During the post Kutch (Bhuj) earthquake (2001) surveys a strange phenomenon came to the notice of the seismologists of Geological Survey Of India (GSI). They noticed in Ahmedabad that some buildings, like Akshay Apartments, a building comprising 12 flats came down like a pack of cards. All occupants except an old lady came out before it came down and survived. Barely 30 m away from this building stood Gautam Krupa complex with 16 flats. These flats received only minor cracks. Scores of such incidents were reported by the GSI team from several localities.
In the recent past after major earthquakes in Mexico City (1985), San Francisco (1989) and Los Angles (1995) similar phenomenon had been noticed. This set the ball rolling. Was it because of some supernatural power that saved selected buildings from destruction or was it due to some other reason? Science does not believe in superstitions. It has to have an answer for every query.
The answer was not lagging behind. Those who have watched the 1957 Movie Bridge on the River Kwai by David Lean would recall that the British soldiers as they marched in a column in unison began to march haphazardly as they crossed the Bridge. Why? The uniform footfall of the soldiers creates ripples on the ground. While marching through the bridge, such ripples can make the bridge collapse if the structure starts to resonate at the same wave length. Knowing well the fact the structural engineers have been using design parameters that ensure that a colossal civil structure does not resonate with earthquake waves at the same wave length.
It is easier said than done. In case of a structure like a major dam or a bridge across a river all such parameters are taken in to account and the structures are designed to face the maximum possible consequences. However, in case of individual houses or even multistoried apartments, in India except earthquake resistant designs as per the earthquake zone in which the apartment lies, nothing much is done.
Recent studies have shown that merely following the earthquake design code in an alluvial country uniformly within a zone is not enough. Wide spread damage at Ahmedabad with lucky escape for a few is one of the recent examples from India.
The seismologists found that as the earthquake waves traverse through different sub-soil layers of sand, clay, gravel etc. their behavior varies in different layers. This causes variation in ground amplification and dynamic site period resulting in large variation in the severity of damage to buildings, roads, railway lines and other lifeline structures.
Alarmed by possibilities of great damage in case of a powerful earthquake occurring within a radius of 200 km, the seismologists in India too have started a micro-seismic zonation of vulnerable areas.
Dehradun is one such area in North India which is at present highly vulnerable in case of an earthquake occurring in the central Himalayas. Firstly the city has witnessed a lot of damage during Kangra (1905), Uttarkashi (1991) and Chamoli (1999) earthquakes says Dr. A.K. Mahajan of Wadia Institute of Himalayan Geology, Dehradun secondly being the present capital of Uttarakhand, the city is going through a construction boom. Suddenly the population of the city has increased multifold and housing is one of the priorities.
An earthquake never kills anyone, people die of house collapse. That is why the need for houses which can withstand the impact of the Shear or S waves of the earthquake. These waves travel at a slower rate through a rocky medium, but generate a ripple effect in an elastic medium. The unconsolidated sand and gravel layers in the subsurface in an alluvial terrain are ideal propagators of S waves over long distances.
Dehradun valley is unique as it is parallel to the mountains. Normally valleys are carved across the mountains. Dehradun lies between the Siwalik Hills and the Himalayas. Since it is like a bowl the streams from north and south drain in to the valley. Layers of sediments brought by these rivers have been deposited in the valley. These sediments are coarse gravels and in depth they become coarser with occasional patches of clay or mudstone. In the northern part of the city these gravels are absent, instead rocs of Siwalik are found in the subsurface. Towards the east there is more lime present in the gravel.
Seismic hazard wise, Dehradun is quite vulnerable. It falls in the zone IV of the seismic zonation map of India. Based on the past earthquake evidences, active tectonics or study of fault planes along which movement is still going on and seismic vulnerability studies indicate one conclusion, that is an earthquake of magnitude of 1905 Kangra earthquake is overdue in the Himalayan kingdom. If such an earthquake occurs Dehradun may have as much as 200,000 fatalities. Though it is not yet possible to predict 'when', but a powerful earthquake is due for sure.
The S waves of an earthquake develop a greater magnification while passing through lose and unconsolidated sediments. Problem with Dehradun city, is its location on lose deposits of sand and gravel. These are called as the fan deposits-that is material carried by streams down the steam gradients gets fanned out as soon as the stream hits a plain. This material has undergone several cycles of weathering, erosion and deposition. Studies by Mahajan have shown that the peak amplification function between the free surface and the bedrock is at 3 Hz in the northern part of the city, 2.5 Hz in the middle and 1.5 Hz in the south and southwestern parts of the city. This confirms much less thickness of lose alluvial sediments in the northern part of the city compared to the southern and southwestern part.
Geological Survey of India (GSI), having seen the selected damage at Ahmedabad after Kutch (Bhuj) earthquake 2001 has already started a systematic micro-zonation of most vulnerable areas from severe earthquakes point of view. Dehradun being a vulnerable area and one of the densely populated areas has been studied in detail. A ground vulnerability index of Dehradun urban complex has been prepared by the GSI. As per that southern part of Dehradun around Pithuwala, Nayanagar, Tuntawala, Harbhajwala etc showed very high values of vulnerability index and hence fell in very high earthquake damageable zone. Most of the central part of Dehradun shows low level of vulnerability. Southwestern part of city also shows a higher intensity increment. In other words that area will have worse affect in the event of a high intensity earthquake.
In a subsurface set up like Dehradun's the possibility of ground underneath getting liquefied is more. Under similar situation several multistoried apartments crumbled at Ahmedabad. GSI's studies in Dehradun indicate that chances of liquefaction at Dehradun are quite remote. However ground motion can get amplified three to four times in the event of an earthquake of magnitude around 8 within 100 km radius of Dehradun. Such amplifications mean that while constructing multistoried apartments expert advice must be sought and building designed accordingly.
It is important that in vulnerable areas all public utility buildings including schools, hospitals and offices where a larger congregation of people is anticipated should be made absolutely earthquake safe.
Strange are the nature's ways. Indian plate is constantly moving underneath the Asian plate since past 20 million years. Consequently the Himalayas are constantly rising. The Himalayan belt is under a constant stress. It is the release of this stress that leads to major earthquakes in the region. All leading seismologists postulate release of stress this time in the Central Kumaon sector. Such observations make Dehradun all the more vulnerable. It is not only for the present for all times to come that building codes are strictly followed as per the threat envisaged in particular areas of urban habitats like Dehradun.