By Elaine Lies

TOKYO (Reuters) - Japan's massive 2011 earthquake may trigger more, and larger, volcanic eruptions over the next few decades, perhaps even that of Mount Fuji - but predicting them remains close to impossible, a volcano expert said on Friday.

The nation last month suffered its worst volcanic disaster in nearly 90 years when Mount Ontake, its second tallest active volcano at 3,067 meters (10,062 feet), suddenly erupted, raining down ash and stone on hikers crowding the summit.

The eruption killed 56 people, exceeding the deaths in the 1980 eruption of Mount St Helens in the United States. Seven victims remain missing, and recovery efforts have been suspended until the spring.

Japan may well be moving into a period of increased volcanic activity touched off by the 9.0 magnitude earthquake of March 11, 2011, said Toshitsugu Fujii, a volcanologist and professor emeritus at the University of Tokyo.

"The 2011 quake convulsed all of underground Japan quite sharply, and due to that influence Japan's volcanoes may also become much more active," Fujii told reporters.

"It has been much too quiet here over the last century, so we can reasonably expect that there will be a number of large eruptions in the near future."

Earthquakes of magnitude 9.0 or greater around the world have led to repeated volcanic eruptions in the last 50 years, sometimes within days, Fujii said.

Previous periods of seismic activity in Japan have seen major quakes interspersed with eruptions, he added.

One noteworthy candidate for eruption is iconic Mount Fuji, which used to erupt roughly every 30 years, but has been silent since its last eruption in 1707.

A catastrophic eruption at the 3,776-metre-high (12,390-ft-) snow-capped peak could rain some 10 cm of ash on the capital Tokyo, located 100 km (60 miles) to the northeast.

"The last eruption was 300 years ago, ten times longer than before," Fujii said. "So it could erupt at any time."

Ontake, thought to have been dormant until it erupted in 1979, had been quiet since a minor eruption seven years ago. Small tremors were recorded several weeks before last month's eruption, but in the absence of other signs an eruption could not be predicted, experts said.

Japan monitors 47 of its 110 active volcanoes around the clock, but the research budget has always been less than for earthquakes and critics say there is insufficient equipment.

Fujii said the nation was suffering from a shortage of volcano experts that has left most peaks without observers at the site to constantly keep watch for small changes.

Even with the best measurement, prediction is hard. Of Japan's nine major eruptions since 1977, the longest warning was a week. Most were a matter of hours.

"Fuji is showing absolutely no signs of eruption at this point," Fujii said. "But that says nothing about next year."


(Reporting by Elaine Lies; Editing by Clarence Fernandez)