No reliable historical seismic record in the region before 1800 A.D.

While the government is determined to build a nuclear power plant at Jaitapur in Ratnagiri district, a new paper in journal Current Science has said that the apparent seismic silence of Jaitapur does not mean that an 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,” say Roger Bilham, Professor of Geological Sciences, University of Colorado; Vinod K. Gaur, Professor at the CSIR (Council of Scientific and Industrial Research) Centre for Mathematical Modelling and Computer Simulation; and Indian Institute of Astrophysics, Bangalore, in their paper (Vol. 101, NO. 10, 25 November, 2011)

The journal is published by the Indian Academy of Sciences, Bangalore.

The paper says,” “It is our opinion that insufficient data are available to exclude this possibility. With the possible exception of Koyna and Latur, which have recently been relieved of local tectonics stresses, no shallow fault between lat. 16°N and 19°N may be invulnerable to future M ≥ 6 rupture [an earthquake with a magnitude of 6 on the Richter scale] . 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 paper notes that the historical seismic record near Jaitapur extends reliably back to only 200 years, with scant additional data prior to the year 1800. Due to the long interregnum between earthquakes in continental India, spanning millennia, seismic data from a few hundred years cannot be taken as a guide to future seismic hazard. Because of low strain rates, and, therefore, low seismic productivity in the plate interior, reliable hazard figures are difficult to estimate as these require a record of earthquake history over at least a 1,000 years, preferably more. Geological studies are unavailable to characterise the millennia-long seismic history.

Mapped surface faults in India are rare and their absence in geological maps may be inconclusive since they are commonly associated with insignificant slips that have accumulated at infrequent intervals. Traces of recent activity are quickly covered by surface soils. The Latur earthquake, for example occurred on an unmapped surface fault, according to the paper.

Difficult to study

“Subsurface faults [deeper than 5 km, say] are difficult to study using traditional paleoseismic methods. We have found no publicly available information on seismic imaging near Jaitapur that may have mapped its subsurface structures,” the paper says.

The paper explains that the Jaitapur site lies at a distance of about 110 km from the 6.4-magnitude Koyna earthquake of 1967, which was induced by the impounding of the Koyna reservoir. The importance of this earthquake lies in the implication that stresses in the nearby Jaitapur region are likely to be sufficiently high enough to produce earthquakes, despite the lack of historical precedent.

The Nuclear Power Corporation of India Limited, which is building the Jaitapur plant, and the Atomic Energy Commission have been repeatedly saying that Jaitapur lies in seismic zone three, which is relatively less prone to danger, and that the plant would have an earthquake-resistant design. However, the Fukushima disaster, where the nuclear plants were built to withstand earthquakes alone, and not both an earthquake and a tsunami, has cast a shadow over the Jaitapur plant.