As Japan is rocked by a powerful 6.6-magnitude earthquake shortly after holding a minute's silence to mark a month since the massive March 11 quake and tsunami which sparked a nuclear emergency, here is a list of the key events since then:

Friday, March 11: A 9.0 magnitude undersea earthquake, the most powerful ever recorded in Japan, strikes off the Northeast coast just before 3.00 pm local time (0600 GMT), triggering a massive tsunami which destroys whole towns and villages.

The power supply and cooling systems at the Fukushima Daiichi nuclear power plant 250 kilometres (155 miles) North of Tokyo are damaged.

Saturday, March 12:

The government orders the evacuation of residents living close to the plant, where an explosion occurs in a building housing one of the reactors.

Sunday, March 13:

The government says 230,000 people have been evacuated from the vicinity of the crippled nuclear reactors.

Prime Minister Naoto Kan says Japan is facing its worst crisis since the end of World War II and millions of residents are without any power or water.

Monday, March 14:

A second explosion occurs at the stricken Fukushima plant.

Tuesday, March 15:

Two more blasts and a fire rock the Fukushima plant and radiation levels around the facility reach dangerous levels, prompting the government to advise people up to 30 kilometres (20 miles) from the reactor to stay indoors.

Thursday, March 17

Foreign governments urge their citizens to leave Tokyo and the US state department authorises the voluntary departure of US embassy family members.

Friday, March 18

The core of the number three reactor is damaged.

Japan's nuclear safety agency raises the Fukushima crisis level from four to five on the international scale of gravity for atomic accidents, which goes up to seven.

Saturday, March 19

Abnormal levels of radiation are detected in milk and spinach from areas near the stricken plant and traces of radioactive iodine are reportedly found in tap water in Tokyo.

Monday, March 21

Japan orders a halt to shipments of certain foods from four prefectures after abnormal radiation levels are detected in products from the area.

Tuesday, March 22

Tokyo electric power company, the operator of the crippled nuclear plant, apologises for the "anxiety and nuisance" caused to residents forced to flee the high-radiation zone around the plant and to wider society.

Wednesday, March 23

Government warns radiation levels in the tap water in Tokyo and neighbouring towns are above levels safe for babies.

Ban on milk and fresh produce from the Fukushima region.

Thursday, March 24

Three workers are exposed to high levels of radiation at Fukushima, with two hospitalised after stepping in a puddle of radioactive water. They are discharged on March 27 with no sign of serious injuries.

Tuesday, March 29

"Maximum alert" after plutonium discovered in the soil.

At least 19 people are exposed to high levels of radiation at the site.

Thursday, March 31

Radioactivity detected in groundwater reaches a new high of 10,000 times the legal limit.

Concentrations of radioactive iodine 4,385 times higher than normal are recorded in the sea.

The government does not expand the evacuation zone, as recommended by the UN nuclear watchdog, the IAEA.

French President Nicolas Sarkozy arrives in Tokyo, the first foreign leader to visit since the disaster.

Friday, April 1

Japan announces start of "rehabilitation and reconstruction phase".

About 25,000 Japanese and US troops are deployed to the coast in the search for victims.

Saturday, April 2

Radioactive water leaks into the sea from a crack in the damaged nuclear plant.

First visit by Prime Minister Naoto Kan to the devastated northeast.

Monday, April 4

Start of operations to dump 11,500 tonnes of radioactive water into the sea.

Tuesday, April 5

India becomes the first country to impose a total ban on the import of food products from Japan.

Wednesday, April 6

The radioactive water leak at the plant is plugged.

Thursday, April 7

Powerful 7.1 magnitude aftershock hits the northeast, leaving two dead. More than 400 strong tremors have struck since March 11.

Mildly radioactive water slops from fuel cooling pools at a power plant in Onagawa, Miyagi prefecture.

Nuclear plant operator Tepco starts injecting nitrogen into the number one reactor of the Fukushima plant to prevent a hydrogen explosion.

Friday, April 8

The European Union tightens radiation limits on food imported from Japan.

Japan announces it will lift some of the restrictions on selling fresh produce from areas near the Fukushima plant.

India lifts its ban on food imports from Japan but demands "radiation free" certificates with Japanese shipments of produce.

China expresses concern over the release of radioactive water from the plant into the sea.

Saturday, April 9

Industry minister Banri Kaieda becomes the first government official to visit the crippled nuclear plant.

Monday, April 11

A strong 6.6-magnitude earthquake hits eastern Japan, causing a brief loss of power and evacuation of workers at the stricken nuclear plant. Buildings sway in Tokyo but a tsunami alert is later lifted.

The quake comes shortly after the the nation holds a minute's silence to remember the 28,000 dead and missing from the twin disasters a month earlier.

Government says it will widen the evacuation zone around the Fukushima plant because of long-term health concerns.

Before the strong aftershock the government says the risk of a massive radiation leak is becoming "significantly smaller".

President of Tepco Masataka Shimizu visits Fukushima and apologises "from the bottom of my heart" for the atomic emergency.