Delhi -- Another Ahmedabad?

Delhi -- Another Ahmedabad?
Arun Bapat

Just over a month ago, the nation witnessed the Gujarat earthquake. Its epicenter was near Bhuj and, seismologically speaking, the maximum destruction should have been confined to an area of 20 to 30 km from here. However, in Ahmedabad, which is about 320 km from Bhuj, tall buildings were extensively damaged. In seismological terms, this destruction was due to “surface waves”.

When an earthquake occurs, waves are emitted from the hypocentre, the underground location where the actual rupture takes place. The waves, which travel from the hypocentre to the surface of the earth, within solid earth, are known as ‘‘body waves’’. When these waves reach the surface of the earth, they travel partly on the surface of the earth and partly in the air. That’s why they are known as surface waves. These waves travel slowly and go around the earth several times. One characteristic of these waves is that they are highly predominant within a distance of 200 to 400 km from the epicenter and are extremely harmful to structures that are taller than 15 m, which are located in the distance strip of 200 km long, between 200 to 400 km from the epicenter. These waves do not adversely affect lower structures in the 200 km strip and do not cause any damage beyond 400 km.

Now just compare the situation in the vicinity of the national capital, New Delhi. The seismically active Himalayan fault is 230 to 350 km away from Delhi. The Himalayan fault is known to have seismogenic potential to generate earthquakes up to a magnitude 8.0 or more. Tall buildings in Delhi, like Ahmedabad, have not been designed to withstand seismic forces. The foundations in Delhi are not on solid rock but there is an excess of soil and crushed rock (murrum) on the ground. In case a powerful earthquake occurs in the Himalayas, then the 200 km long seismic scissors of surface waves will wildly chop the tall concrete structures of Delhi and the possibility of some of them collapsing cannot be ruled out. Seismic archives tell us that the Qutab Minar was deprived of its upper portion by a powerful earthquake in the Himalayas on September 1, 1803. The Kangra earthquake of April 4, 1905, was strongly felt in Delhi.

The most recent Delhi earthquake of magnitude 5.5 occurred on August 27, 1960. At that time, the area, population and number of buildings in Delhi were limited. As per the report of the Geological Survey of India, this earthquake caused extensive damage to houses in Delhi, Delhi Cantonment, Muhammudpur, Palam, Gurgaon, and so on. In New Delhi Municipal Committee area, damage occurred in Irwin, Ashoka, Mahadeo, Talkatora, Feroshah, Shershah, Hastings Roads and in the South and North Avenues. The upper portion of the clock tower in Rashtrapathi Bhavan fell and hairline cracks appeared in the buildings of the Reserve Bank of India, Ashoka Hotel, Janpath Hotel, Eastern Court, and so on. During this quake about one hundred persons were injured and two persons died. It also resulted in the discharge of hot water at Sohna being diminished. Total losses were estimated at Rs 5 lakh.

The above figures are for an earthquake of the magnitude 5.5. Remembering that the Richter Scale is logarithmic in nature, the effect of magnitude 7.5 would be about one thousand times greater. This warrants immediate executive actions. For one, all tall buildings should immediately undertake strengthening measures so that they would be able to withstand seismic forces. For the purpose of repair measures, a definite time frame must be prescribed. Second, an interim stay could be enforced for structures under construction. Builders should be ordered to change their designs. For tall buildings the completion certificates should be given on a floor-wise basis. Only after the lower floor is certified as aseismic, should permission for the next floor be given. Thirdly, all new structures must be designed aseismically. Last, but not least, all authorities in government and private offices must undertake a programme of earthquake education. If suitable measures are not undertaken on a war footing, the possibility ofthe Ahmedabad story being repeated in Delhi is very high. We can ignore Mother Earth only at our own peril.

The writer is a research seismologist at Pune