The deadly 9.0-magnitude quake that struck off northeastern Japan on March 11 ruptured a relatively small part of a notorious fault that straddles the Pacific seabed, Japanese scientists reported on Wednesday.

The earthquake occurred in part of the so-called Japan Trench, where the Pacific plate dives beneath the Okhotsk plate on which the Japanese archipelago lies.

Data supplied by a network of Global Positioning System (GPS) stations, dotted across Japan and called GeoNet, have helped unlock details of where the quake took place and what happened.

Modelling of the stresses and strains placed upon Honshu island as the fault was torn apart indicates the epicentre was about 200 kilometres (120 miles) east of Sendai, at the heart of an extraordinarily compact, lozenge-shaped area of ocean floor.

Only a handful of earthquakes measuring 9.0 magnitude or more have ever been recorded, and they can rip open the sea floor over many hundreds of kilometres (miles).

The biggest quake ever detected, a 9.5-magnitude event that occurred off southern Chile in 1960, ruptured the plate boundary for more than 1,000 kilometres (600 miles).

The March 11 quake, though, points to a slippage zone that measures 400 kms (250 miles) by 200 kms (120 miles) wide.

But what it lacked in size it more than compensated in terms of movement, for the energy release occurred less than 20 kms (12 miles) below the ocean floor.

The sea bottom at the epicentre shifted by an astonishing 27 metres (88 feet), inflicting a water displacement which explains why the tsunami was so great.

The GeoNet system uses positioning sensors to provide millimetric mapping of land movements.

In the 15 years preceding the March 11 quake, the system showed a slow buildup of strain across Honshu as the mighty Pacific plate squeezed and dragged on the island's eastern flank.

The technology could be useful in monitoring faults where a massive quake occurs at gaps spanning centuries or even longer, after pent-up strain builds to breaking point.

Geological evidence from the distant past had suggested that the Japanese Trench was prone to massive, but very rare, tsunami-generating quakes.

But with the possible exception of a quake in AD 869, there was no documented evidence to support this suspicion, and so the risk was downplayed or ignored, says the study.

The paper, published in the British weekly science journal Nature, is lead-authored by Shinzaburo Ozawa of the Geospatial Information Authority of Japan, located in Tsukuba.

In a commentary, Jean-Philippe Avouac of the California Institute of Technology (Caletch) said that new data, from GeoNet and from sea-bottom pressure caused by the tsunami waves, indicated the epicentral slip on March 11 could be even more than 50 metres (165 feet).

If so, that would make it the biggest slip ever recorded, he said.

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