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TOKYO, Aug 22 (Reuters) - An earthquake with a preliminary magnitude of 5.1 jolted central Japan on Friday, a Japanese government agency said.

The quake, which occurred at 7:59 p.m. (1059 GMT), was also felt in Tokyo.

There were no immediate reports of damage, and no tsunami warning was issued.

The focus of the tremor was 50 km (31 miles) below the surface of the earth, in Ibaraki prefecture, just north of Tokyo, the Japan Meteorological Agency said.

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 a magnitude of 6.8 struck the Niigata region in northern Japan, killing 65 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. (Reporting by Yoko Kubota; Editing by Valerie Lee)

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