A strong earthquake with a preliminary magnitude of 6.1 jolted the Tokyo region this morning.
No tsunami warning was issued, however, and there were no reports of serious damage or injuries.
Two runways at Narita International Airport near Tokyo were closed for checks but soon reopened.
Some high-speed bullet train lines experienced minor delays as power stopped briefly, and some local lines were halted. Officials warned of possible aftershocks and landslides.
The focus of the quake was about 37 miles below the earth's surface in Chiba prefecture, east of Tokyo, the Japan Meteorological Agency said.
Television pictures showed passengers being helped from a train that had come to a halt between stations in rural Chiba, and a convenience store with goods scattered over the floor.
Earthquakes are common in Japan, one of the world's most seismically active areas, and the country accounts for about 20 per cent of the world's earthquakes of magnitude six or greater.