An earthquake as big as the one that jolted northern Japan last year could kill more than 320,000 people if it happened off central and western Japan, though the probability of such an occurrence is extremely low, the government said today.

The report, compiled by a government panel of experts to help devise a new disaster response policy, was commissioned following the March 2011 magnitude 9 earthquake off northern Japan. The official toll from that calamity stands at the moment at 15,869 dead and 2,847 missing.

In the worst case scenario, in which a magnitude 9 earthquake hits in the middle of the night in winter, up to 323,000 people would likely be killed, with the tsunami accounting for 71 percent of the deaths, the report said.

It generally takes more time for residents to flee from a tsunami in the night. And some heating devices, such as kerosene stoves, increase the number of casualties through the risk of fire.

The projected death toll compares to 24,700 in a 2003 estimate based on an 8.7 earthquake happening in the same area, near major industrial cities, including Nagoya and Hamamatsu.

A magnitude 9 quake is about three times as big as a magnitude 8.7 tremor in terms of the amount of energy it vents.

The government plans to map out next year new earthquake response steps, including how many military personnel and firefighters need to be mobilised to handle the aftermath of a major earthquake, a Cabinet Office official said.