Even as the government is going all out to pacify the people affected by the tremors experienced in parts of Idukki district last Friday, a section of earth scientists feel that the increased seismic activity in the area can be attributed to the reservoir of the Idukki arch dam.
They have called for detailed scientific investigations into the possibility of reservoir-induced seismicity.
C.P. Rajendran, Fellow at the Indian Institute of Science, Bangalore, said the pattern of occurrence of the tremors and their frequency pointed to reservoir-induced seismicity.
“The smaller quakes have increased in frequency over the years. In September, there were two tremors of magnitude 3.9 and 2.2 respectively. The sequence and timing of the seismic activity also show that it is related to the reservoir. The fact that most of the tremors have been reported in the vicinity of the dam also point to the role of the reservoir.”
Dr. Rajendran, who was a scientist at the Centre for Earth Science Studies here till he moved to Bangalore, said the maximum number of tremors in the region was reported in the post-monsoon period when the reservoir held the maximum volume of water. “From December to March, there is a decrease in seismic activity. It is like breathing in and breathing out. The pattern is clear.”
He, however, said the situation in Idukki gave no reason for panic. “So far, there has been no tremor above magnitude 4.”
Dr. Rajendran, who is currently studying seismic activity in the Andamans and the Himalayas, said the ancient faults present in the hilly terrain in central Kerala, could generate quakes up to 4.5 intensity though a magnitude 6 temblor could not be ruled out due to the density of the faults.
“Constructing a dam over such a fault zone is bound to have an impact on the geology of the region. The enormous pressure of the water above changes the stress levels and the ancient faults start moving. The crustal readjustment leads to quakes. The intensity of seismic activity depends on the dimension of the faults and the length of the rupture”.
The weight of the reservoir can also force water down fissures and faults under the reservoir. The water acts as a lubricant, causing the faults to slip.
Dr. Rajendran stressed the need for focussed studies on reservoir-induced seismicity in Idukki.
Scientific data obtained from various locations across the world point to the link between the filling up of reservoirs and the increase in seismicity. In India, the subject gained academic interest following the disastrous earthquake in Koyna, Maharashtra, in December 1967 that destroyed a village, killing 180 people and rendering thousands homeless. Scientists believe that the quake was catalysed by the Koyna dam.
V. Nandakumar, senior scientist, CESS, feels that the State government should initiate an indicator study at Idukki using expert teams who have done detailed studies of this nature elsewhere in India or abroad. Such a study, he observed, would require hi-tech methods and equipment.
Change in land-use pattern
Dr. Nandakumar said the changes in land-utility pattern in the State, including the construction boom, could have created an imbalance in the structure of the crust. This, he said, might be leading to stress accumulation and fault movements. “Only a detailed analysis can reveal the extent of strain.”
