Plate and some small tectonic plates inbetween. This results in many earthquakes as these plates interact. In 2004 a 6.6 M quake in the same general area killed 40 people and injured 3,000. The US Geological Service account of the quake describes the geology in more detail. (See the Plate Tectonics page for more information on these processes.) The US Geological Survey (USGS) is an invaluable resource in understanding and tracking earthquakes.
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