New Delhi, Feb. 12: A new study has corroborated earlier predictions that a stretch of the central Himalayan region is ripe for an earthquake more powerful than any it has experienced over the past 700 years. But the best available science cannot predict exactly when it will occur.

Three Indian geophysicists have excavated fresh evidence for the build-up of tectonic strain along a 600km stretch of the central Himalayas long overdue for release through an earthquake that they say could flatten unprotected structures hundreds of kilometres away.

Their findings, just published in the Journal of Geophysical Research, suggest that this central Himalyan region which extends from Kangra in Himachal Pradesh to the Bihar-Nepal border has not experienced a magnitude-8 earthquake since the last one between 1259 and 1433.

While thousands of smaller earthquakes have occurred there since then, the scientists say, the absence of a magnitude-8 event implies that the tectonic strain resulting from the Indian plate pushing against the Asian plate has accumulated there without sufficient release for 700 years.

"This segment is mature. It's ready for a really big earthquake," said Chittenipattu Rajendran, professor of geodynamics at the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, who led the study. "We can say that now with even greater confidence than before. But not when."

Some geophysicists have questioned that confidence. But Rajendran said the findings along with the results of earlier studies should be enough to stir governments and civic authorities into bolstering measures to protect structures in the region and in neighbouring areas in the Indo-Gangetic plains.

Rajendran and his colleagues Kusala Rajendran at the Indian Institute of Science, Bangalore, and Biju John at the National Institute of Rock Mechanics, Kolar, excavated a trench 30-metres long and four-metres deep across a section of the central Himalayan fault - the collision zone between the Indian and Asian plates.

"This was like an archaeological excavation. We were looking for not ancient settlements or artefacts, but for evidence of a really large earthquake," Rajendran said. A large earthquake would have left telltale marks such as dislocations in sedimentary layers in the excavation zone. The scientists couldn't find any such signs beyond the 1400s.

About 15 years ago, Indian geophysicist Vinod Gaur and his American collaborator Roger Bilham had measured plate movements, computed the strain build-up, examined historical records of references to big quakes and predicted that a large tremblor in the central Himalayan region was overdue.

They had used ancient Tibetan documents to suggest that an earthquake had occurred in 1505.

But the excavations by the Bangalore scientists have revealed no evidence for a large earthquake in 1505. "This means the strain has been building up even longer than the period they had assumed," Rajendran said. "That would make us closer to what is overdue."

Geophysicists point out that subterranean strain build-up and rupture - the abrupt release of accumulated strain through earthquakes - fall in what physicists call a class of non-linear phenomena, processes that cannot be predicted with the existing state-of-art tools of science.

A useful earthquake prediction demands three parameters - the exact time, exact location and exact magnitude. The studies from the central Himalayas only predict the magnitude of the imminent release - about magnitude 8 - and an epicentre somewhere along the 600km long stretch of the region.

When it will strike, no one can tell. But, the scientists say, that is no reason for inaction.

"Major cities in the region need to be prepared for what is coming," Rajendran said. "Hazard-reduction strategies, ranging from retrofitting existing structures and buildings to enforcing building codes on new structures, should be rigorously adopted."

A senior geophysicist in Pune who was not associated with the study cautioned that the accounting of strain build-up should be viewed as incomplete without a knowledge of how much strain smaller earthquakes have released over the centuries.

"This expectation of a large earthquake is based on presumptions that the smaller earthquakes do not release enough stored energy," said Shyam Rai, the chair of earth and climate science division at the Indian Institute of Science Education and Research, Pune.

"The truth is no one knows how much energy small earthquakes release on an episodic basis," Rai told The Telegraph.

Over a seven-year period, between 2002 and 2008, the central Himalayan region recorded over 1,100 magnitude-3 earthquakes, several dozens of magnitude-4 earthquakes and nine earthquakes between magnitude 5 and 6. Rai and his colleagues are currently trying to estimate the strain released through such smaller earthquakes.

The smaller earthquakes would not have left imprints that could be detected through excavations. Over a period of 700 years, Rai said, the central Himalayan region would have had several thousands of earthquakes between magnitude 4 and 6.