TOKYO, July 16 (Xinhua) -- An earthquake measuring 6.6 on the Richter scale rocked Niigata prefecture of central western Japan and surrounding areas on Monday morning, injuring over a dozen people.

    Dozens of houses in Kashiwazaki in Niigata collapsed due to the 10:13 a.m. (0113 GMT) quake, injuring dozens of people, Kyodo News said. In nearby Nagano prefecture, at least four people suffered injuries.

    The tsunami warning issued from the strong quake was relieved about an hour later. As many as eight aftershocks were registered from 10:34 a.m. (0134 GMT) to 11:43 a.m. (0243 GMT) in Niigata, with magnitudes around 3 on the Richter scale, according to Japan Meteorological Agency.

    The epicenter of the quake was around 10 kilometers below sea level in waters off Niigata prefecture, the agency said.

    Due to the strong quake, four reactors at Kashiwazaki-Kariwa nuclear plant automatically shut down for safety, public broadcaster NHK said.

    The quake also caused power failures, forcing East Japan Railway Co. to halt Shinkansen bullet train services. JR East said it is halting train services on the Tohoku, Joetsu and Nagano Shinkansen lines as well as other train services. Several parts of highway in Niigata were also closed after the quake.

    Niigata airport shut down its runways immediately after the quake to check for damage, airport officials said.

    The government has set up an emergency management center to handle the quake. Prime Minister Shinzo Abe, who has been in Kyushu of southern Japan, is coming back to Tokyo due to the quake, local reports said.

    The quake measured upper 6 on the Japanese seismic intensity scale of 7 in Kashiwazaki and Kariwa in Niigata prefecture and Iizuna in Nagano prefecture. The earthquake also affected nearby prefectures including Ishikawa, Yamagata, Tochigi, Saitama, Fukushima and others.