Scientists say last year's earthquake in Japan has greatly increased the pressure of the magma chamber underneath Mount Fuji.

The volcano last erupted in 1707, less than two months after a large earthquake.

That blast scattered ash and cinders as far away as Tokyo, 100 kilometres from the volcano.

Experts at Japan's National Research Institute for Earth Science and Disaster Prevention say there is heightened pressure under Mount Fuji due to last year's devastating earthquake and aftershocks.

The magma chamber is about 15 kilometres underground and it is feared the increase in pressure could trigger an eruption.

When Mount Fuji last erupted, the pressure caused by the prior earthquake was weaker than that of last year, the researchers said, warning that eruptions could occur some years after earthquakes.

Based on the tectonic movements caused by the two earthquakes in March 2011, the researchers estimated about 1.6 megapascals of pressure, equivalent to atmospheric pressure of 15.8 kilograms per square centimetre, has been placed on the magma chamber.

In the past, 0.1 to several megapascals of pressure have triggered volcanic eruptions.

Although conditions in the magma chamber vary, 1.6 megapascals is "not a small figure", Eisuke Fujita, a senior researcher at the institute, said.

In 2004, the government issued a report estimating the maximum economic damage if Mount Fuji erupts at about 2.5 trillion yen ($31 billion).

But the researchers say heightened pressure is not the only factor which can cause volcanic eruptions and no signs of eruption have been detected so far.

ABC/Kyodo