Japan hit by 7.1 aftershock on quake anniversary

SENDAI, Japan // Japan on Monday widened the evacuation zone around a stricken nuclear plant exactly a month on from a huge natural disaster as another 7.1 magnitude quake and tsunami alert strained nerves anew.

The latest aftershock caused buildings to sway in Tokyo, shortly after the nation had observed a minute's silence to remember the 13,000 people killed in the March 11 disaster and the 15,000 who officially remain missing.

The US Geological Survey said the 7.1 onshore quake hit at 5.16pm at a depth of just 13 kilometres. Its epicentre was 81 kilometres south of Fukushima city, near the troubled nuclear plant.

Japan's meteorological agency warned that a one-metre (three foot) wave could hit Ibaraki prefecture, one of the areas pummelled by last month's massive tsunami, before cancelling the alert less than an hour later.

Another tremor of 7.1 on April 7 - just one of thousands of aftershocks to hit the traumatised country - killed at least two people and cut electricity across a huge area of northern Japan.

Workers battling to contain the crisis at the Fukushima nuclear plant were evacuated after the latest quake on Monday, which briefly knocked out power to crippled reactors before electricity was restored.

People across the country had paused at 2.46pm, the moment Japan's biggest ever recorded earthquake struck, setting off a chain of events that has left workers scrambling to tame runaway atomic reactors at the Fukushima plant.

Underlining the threat of long-term health damage from radiation, the government on Monday said it was to widen the evacuation area around the atomic plant to include some towns outside the current 20-kilometre exclusion zone.

Those areas were liable to receive potentially hazardous radiation levels of 20 millisieverts per year, the top government spokesman. Yukio Edano. said, while stressing there was no deterioration at the Fukushima plant.

Engineers at Fukushima who last week sealed a leak spewing highly contaminated water into the sea have begun installing a "silt curtain" to try to prevent radioactive mud from spreading around the ocean.

But at the same time, plant operator Tepco is deliberately dumping more than 10,000 tonnes of mildly radioactive water into the ocean to free up urgently needed storage space for highly toxic liquid.

Masataka Shimizu, the president of Tepco, on Monday visited Fukushima and apologised for the atomic emergency engulfing the area.

"I offer my personal apology from the bottom of my heart once again to the people in Fukushima prefecture and residents near the nuclear plant for having imposed such awful physical and mental burdens," he said.