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Geologists differ on seismic risk to Jaitapur nuclear plant

By K.S.Jayaraman, IANS,

Bangalore : Is Jaitapur a safe site for a nuclear power plant from the earthquake risk point of view? A debate on this among geologists with opposing views that started a year ago has now intensified, with each group claiming the other is wrong. Common people are confounded.

Jaitapur is in the west coast of India, in Maharashtra. It is the site proposed by the Nuclear Power Corporation of India (NPCIL) for housing six French reactors with a combined power capacity of 9,900 MW, which would make it India’s largest nuclear power station.

The NPCIL has said that “extensive studies carried out by various government organisations have found no active geological fault in a 30-km radius from the proposed site, thus validating that the site is not earthquake-prone”.

Doubts about this assertion were first raised in November last year by two renowned geologists, Roger Bilham of the University of Colorado in the United States and Vinod Gaur in the Centre for Mathematical Modelling and Computer Simulation in Bangalore.

After a detailed analysis of existing data, they published a paper in the journal Current Science, sounding a warning that an earthquake of above-6 magnitude on the Richter scale could not be ruled out in Jaitapur.

According to them, 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 Himalayas that extend southward, deep into the plate interior.

This “flexural depression,” they said, resulted in high compressional stresses responsible for the thrust faulting that produced the magnitude 6.3 Latur earthquake in 1991 and the 6.4 Koyna earthquake in 1967.

Because the Jaitapur region lies “in this same compressional downwarp” responsible for generating these two earthquakes, they argued: “A similar-sized earthquake could possibly occur directly beneath the nuclear power plant.”

Moreover, they pointed out that 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.”

Bilham and Gaur also noted that the historical seismic record near Jaitapur extends reliably back for only 200 years; estimates of risk assessed from a short data set of only the past few centuries “may not represent the true risk to the plant,” the two said.

Considering that the Latur earthquake occurred on an unmapped surface fault, Gaur and Bilham argued that “there presumably exist numerous faults that represent seismic hazards we know nothing about.”

B.K. Rastogi, director of the Institute of Seismological Research in Gandhinagar, Gujarat, dismissed all the arguments of Gaur and Bilham saying their paper had “many errors and misjudgements.”

In a rebuttal published in the July issue of Current Science, Rastogi said that Koyna and Latur are “in a postulated Koyna-Kurdwadi Rift” more than 50 km north of Jaitapur and therefore “they are not in the same geological setting,” as claimed by Gaur and Bilham.

Secondly, Rastogi argued, the seismic risk for nuclear plants is not assessed by historical earthquake data alone but based on geological studies and review of geophysical and seismotectonic studies.

He said all such studies have been done in the case of Jaitapur and all apprehensions raised (by Gaur and Bilham) “have been considered by the geologists and seismologists involved in site investigations.”

For instance Rastogi claimed that during March 2012 a team of 30 geologists from his institute (including himself) as well as from the Geological Survey of India and the Oil and Natural Gas Corporation Limited thoroughly studied the areas around Jaitapur and up to eight kilometres south of it and found no anomaly supporting the presence of faults.

Furthermore, he said, the Vijaydurg fort and several watch towers built on nearby hillocks 400-500 years ago are intact “indicating absence of any severe earthquakes” at least in the past 500 years.

Rastogi’s rebuttal has evoked a counter-rebuttal from Gaur and Bilham.

In a paper published Dec 10 in Current Science, they presented several lines of evidence to show that “Rastogi’s assurances concerning the impossibility of future earthquakes near Jaitapur are oversimplifications of the current state of our knowledge”.

“First, Rastogi’s dismissal of flexural stresses in the Indian subcontinent is inconsistent with numerous theoretical and observational data supporting their existence,” the authors said.

Secondly, they said that Rastogi has presented no authoritative documentation in support of his claim that there is no active fault within 10 km of Jaitapur.

“In the absence of supporting evidence, we contest his claim,” Gaur and Bilham said, reiterating their warning: “Jaitapur lies in a region where plate tectonic stresses are locally close to critical failure, and where minor perturbations in stress can trigger earthquakes.”

According to them, “tectonically, the Jaitapur region is precisely in the same state of seismic quiescence and historical ignorance as the regions of Latur or Koyna were prior to the damaging earthquakes for which they are now famous.”

“We maintain that Rastogi’s confidence in future regional seismic quiescence near latitude 16 degree N (Jaitapur) is not supported by India’s recent seismic history and that routine palaeoseismic studies that might support or refute his claims are incomplete or have yet to be undertaken,” Gaur and Bilham said.

(K.S. Jayaraman can be contacted at [email protected]ail.com)