Big Tokyo earthquake likely 'within the next few years'
The chance of a big earthquake hitting the Japanese capital in the next few years is much greater than official predictions suggest, researchers say.
The team, from the University of Tokyo, said there was a 75% probability that a magnitude seven quake would strike the region in the next four years.
The government says the chances of such an event are 70% in the next 30 years.
The warning comes less than a year after a massive earthquake and tsunami devastated Japan's north-eastern coast.
The last time Tokyo was hit by a big earthquake was in 1923, when a 7.9 magnitude quake killed more than 100,000 people, many of them in fires.
Researchers at the University of Tokyo's earthquake research institute based their figures on data from the growing number of tremors in the capital since the 11 March 2011 quake.
They say that compared with normal years, there has been a five-fold increase in the number of quakes in the Tokyo metropolitan area since the March disaster.
They based their calculations on data from Japan's Meteorological Agency, They said their results show that seismic activity had increased in the area around the capital, which in turn leads to a higher probability of a major quake.
The researchers say that while it is "hard to predict" the casualty impact of a major quake on Tokyo, the government and individuals should be prepared for it.
Correspondents say that while the university calculations take account of greater seismic activity since March, government calculations may use different or less up-to-date data and different modelling techniques.
The 9.0 magnitude earthquake last year also crippled the cooling systems at the Fukushima nuclear power station, causing meltdowns in some of its reactors.
Japan is located on a tectonic crossroads dubbed the "Pacific Ring of Fire" which is why its is commonly regarded as one of the world's most quake-prone countries, with Tokyo located in one of the most dangerous areas.