UPDATE 2-Quake hits northern Japan, injuries reported
TOKYO, Aug 16 (Reuters) - A strong earthquake with a preliminary magnitude of 6.8 jolted northern Japan on Tuesday, and media reports said many people had been injured.
Buildings also swayed in Tokyo when the quake, measuring 6 on the Japanese scale of 7, struck at 11:46 a.m. (0246 GMT).
A tsunami warning was issued for the coast of Miyagi prefecture, about 300 km (190 miles) north of Tokyo, and some buildings had been damaged, TV reports said.
Many people had been injured in Sendai, a regional city with a population of about one million, Kyodo news agency reported, citing local fire department officials.
"It shook for a long time, but things didn't fall down or fall from shelves. And as far as I can see, no windows are broken," a Sendai city official told NHK.
The focus of the tremor was 20 km below the sea off the coast of Miyage, the Japan Meteorological Agency said.
Tohoku Electric said all three nuclear power generators at its Onagawa plant stopped automatically after the quake and Nippon Oil said it suspended operations at its refinery in the district. Trains were also stopped in the region.
Sony Corp, which has a production facility in Sendai for recording media and batteries, said there was no impact from the earthquake on its facilities and no staff were injured.
The magnitude of the earthquakes was measured according to a technique similar to the Richter scale, but adjusted for Japan's geological characteristics.
Earthquakes are common in Japan, one of the world's most seismically active areas. The country accounts for about 20 percent of the world's earthquakes of magnitude 6 or greater.
In October 2004, an earthquake with the same magnitude struck the Niigata region in northern Japan, killing 40 people and injuring more than 3,000.
That was the deadliest quake since a magnitude 7.3 tremor hit the city of Kobe in 1995, killing more than 6,400.