Authorities issued a warning for a three feet high tsunami after today's earthquake, which had its epicentre in Fukushima prefecture. The warning was lifted after an hour.
Workers battling to contain a crisis at the stricken Fukushima nuclear plant were ordered to evacuate after the powerful aftershock, operator TEPCO said.
The earthquake briefly knocked out power to crippled reactors before electricity was restored.
"The company ordered workers to withdraw and stay in a quake-proof building," a spokesman for the operator said. "We don't know many workers were involved."