A magnitude 6.8 earthquake hit northern Japan Tuesday just before noon, triggering a tsunami alert along the Pacific coast, the Meteorological Agency reported.

The agency said the quake's epicenter was located about 50 miles off the coast of Sendai, Miyagi prefecture, about 186 miles northeast of Tokyo.

TV reports indicated about 80 people were injured as the roof on an indoor pool collapsed in Sendai.

The shaking alerted the East Japan Railway Co., which immediately suspended bullet train services, causing major transportation disruptions amid the rush of returning holidaymakers. Sendai's subway system was also disrupted.

News reports said that a nuclear power plant in Onagawa, Miyagi prefecture, shut down automatically, leaving thousands of households in the area without electricity.

According to the agency, the earthquake struck around 11:46 a.m. and was centered 12.4 miles below the ocean floor.