An earthquake measuring 6.1 on the Richter scale jolted off the east coast of Honshu, Japan Thursday at 23:58 local time, the U.S. Geological Survey said.
Japanese workers entered the No.1 reactor building at the crippled Fukushima Daiichi nuclear plant for the first time since a hydrogen explosion ripped off.
Level 7 - Major release of radioactive material. Example: Chernobyl, Ukraine, 1986
Level 6 - Significant release of radioactive material. Example: Kyshtym, Russia, 1957
Level 5 - Limited release of radioactive material. Example: Three Mile Island, US, 1979, and Windscale, UK, 1957
Level 4 - Minor release of radioactive material with at least one death from radiation. Example: Tokaimura, Japan, 1999
Level 3 - Exposure in excess of 10 times the statutory annual limit for workers
Level 2 - Exposure of a member of the public in excess of 10mSv (average annual dose is 1mSv)
Level 1 - Exposure of a member of public above statutory annual limit. Minor safety problems