A massive slab of rock lurking beneath the Kanto Plain on the central Japanese island of Honshu is a major source of the earthquake threat that dogs Tokyo, scientists said on Sunday.

Around 100 kilometres (60 miles) wide and 25 kms (15 miles) thick, the chunk is jammed between tectonic plates that converge beneath the flat, densely-populated plain.

The giant fragment is a potent trigger for a hugely destructive kind of quake, for it wedges between two of the plates and prevents them from sliding smoothly over one other.

As a result, tensions build up until the stored energy is released catastrophically, rather than in smaller, safer movements, the experts say.

Tokyo is built atop the Eurasian plate, one of the two dozen or so tectonic plates that, like jostling pieces on a jigsaw puzzle, comprise Earth's surface.

Around 100 kms (60 miles) to the northwest of the city, on the Kanto Plain, the local geology becomes complex, turning into a triple-layer subterranean sandwich.

Beneath the Eurasian plate at this point, two other plates -- the Pacific plate and the Philippine Sea plate -- converge and descend towards the planet's interior.

A team led by Shinji Toda of the National Institute of Advanced Industrial Science and Technology (AIST) in Tsukua sifted through piles of data from 300,000 micro-quakes to build up a 3D picture of what was happening and where in this trouble spot.

They point the finger at a large fragment broke away from the descending Pacific slab between two and three million years ago.

The main cause appears to have been a collision of two chains of seabed mountains into the Japan Trench on the ocean floor about 200 kms (120 miles) east of modern-day Tokyo.

"The fragment is now jammed between the Pacific and Philippine Sea slabs, like a pill that can't be fully swallowed," they said.

The Tokyo region has been hit by major earthquakes in 1703, 1855 and 1923, the last of which claimed 105,000 lives.

According to a 2005 estimate by Japan's Central Disaster Management Council, recurrence of any quake of a similar size would inflict costs today of around 100 trillion yen, or about 100 billion dollars.

The top of the proposed "Kanto Fragment" lies about 40 kms (25 miles) beneath the surface, the authors calculate.

They add it is not to blame for all of Tokyo's major quakes, but is particularly associated with "deep-thrust" types, which are highly destructive because of their closeness to the city.

The 1855 quake, which had a magnitude of 7.3, was one of this type.

The paper is published online by Nature Geoscience, a journal of the Nature group, based in London.