(Adds no reports of casualties, details)
TOKYO, May 12 (Reuters) - A magnitude 5.1 earthquake hit central Japan on Saturday morning but no tsunami is expected, the Japan Meteorological Agency said.
Police said they have heard no reports of damage or casualties.
The quake's epicentre was in northern Nagano prefecture in central Japan, and tremor occurred at around 10:29 a.m. (0129 GMT) at a depth of around 10 km (6 miles), the weather agency said.
No abnormalities have been observed at Tokyo Electric Power Company Holdings' (TEPCO) Kashiwazaki-Kariwa nuclear power plant in neighboring Niigata prefecture, public broadcaster NHK quoted the electric utilities as saying. (Reporting by Kiyoshi Takenaka; Editing by Simon Cameron-Moore)