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.