TOKYO, Japan (CNN) -- A magnitude-6.8 earthquake, centered 17 kilometers below sea level, struck just off the northwestern Japanese coast Monday morning, knocking down several small buildings and resulting in at least 20 injuries, authorities reported.
The quake struck at 10:13 a.m. (0113 GMT), the Japanese Meteorological Agency reported, and caused minor shaking in Tokyo, about 240 kilometers (150 miles) south of its epicenter.
According to The Associated Press, the Meteorological Agency said small tsunamis as high as 20 inches were believed to have hit coasts in the area.
The Meteorological Agency issued a tsunami warning for Sado Island and the coast of Niigata prefecture, where waves were expected to rise about 50 centimeters (20 inches) because of the quake, but the warning was lifted about an hour later.
In Kashiwazaki, in Niigata prefecture, police told Japanese broadcaster NHK that several buildings collapsed and 20 people had been treated at area hospitals. One person was rescued from beneath a collapsed home, and another was still trapped late Monday morning.
The temblor triggered an automatic shutdown of the three reactors at a nuclear power plant outside Kashiwazaki, Tokyo Electric Power reported. A small fire broke out at the plant's electrical transmission facility shortly after the quake, sending a plume of black smoke over the plant, but there was no release of radiation, company spokesman Kaoru Yoshida reported.
The same area was hit by a trio of earthquakes in 2004 that killed 25 people and injured more than 1,200.
Prime Minister Shinzo Abe was campaigning in Nagasaki at the time, but flew back to Tokyo to lead a government task force assessing the damage, the Japanese government reported. E-mail to a friend
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