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.
But continental crust is too light and buoyant to be subducted. So when India itself finally bulldozed into Eurasia 50 million years ago, it hit a wall. Both continents had to give. On the Eurasian side of the collision zone, the crust was thickened and squeezed upward, creating the high plateau of Tibet. On the Indian side, a great slab of rock was sheared off the leading edge of the subcontinent, along a thrust fault that dipped gently northward toward Eurasia, like a beach toward the sea. As India ground forward the slab was forced back up onto the subcontinent. Eventually that fault became inactive; at that point another detachment fault formed underneath and parallel to the first one, and another slab was shoved underneath the first, lifting it up. Over millions of years, those two slabs became the Himalayas.
Today India still piles headlong into Eurasia, and the Himalayas are still under construction. The active detachment fault is around 1,500 miles long and between 60 and 120 miles wide; from the plains of northern India it dips north underneath the mountains. Most of the time the fault is stuck. But every so often, after enough elastic energy has built up in the fault to force the blocked continent forward, it slips 30 feet or more all at once. That generates a great earthquake: a quake of magnitude 8 or greater.
There have been four such quakes along the detachment fault in just the past 100 years. The quakes–a magnitude 8.7 in 1897, in Assam; a magnitude 8 in 1905, in Kangra; a magnitude 8.4 in 1934, in Bihar; and a magnitude 8.7 in 1950, again in Assam–each ruptured a different segment of the fault. But one segment, located between the Kangra and Bihar rupture zones (roughly between the longitudes of Kathmandu and Delhi), has been strangely reserved. Between 300 and 500 miles long, it is called the Central Himalayan seismic gap.
Most seismologists believe quakes are more likely to occur in seismic gaps, where they haven’t occurred recently. If so, the Central Himalayan gap is more than due. According to the available historical records–mostly Portuguese and English records from India’s colonial days– it has not had a great quake for at least 300 years. Notes scribbled on some Nepalese religious tracts indicate that a big quake did strike Kathmandu in June 1255. The quake killed one-third of the population of the country, including its king. Assuming it ruptured the detachment fault in the Central gap (which isn’t certain), and assuming it was the last major quake in that segment (ditto), then 740 years’ worth of strain has built up in the gap.
Last year seismologists Roger Bilham of the University of Colorado in Boulder, Roland Bürgmann of Stanford, and their colleagues determined the amount of strain, using the Global Positioning System, a network of satellites stationed around the globe that can pinpoint, within a fraction of an inch, the distances between receivers on the ground. The researchers found that India and Asia are converging at the rate of around eight-tenths of an inch per year. But nearly all that convergence is happening between central Nepal and Tibet, in the Himalayas themselves; Nepal and India are barely getting closer at all–because the detachment fault that separates them is stuck. At the moment, the energy of the continental collision seems to be going into squeezing the rocks in the mountains; that is, it’s being stored as elastic strain. Someday, says Bilham, the rocks will rebound, and the strain will be released as motion along the fault. If the last quake in the Central gap was indeed in 1255, some 50 feet of motion–eight-tenths of an inch per year times 740 years– has built up and needs to be released.
What would it take to release all that strain? Hundreds of quakes comparable to the magnitude 6.8 temblor that devastated Kobe, Japan, last January, or the magnitude 6.7 that rocked Los Angeles in January 1994–but if the Central Himalayan gap were prone to minor quakes in such number, they would be happening already. It’s far more likely, says Bilham, that the strain will be uncorked by quakes of magnitude 8 or greater, as it has been elsewhere on the detachment fault. Bilham estimates that four magnitude 8.2 quakes would do the job, or three 8.5’s, or a single 8.9. (The energy released by a quake increases by a factor of 30 with each added point in magnitude.)
The extent of the damage would be enormous. A single 8.2 quake in the Central gap, says Bilham, would be felt as far away as Calcutta and Bombay, some 850 miles distant. We’re talking 100 or 200 million people affected. All along the Ganges plain, where there are about a dozen cities with populations of at least a million, including the capital of Delhi, with nearly 9 million inhabitants, the shaking would be intense, because the soft soil would tend to amplify it. And it would be intense too, of course, in the Central gap itself, directly above the ruptured fault segment.
The Tehri High Dam, 200 miles northeast of Delhi, is being built on the edge of the Central gap.
The site was chosen in 1961, before plate tectonics had revolutionized geology–at a time when geologists didn’t understand the fundamental mechanism driving earthquakes in the Himalayan region. When the original design for the dam was approved by the Indian government’s Planning Commission in 1972, engineers still believed the dam would be at risk only from a quake on one of the smaller faults that crisscross the area. Thus they designed the dam to withstand only a magnitude 7.2 quake, which they figured would produce ground accelerations–fast back-and-forth shaking triggered by high-frequency seismic waves–of .25 g, or one-fourth the acceleration due to gravity.
Construction of the Tehri dam began in 1978, but it has proceeded slowly. The tunnels that will house the turbines have been dug, and a small cofferdam has been erected to divert the Bhagirathi while the main dam is being built just downstream. Today the main dam rises just 50 feet off the canyon floor. It is being constructed of rock fill–considered more stable in an earthquake than concrete, which cracks, or soil, which can become saturated with water and liquefy–over a core of impervious claylike material. The dam will have a triangular cross section: it will be more than half a mile wide (down the valley) at its base and just 65 feet wide at the crest. When it reaches its full height of 850 feet, its reservoir will submerge, partially or entirely, the town of Tehri and over a hundred villages, with a combined population of 70,000. Opposition from these people and from environmental groups has slowed the dam project. So has opposition from seismologists.
One of those seismologists is Vinod Gaur, formerly the director of the National Geophysical Research Institute of India and now at the Center for Mathematical Modeling and Computer Simulation in Bangalore. In 1990, Gaur served on an expert committee convened by the government to review the troubled dam project. He succeeded in convincing the committee that a quake much larger than a magnitude 7.2–at least a magnitude 8.5– was quite likely to occur at Tehri. Such a quake, rupturing the detachment fault under the dam, would be capable of producing a 1-g acceleration, an acceleration that could literally throw a building off its foundation.
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