A strong earthquake with a preliminary magnitude of 6.8 jolted northeastern Japan off Fukushima prefecture Friday and a 50 cm tsunami advisory was issued for the coast of Miyagi and Fukushima prefectures, the Japan Meteorological Agency said.

The tsunami advisory was issued for the coastal areas that suffered severe damage from the massive quake and tsunami March 11.

Tokyo Electric Power Co said no abnormalities had been found at radiation monitoring posts at the crippled Fukushima Daiichi nuclear power plant and that cooling operations at the damaged reactors were continuing.