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Friday, December 23, 2011
OP-ED

Seismo-tectonic risk of Tipaimukh Dam

India is going ahead with a plan to construct a dam across the Barak River in Monipur state for flood control and construction of a 1,500 MW hydroelectric generation plant. This project has created controversy between Bangladesh and India over water sharing rights as per international law. It has led to discontent among and protest by the people of the lower riparian Bangladesh over the serious negative impacts on the socio-economy, hydrology, agriculture, fisheries, ecology, etc.

Nobody can deny that the unilateral decision of India to control and regulate water flow of the Barak river will have devastating effects of the life and economy of the people the north- east and eastern part, as witnessed by the people of the northern and southern regions of Bangladesh due to the Farraka Barrage. More than 67 villages of Monipur will be deprived of their source of livelihood, 1,461 tribal Hmar families will have to be evacuated from their paternal property, and about 60 kms of National Highway No-53 of Monipur will be submerged.

The people of Monipur fought legally to stop the project but failed. The Sinlung Indigenous People's Human Rights Organisation (SIPHRO) of India opined that the process of choosing the dam site ignored both the indigenous people and recommendations of the World Commission on Dams (WCD). About 20 influential socio-political organisations in Manipur have united under the banner of "Action Committee against Tipaimukh Project" and have been protesting against the project.

It seems that India has ignored all these protests and discontent. Strong determination declared by Indian Prime Minister Manmohan Singh in Monipur for implementation of Tipaimukh project leaves no ambiguity of their intention. India is not even considering the fact that the dam site is located in a topographically fragile, ecologically sensitive and seismo-tectonically highest risky zone.

The proposed Tipaimukh Dam site is at Monipur-Mizoram boarder in India. The area is surrounded by regions of high seismicity, which include the Himalayan Arc and Shillong Plateau in the north, Burmese Arc, Arakan Yoma anticlinorium in the east and complex Naga-Disang-Haflong thrust Zones in the northeast. The major Dauki Fault system along with numerous subsurface active faults and a flexure zone -- called Hinge Zone -- lie in the vicinity of the dam site. Monipur-Mizoram states are part of India Myanmar Hill Range formed by the interaction of the Indian, Eurasian and Myanmar Plates. These weak regions are believed to provide the necessary zones for movements in the North-East India Region, which experienced many major earthquakes during last 150 years and has been affected by small earthquakes occasionally.

The dam site and adjoining areas lie in the most seismically active zone in the world. Historical records show that at least eight large earthquakes in the region have occurred during last one hundred and fifty years, with three of them having magnitude of more than 8.

The northern and eastern regions of India covering Meghalaya, Assam, Monipur, Mizoram and north-eastern Bangladesh are so related morphotectonically that the analysis of seismicity of the Bangladesh region without considering adjoining areas will be incomplete and unrealistic. All the great earthquakes located in India have affected the north-eastern region of India and caused enormous damage. The western Assam earthquake of 1897 is probably the most documented. The intensity distribution of its surface effects shows that the damage represented by Mercali scale (ME) VIII-IX lies in the vicinity of the Tipaimukh dam site. Return of such an earthquake in the region is not at all unexpected since the dam site is situated in the most fragile ge0-tectonic region.

Analysis of the epicentres of earthquakes obtained from the NOAA catalogue for the period 1868-2011shows that they are distributed in the weak zones comprising surface or subsurface faults. Most of the events are of moderate rank (Mb = 4-6) and lie at a shallow depth, which indicates the recent movements in the sediments overlying the basement rocks. In the north-eastern region (Surma basin), major events are controlled by the Dauki Fault system and hence, it should be observed carefully. The earthquakes in the folded belt demonstrate shallow and low angled thrust behaviour, which is conformable with the tectonic configuration of the region.

The average return period of earth-quakes in the northeast Indian region has been studied, and it was predicted that the largest earthquakes expected to occur in 50 years would be about M = 7.8-8.8 and in every 100 years would be about M = 8.3-9.3 (type-I distribution). 1897 type of earthquake (M > 8) has a return period of about 100 years and it is seen from the record that it has not occurred in the region since then. It is also observed from the seismicity that the Dauki Fault region is relatively quite, while Naga-Disang-Haflong thrust zone in the northeast of the dam site is seismically more active. Activities in the thrust zone may develop stress concentration in the seismo-tectonically active north-east India region.

Seismicity records and tectonics of north-eastern India demonstrate that the region is in a highly seismic zone. The morphotectonic behaviour of north-east India shows that the area has been affected by the movements of the Dauki Fault system and relative upliftment of the Shillong Plateau. The Dauki Fault system is quiet compared to the Naga-Disng-Haflong thrust belt, giving rise to the probability of sudden rupture in the vicinity of the Dauki Fault system. On the basis of the predicted average return period of a large earthquake, a possibility of reoccurrence of 1897 type in the vicinity of Tipaimukh Dam should be considered seriously. Huge volume of water in the reservoir of the dam may also enhance seismic activities of the existing faults and fissures in the region and can trigger earthquakes.

Bangladesh has no idea whether India has studied the seismo-tectonic risk of the dam site with due importance. Avoiding or ignoring such important aspect of the dam site may cause unimaginable destruction in lower Surma-Kushiyara-Maghna basin in the event of damage to the dam due to large earthquakes. In case the dam is damaged, the tsunami-like rush of water will wash out the downstream districts of Sylhet, Habigonj, Moulabi Bazar, Sunamgonj, Brahmanbaria and Kishoregonj in Bangladesh. It is, therefore, suggested that in-depth study evaluating seismo-tectonic risk of the dam is necessary for the interest of the dam, Manipur state and lower riparian Bangladesh.

The writer is a former Minister and former Professor of Dept. Geology, Dhaka University.

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