TOKYO, Aug 9 (Reuters) - A strong earthquake with a preliminary magnitude of 6.9 jolted central Japan on Sunday, halting some train services briefly but there were no immediate reports of damage.

The lengthy quake, felt across the capital just before 8 p.m. (1100 GMT), prompted railway services to halt some high-speed services briefly while tracks were checked, national broadcaster NHK said, but there were no immediate reports of damage.

The tremor was centred 340 km (210 miles) deep under the sea south of Tokyo but, based on a Japanese scale of ground shaking from the Japan Meteorological Agency, it was unlikely there was much damage.

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 Rodney Joyce and Yoko Nishikawa; Editing by Sanjeev Miglani)