Wednesday, November 28, 2007

Thank your stars, Delhi (Mail Today 27.11.07)

THE earthquake that shook the Capital early on Monday is yet another reminder of a seismic disaster waiting to happen. The threat of a major earthquake looms large. Scientists say there is high probability of a major quake rocking Delhi before 2033.
Although no damage to property was reported from anywhere, many residents felt the tremors and heard sounds of ground motion in several parts of the city. The area near Bahadurgarh town, the epicentre of the quake, is not new to this kind of seismic activity. It has recorded nearly two dozen quakes of minor intensity in the past two years. Monday morning’s was the strongest in this period.
Sohna fault, the cause of ground motion in the area, is one of the six major geological faults which are the main source of seismicity in Delhi and surroundings areas.
A fault is a fracture zone between two rocks — the movement of
which releases energy, causing an earthquake.
Overall, movements in about 20 faults — including those in the Himalayan region — can result in ground vibration in Delhi. That is why Delhi has been placed in the zone of highest seismicity in India.
The likelihood of Delhi facing a serious earthquake cannot be ruled out, said professor R.N. Iyengar, Raja Ramanna Fellow at the Indian Institute of Science, Bangalore. He has studied the earthquake scenario in Delhi for several years. The civil engineering professor has mapped epicenters of over 278 earthquakes between 1720 and 2001 — which either originated in the Himalayan region or were of local origin — to work out Delhi’s susceptibility to quakes.
Another study of 100 quakes in the region showed that the epicentres have a pattern of clustering in two belts between Rohtak and Delhi. Based on his work and other scientific evidence, scientists have predicted that in a time window of 50 years, between 1983 and 2033, there is high probability of a large earthquake of magnitude of 7 or more on the Richter scale shaking the ground in Delhi.
“I do not want to sound alarmist but there is reasonable chance for Delhi to experience a major earthquake,” Iyengar told Mail Today in Bangalore.

While all of Delhi and surrounding areas fall in the highest seismic zone in India, there are some areas which could feel the impact more than others. The city’s northern Yamuna belt causes the most concern. It covers trans-Yamuna areas such as Patparganj, Vaishali and, across the UP border, Noida. The concern is because of the soil condition in the region. Local soil conditions can dramatically alter amplitude and frequency of ground motion, and determine the extent of damage an earthquake can cause. That is why, scientists point out, macro-level zoning of earthquake-prone areas is not too useful. The effort now is to develop microzonation maps of different quake-prone cities. Iyengar is heading this effort under the aegis of the National Disaster Management Authority.
Because of typical soil conditions in areas around the Yamuna basin in Delhi, conventional mitigation measures, such as reinforcing upper structures on the ground, may not prove to be effective. Trans-Yamuna areas have extensive tracts of silt and loose sand.
Researchers at IIT Delhi, have specifically studied the phenomenon of liquefaction, which occurs in saturated soil (filled with water). In the event of an earthquake, soil particles move readily, causing landslides and shaking the foundations of buildings and other structures.
The IIT scientists dug 1,200 boreholes at various locations in Delhi to study soil conditions and prepare a map of liquefaction hazard for the city. They found that liquefaction potential is severe in trans-Yamuna areas — Yamuna Vihar, Geeta Colony, Mayur Vihar, Preet Vihar and Vinod Nagar.
The probability of liquefaction is ‘moderately severe’ in Punjabi Bagh, Shahdara and Pashchim Vihar, while it is relatively low in areas in northern Delhi such as Rithala, Vijay Vihar, Anand Vihar and Haidarpur. The probability is very low in areas such as Golf Links, Lodi Road, Connaught Place and Dhaula Kuan due to rocky conditions and gravelly sands.
The information, scientists say, can be used to select suitable techniques for laying the foundation of buildings in different parts of the city.
(with Max Martin in Bangalore and Abhishek Shukla in Delhi)

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