An earthquake of magnitude .mw-parser-output .tooltip-dotted{border-bottom:1px dotted;cursor:help}Mw6.6 struck Niigata Prefecture, Japan, causing a leak of radioactive gases from the Kashiwazaki-Kariwa Nuclear Power Plant.

The Chūetsu offshore earthquake was a powerful magnitude 6.6 earthquake that occurred 10:13 local time on July 16, 2007, in the northwest Niigata Prefecture of Japan. The earthquake, which occurred at a previously unknown offshore fault shook Niigata and neighbouring prefectures. The city of Kashiwazaki and the villages of Iizuna and Kariwa registered the highest seismic intensity of a strong 6 on Japan's shindo scale, and the quake was felt as far away as Tokyo. Eleven deaths and at least 1,000 injuries were reported, and 342 buildings were completely destroyed, mostly older wooden structures. Prime Minister Shinzō Abe broke off from his election campaign to visit Kashiwazaki and promised to "make every effort towards rescue and also to restore services such as gas and electricity".

Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake. These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking (quaking) caused by an earthquake at a given location. Magnitudes are usually determined from measurements of an earthquake's seismic waves as recorded on a seismogram. Magnitude scales vary based on what aspect of the seismic waves are measured and how they are measured. Different magnitude scales are necessary because of differences in earthquakes, the information available, and the purposes for which the magnitudes are used.

Niigata Prefecture is a prefecture in the Chūbu region of Honshu of Japan. Niigata Prefecture has a population of 2,131,009 and is the fifth-largest prefecture of Japan by geographic area at 12,584.18 km² (4,858.78 sq mi). Niigata Prefecture borders Toyama Prefecture and Nagano Prefecture to the southwest, Gunma Prefecture to the south, Fukushima Prefecture to the east, and Yamagata Prefecture to the northeast.