A state of emergency was declared at one of the country’s nuclear power plants after the quake.
The Fukushima reactor, which is around 30 miles inland from the coast in north-east Japan, suffered a failure in its cooling system.
Chief Cabinet Secretary Yukio Edano, the top government spokesman, said the nuclear power plant developed a mechanical failure in the system needed to cool the reactor after it was shut down.
Japanese authorities were last night planning to release slightly radioactive vapour to ease the pressure within the nuclear reactor whose cooling system failed.
Japan’s nuclear safety agency said pressure inside one of six boiling water reactors at the plant had risen to 1.5 times the level considered normal.
The agency said the radioactive element in the vapour that will be released would not affect the environment or human health.
Professor Tim Abram, a nuclear fuel technology expert at Manchester University, said that as long as a reactor was shut down, it was considered “benign” until bosses decided it was safe to be turned back on.
He said: “All nuclear facilities are designed to withstand seismic events.
“The magnitude of the seismic event that they are designed to withstand varies from country to country.
“It’s not done on a case of a particular point on the Richter scale, but instead on the basis of probability of earthquakes in particular countries.
“In somewhere like Japan, the probability will be much, much higher.”