In a deep canyon in the foothills of the Himalayas, near the Indian town of Tehri, a thousand-odd workers are building a hydroelectric dam. When it is complete in four years or so, the Tehri High Dam will deliver 2,400 megawatts of electricity to energy-starved northern India. Its location seems perfect: a hydroelectric plant draws its energy from falling water, after all, and the greater the drop, the better. Once the Tehri dam has blocked off the narrow gorge of the Bhagirathi, interrupting that river’s rush toward the Ganges, the water will have a long drop indeed--850 feet from the top of the dam to the canyon floor. Moreover, the 30-mile-long reservoir behind the dam will provide the drinking and irrigation water that Delhi and the rest of the region need to weather the arid months before the summer monsoon. To Indian planners, the Tehri dam makes doubly good sense.
But the very forces that make it such a good idea--the geologic forces that built the deep gorges and high peaks of the Himalayas--might also be the dam’s undoing. The Tehri dam will stand in one of the world’s most active earthquake zones. Less than ten miles beneath the dam site lies a fault, the geologic boundary between India and the Asian continent, that is capable of unleashing a catastrophic quake. Some seismologists think the fault segment around Tehri is overdue for a quake whose magnitude could be as large as 8.9. And they fear the dam will not survive it.
The geologic stage for this human drama was set some 180 million years ago, with the breakup of the supercontinent of Gondwanaland. One of Gondwanaland’s fragments was a crustal plate that carried what is now the Indian subcontinent. Over the next 130 million years, this plate tracked steadily northward, erasing the ocean that then separated India from Eurasia. The oceanic crust was subducted--forced down into the underlying mantle.
