Saturday, May 5, 2012

Dams and Earthquakes

China has recently undertaken a lot of research on the link between dams and earthquakes as shown in the article published in the magazine Science (posted below).
It is not the case in India, which is full-swing building dams in the North-east without the necessary scientific knowledge of the seismicity of the area. 
Who remember the earthquake of August 15, 1950?
I have often written about this on this blog.
Today, I am posting a text describing the August 15, 1950 earthquake which had its epicenter in Rima in Tibet, just north of the McMahon Line:
Seismographs the world over one day last fortnight registered an earthquake so violent that the record of its convulsions ran off the paper. Because of the incomplete recordings, seismologists were unable at first to determine the quake's location, later reckoned that it must have hit hardest in southeastern Tibet and in northern Assam (a province of India). Communications with this wild mountain region, always poor, had stopped abruptly; the extent of the damage done by the quake could only be guessed at. By last week, frightening evidence of the quake's dreadful work in Assam had begun to reach the outer world. Shipping on the lower Brahmaputra River (whose source is in the Himalayas) was dislocated by a tide of tens of thousands of uprooted trees and the bodies of tigers, elephants and other wild life borne down the river from the earthquake area. The waters of the Brahmaputra, blackened with sulphur that the quake had churned up from the earth's innards, cast up millions of dead and dying fish. Up in the mountains the river had been dammed by landslides. Indian air force pilots in Liberator bombers were sent to blast the river free, but before they could go to work, the water burst free and inundated vast stretches of land. Ledo airfield, built as a U.S. Air Force base during World War II, was destroyed. Frightened travelers who managed to make their way out of the earthquake region told of entire villages that had been swallowed. Thousands of people and cattle were marooned on islands in flooded areas. Brahmin priests performed propitiation ceremonies to the goddess of earth and the god of destruction. The quake had not yet spent itself. Hills were still disappearing and new hills were rising out of the laboring earth. Peasants in the area sat in numb and sleepless terror, watching tumblers half-filled with water for signs of further tremors. At the slightest trembling of the water, they would rush frantically to open ground. At week's end New Delhi heard that the death toll was close to 5,000.
Colin S Jackson wrote on 1 May 2006 about the Assam earthquake of 1950 which appeared in August 15th 1950, the anniversary of India's Independence.
I had arrived on Nya Gogra Tea Estate, the most easterly garden in Darrang District Assam, fresh from the UK a few weeks earlier on about 9th June. I was sharing a bungalow with another assistant named John de Jonghe, we had had our baths and were sitting pyjama clad and dressing-gowned in the sitting room awaiting dinner. At the time I had not really taken note, but all the crickets, jackals etc had gone completely silent, and John was sitting watching a bell. The bell was one with a central striker, and he had loosened the bell so that it was loose and rocking, occasionally hitting the striker and so sounding the bell. I don't know how long he had been watching this, but he obviously knew something which I didn't.
When the earthquake struck, the bungalow was rocking so badly that it was not possible to stand. The axis of the bungalow seemed to be at right angles to the direction from which the tremors were coming, and as the bungalow's central wall tilted away from us we rushed to lean against it as the wall tilted back. As the wall tilted away again we moved as fast as we could towards the doorway onto the verandah, leaning against the wall as it tilted back. As the wall tilted away so we made further rushes until we got down the bungalow steps into the compound.
It was not possible to stand, and crouching/ squatting on the grass we saw the plinth bungalow, all lit up, rocking rather like a boat as it rocks in the wash as a speed boat drives past creating waves which the ground was making.
I don't think that the actual shock lasted for all that long a time, although it seemed an eternity. Shortly after that shock was over a Land Rover drove up with the East Boroi area doctor, Doctor
Summers aboard, and he said that as he drove it was as if the steering on the vehicle had gone, and he had stopped to find that he was in an earthquake.
On checking it was found that nobody had suffered any harm beyond the fright, and there was no damage to buildings offices, bungalows, factory withering houses, hospital and so on beyond the odd cracks.
Shocks continued through the night reducing in duration and intensity, though minor shocks continued over the next ten days or so. I was intrigued one day to see water slopping about in a ditch although there was no apparent shock, even when I leaned against a shade tree everything was quite still with not a breath of wind.
What was frightening was the power of the quake, I believe 8 on the Richter scale, the forces so much greater than any man-made explosion of that time.
We were some 40 miles from the epicentre where Kingdon Ward, the planthunter was encamped. On Seajulie T.E. in North Lakhimpore, Bill and Marjorie Christie had a terrifying time where the ground opened and the ground rose on one side while it fell away on the other. Bill and Marjorie stood one on each side of the crack passing their baby daughter to the one who was rising, and back as the one sank and the other rose. Ultimately with the `quake over they were all three safe.
Some steamers on the Brahmaputra river were stranded on islands which formed under them, and all river channels had to be resurveyed.
With regard to the minimal damage experienced, I thought about possible reasons for that, and came to the conclusion, rightly or wrongly that; the water table was high at that time, if you dug a hole you would be into water only 18" below the ground surface, in effect the Assam Valley was floating on water.
After the `quake, when we finally went into dinner, I recall saying that "this was not in my contract", and I was off!
The Hindu wrote on  August 21, 1950 about the "The Assam Earthquake":
What has been described as the second fiercest of earthquakes recorded by man-made machines so far has rocked Eastern Tibet and Assam. From the reports coming in after the partial re- establishment of disrupted communications, it is clear that there has been, in addition to much loss of property, some loss of life too and Islands have disappeared into the Brahmaputra. The rumblings of the first, tremendous shock are still being felt and heard. The movement of the 1897 Assam quake was vertical and such is likely to be the case this time too. Geologically, the Himalayas are much younger in age than the Deccan plateau and are still apparently in the process of settling down.
Evidence Mounts for Dam-Quake Link
Jane Qiu
20 April 2012: 291
CHENGDU—Ever since the devastating magnitude-7.9 Wenchuan earthquake killed 80,000 people in 2008, Chinese hydrologists and geologists have wrangled over whether the reservoir behind the 156-meterhigh Zipingpu Dam may be to blame (Science, 5 March 2010, p. 1184).
Today, as China embarks on a spurt of new hydropower development (see main text, p. 288), the debate is getting increasingly heated. Newly published studies present the strongest evidence yet for the link.
Most scientists agree that the Wenchuan hypocenter, or depth of the initial rupture, is directly beneath the reservoir. But just how deep it lies is in dispute. The China Earthquake Networks Center (CENC) and geophysicist Liu Qiyuan of the China Earthquake Administration’s (CEA’s) Institute of Geology in Beijing calculate the hypocenter at 14 and 19 kilometers beneath the surface, respectively. At such depths, some scientists contend,water from the reservoir could not have reached the fault.
Others point out that Liu and CENC based their calculations on seismic data from stations tens of kilometers from the epicenter. Using data from the Zipingpu seismic network, including one station almost directly above the hypocenter, Xu Xiwei, deputy director of CEA’s Institute of Geology, and colleagues reported last year in Seismology and Geology that the hypocenter is as shallow as 6 to 9 kilometers—within easy reach of water from the reservoir.
Some scientists also point to numerical modeling studies looking at stress changes caused by the weight of the reservoir and water infiltration. Xinglin Lei, a geophysicist at the Geological Survey of Japan in Tsukuba, concluded in a study published in the Journal of Asian Earth Sciences last year that the reservoir signifi cantly impacted the Beichuan-Yinxiu fault, which runs along the reservoir, and increased the stress on the hypocenter of the Wenchuan Earthquake. That could hasten the occurrence of the earthquake by tens to hundreds of years, he argues.
Shemin Ge, a hydrogeologist at the University of Colorado, Boulder,and colleagues came to a similar conclusion using a different model in an earlier study.That finding was contradicted in 2010 studies by geophysicists Zhou Shiyong of Peking University in Beijing and Kalpana Gahalaut of the National Geophysical Research Institute in Hyderabad, India. But their results could be explained by the use of different numerical models and geometrical parameters for the faults, among other factors, says Jian Lin, a geophysicist at Woods Hole Oceanographic Institution in Massachusetts, who is not involved with any of the studies.
The strongest evidence for a Zipingpu-Wenchuan link may come from analyses by Xu’s team of nearly 1000 seismic events recorded by the Zipingpu seismic network between August 2004 and May 2008, before the Wenchuan quake. The researchers found three clusters of seismicity—all under magnitude 3.5—after the reservoir was fi lled with water in September 2005. One, the Shuimo swarm, occurred along the Beichuan-Yinxiu fault, where the researchers concluded it was induced by the reservoir.
Despite the controversies, many Chinese geophysicists now agree that the reservoir may be connected to the Wenchuan earthquake. Even if the hypocenter lies 19 kilometers below the surface, Liu says, as his calculation shows, this doesn’t exclude the possibility that Zipingpu hastened the quake because “small incremental stress changes due to the reservoir could rupture critically stressed faults—even without water reaching them.”
The new studies highlight the need to consider reservoir-induced seismicity when building large dams in quake-prone regions, scientists say. With dozens of dams planned for the upper Yangtze, Ge says, “We need to assess whether the proposed reservoirs could significantly speed up the accumulation of stress and cause rock failures.“

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