PARIS (AFP) - Seismologists said on Wednesday they have found clues as to why Japan's 2011 mega-earthquake occurred on a fault previously deemed to be of little threat.

The findings have repercussions for the country's earthquake strategy and for other locations, including California's notorious San Andreas fault, with a similar seismic profile, they said.

Hiroyuki Noda of the Japan Agency for Marine-Earth Science and Technology and Nadia Lapusta of the California Institute of Technology based their findings on a computer model of the March 11, 2011 quake, which triggered a tsunami that killed about 19,000 people and wrecked the Fukushima Daiichi nuclear plant, sparking the world's worst atomic crisis in a generation.

The 9.0-magnitude earthquake struck off north-eastern Japan in part of the so-called Japan Trench, where the Pacific plate ducks beneath the Okhotsk plate, on which the Japanese archipelago lies.

Its epicentre was about 200km east of Honshu island's Sendai, at the heart of a lozenge-shaped area of ocean floor.

This area of the Japan Trench had been generally considered to be stable, as it was a "creeping" segment, meaning that any movement of the plate there was smooth and regular.

According to a commonly accepted theory, this steady movement prevents stress from building up to the point where the fault rips open - rather like a safety valve on a steam engine.

But Mr Noda and Ms Lapusta suggest that fault segments which experience long-term, stable "creep" in fact become weakened when a nearby section of the fault ruptures.

And if the fault is infiltrated by hot geological fluids, this acts as a lubricant, helping a big slip to occur.

"Steadily creeping fault segments are currently considered to be barriers to earthquake rupture. Our study shows that they may join large earthquakes, amplifying seismic hazard," said Mr Noda in an email exchange with AFP.

The authors said they hope their work will be factored into Japan's earthquake awareness programme. Some experts have accused the programme of focussing obsessively on the risk to Tokyo, south of where the 2011 event occurred.

The findings also have implications for risk assessment for the San Andreas fault, which runs down the coast of California, according to the study published in the journal Nature.

The San Andreas also has a creeping segment regarded as a blocker for big earthquakes, said Mr Noda.

"But whether it always acts as a barrier or can join a great earthquake is not a trivial question," he warned.

"Looking for an evidence of rapid slip or frictional heating in the middle of currently creeping segment would be an important project to judge if the conventional scenarios should be revised."