NORTH-EASTERN Japan can expect another monster earthquake large enough to trigger a tsunami within days, the head of the Australian Seismological Centre says.

The director, Kevin McCue, said there had been more than 100 smaller quakes since Friday, but a larger aftershock was likely.

''Normally they happen within days,'' he said. ''The rule of thumb is that you would expect the main aftershock to be one magnitude smaller than the main shock, so you would be expecting a 7.9.

''That's a monster again in its own right that is capable of producing a tsunami and more damage.''

The Japanese quake was the result of a process called thrust faulting. A piece of the Earth's crust broke away at the juncture of the Eurasian and Pacific plates and was thrust underneath the islands of Honshu and Hokkaido. The US Geological Survey estimated the quake moved the Japanese coast about 2.4 metres.

Japanese volcano erupts

A volcano in southwestern Japan erupted Sunday after nearly two weeks of relative silence, sending ash and rocks up to four kilometres (two and a half miles) into the air, a local official says.

It was not immediately clear if the eruption was a direct result of the massive 8.9-magnitude earthquake that rocked northern areas Friday, unleashing a fierce tsunami and sparking fears that more than 10,000 may have been killed.   [ TIMES LIVE ] 

Japan Earthquake and Tsunami overview

•  42 survivors have been pulled out of the rubble
• Official death toll hits 1,597, but many hundreds believed to be buried under rubble or washed away by waves
• Toll will soar after around 2,000 bodies were found on the shores of Miyagi prefecture
• Second explosion at nuclear power plant
• Number of people contaminated with radiation could reach 160
• Region hit by hundreds of aftershocks, some up to 6.8-magnitude
• Rescue operation begins but some areas still cut off by road damage and flood waters
• 70,000 people evacuated to shelters in Sendai

''It basically pushed the sea floor up and down on opposite sides of the fault by 10 metres, causing the tsunami,'' Dr McCue said. ''It is a sudden rupture that has occurred, but it has occurred because the two plates are converging at about eight centimetres a year and have been for about 100 years. That eight metres is released suddenly when the plate snaps and breaks and produces the earthquake.''

Japan's last earthquake on this scale was in 1923, when the magnitude 7.9 Kanto quake killed more than 100,000 people in and around Tokyo and Yokohama.

The latest Japanese disaster is unrelated to the quake that devastated Christchurch last month, which was caused by a fracturing within the Pacific plate.

A seismology research fellow at the University of Melbourne, Gary Gibson, said the world averages one magnitude 8 quake a year, but the rate was inconsistent. The 1980s and 1990s had far fewer large quakes than average, for example.

''There is more variation than you would expect from a random occurrence of earthquakes, and we really don't have a mechanism to describe why that is the case,'' Dr Gibson said. ''But there is no question that the last two years have been very active and well above average.''

Dr McCue dismissed suggestions that melting glaciers due to global warming could escalate the earthquake risk.