Two leading geologists have warned that a magnitude 6-plus earthquake cannot be ruled out in Jaitapur - the proposed site of India's largest 9,900 MW nuclear power plant on the west coast that has seen protests against it for safety reasons - and that it could occur within the lifetime of the power plant.

"Since Jaitapur lies in the same compressional stress regime that has been responsible for generating both the magnitude 6.3 Latur and magnitude 6.4 Koyna earthquakes in the past five decades, it can be argued that a similar sized earthquake could possibly occur directly beneath the power plant," they say in a report in the latest issue of Current Science published by the Indian Academy of Sciences in Bangalore.

"The probability of this earthquake occurring is low but it is nevertheless possible, and is an important consideration in the analysis of power plant safety," say its authors -- Roger Bilham in the department of geological sciences of the University of Colorado, US, and Vinod Gaur in the CSIR Centre for Mathematical Modelling and Computer Simulation in Bangalore.

According to Bilham and Gaur, the Indian Plate is unique among the world's continental plates in that it is flexed by its collision with the Tibetan Plateau resulting in "belts" of buckling parallel to the Himalaya that extend southward, deep into the plate interior.

This "flexural depression", they say, results in high compressional stresses that are believed to be responsible for the thrust faulting that produced the Latur earthquake, and presumably for the faulting in the Koyna region. "The Jaitapur region lies in this same compressional downwarp," the researchers warn.

"The occurrence of earthquakes of up to magnitude 6.5 on faults near Koyna and Latur at approximately the same latitudes of Jaitapur is of considerable concern, since the stress regime near Jaitapur cannot differ substantially from these two areas when viewed from a 'flexural' perspective," the scientists note.

Moreover, the occurrence of the nearby Koyna earthquake has presumably loaded the Jaitapur region closer to failure as a result of a process called "Coulomb stress transfer", they add.

The Jaitapur site is 110 km from the Koyna earthquake of 1967, which was induced by the impounding of the Koyna reservoir. The authors say the Koyna earthquake signifies a region that was highly stressed prior to reservoir impoundment.

"As the stresses in the region are likely to be similar over hundreds of kilometres, the Jaitapur region must be considered to be similarly stressed," they say.

Jaitapur has no record of local seismicity in the past century. However, moderate events such as the Koyna earthquake at distances less than 30 km and larger ones at distances greater than 100 km can also produce significant shaking, they say.

This level of shaking can be easily accommodated by most nuclear power plants, but considering that Jaitapur lies in a tectonic setting similar to Latur and Koyna, an earthquake of magnitude 6.5 is "not unlikely" there, they say. "It could occur within the lifetime of the nuclear power plant."

The Nuclear Power Corporation that plans to house six French reactors in Jaitapur has said the reactors will not face a seismic risk as the site is in seismic zone three and not four. But Bilham and Gaur note that the historical seismic record near Jaitapur extends reliably back for only 200 years and estimates of risk assessed from a short dataset of only the past few centuries, "may not represent the true risk to the plant", they say.

Bilham and Gaur say the low strain rate and the rare incidence of recent or historical earthquakes in the region means "there presumably exist numerous faults that represent seismic hazards we know nothing about". Many of India's faults do not reach the surface, and their examination using traditional palaeo-seismic methods is not feasible, they say.

"A knowledge of the distribution of surface and subsurface faults near Jaitapur is, therefore, an important factor in characterising local seismic hazards considering that the Latur earthquake occurred on an unmapped surface fault," the report says.

The geologists caution that the apparent "seismic quietness" of Jaitapur does not mean that a severe earthquake cannot occur there. "If stress in the region is sufficiently mature to have brought an existing subsurface fault close to failure, an earthquake may be imminent," they said. "It is our opinion that insufficient data are available to exclude this possibility."

"While this may be considered of low probability, it is nevertheless possible, and as the recent earthquake in Japan has demonstrated, it is relevant to plan for all possible futures in the design of nuclear power plants," the geologists conclude.