Geological Setting of Delhi
Delhi, the capital of India is bounded by the Indo-Gangetic alluvial plains in the North and East, by Thar desert in the West and by Aravalli hill ranges in the South. The terrain of Delhi is flat in general except for alow NNE-SSW trending ridge which is considered and extention of the Aravalli hills of Rajasthan. A computer image of the surface topography of Delhi is presented in the figure below.

Seismicity around Delhi appears to be associated with a major geological structure, which is known as theDelhi-Hardwar Ridge. It coincides with the extension of the Aravali Mountain belt beneath the alluvial plains of the Ganga basin to the northeast of Delhi towards the Himalayan mountain (Jain,1996).

Seismic Zoning
The country has been classified into different zones indicating the intensity of damage or frequency of earthquake occurrences. These zoning maps indicate broadly the seismic coefficient that could generally be adopted for design of buildings in different parts of the country. These maps are based on subjective estimates of intensity from available information on earthquake occurrence, geology and tectonics of the country. The zoning of a country is a continuous process which keeps undergoing changes as more and more data on occurrence of earthquakes in that country becomes available.

The region with intensity less than V is designated as Zone 0. Thus, the designation of area as seismic Zone V indicates activity. Delhi is located in zone IV which has fairly high seismicity where the general occurrence of earthquakes is of 5-6 magnitude, a few of magnitude 6-7 and occasionally of 7-8 magnitude. Delhi thus lies among the high-risk areas.

Seismicity in North India, including the Himalayas, is due to collission of the Indian plate with Eurasian plate. This is a continuous process happening for the last 50 million years. These colliding plates flex, storing energy like a spring, and when the plate's margin finally slip to release energy, an earthquake results.

In the past, five earthquakes of Richer Magnitude 5.5 to 6.7 are known to have occurred in the UT of Delhi or close to it since 1720 AD. Two major lineaments namely Delhi-Haridwar ridge and Delhi-Moradabad faults pass through the territory, both having potential of generating earthquakes of magnitude upto MSK VIII will be quite probabale in the Delhi territory. Normal depth of 30 km may be assumed for these earthquakes. It will be prudent to consider the effects of such a potential earthquake for developing a prevention-cum-preparedness plan

Issues
The city's settlement pattern has never been viewed in relation to location and geological characteristics.

Pockets with high rise buildings or ill-designed high-risk areas exist without specific consideration of earthquake resistance. Similarly, unplanned settlements with sub standard structures are also prone to heavy damage even in moderate shaking.

The Central Business District namely Connaught Place, numerous District Centres and sprouting high rise group housing schemes are high risk areas due to the vertical as well as plan configurations. The walled city area, the trans-Yamuna area, and scattered pockets of unplanned settlements also figures as high risk zones due to their substandard structures and high densities.

So far as housing is concerned, vulnerability analysis has never been carried out and preliminary estimate of damages is not available for strengthening of structures under normal improvement development schemes.

The most recent Chamoli earthquake (29 March 1999) was felt all over Delhi. There have been reports of cracks in a few tall buildings located on alluvial deposits in the trans-Yamuna area. This event has been recorded by instruments maintained by CBRI.

Past earthquakes around Delhi
Damaging earthquakes have occurred around Delhi since ancient times. Mahabharata mentions about earthquakes during the war at Kurukshetra (Circa 3000 BC?). More recently, damage to Delhi in the 1720 earthquakes (intensity IX in Delhi) is well discussed by Kafi Khan. Tandon (1953) mentions of damage to the Qutab Minar during the 2803 earthquake near Mathura.

Srivastava and Roy (1982) discuss several more earthquakes in Delhi region. These include: (a) earthquake of year 893 or 894 (Intensity XI XII) which took place not far from Delhi in which many persons died; (b) earthquake of 22 March 1825 near Delhi Intensity VII; earthquake of 17 July 1830 near Delhi (Intensity VIII); and (d) earthquake of 24 October 1831 near Delhi (Intensity VI)

Delhi has also sustained earthquake damage in more recent times. For instance, Srivastava and Somayajulu (1966) mention of (a) Khurja earthquake (M6.7) of 10 October 1956 in which 23 persons were killed in Bulandshahr and some injured in Delhi; (b) M6.0 earthquake of 27 August 1960 near Delhi wherein about 50 persons in Delhi were injurred; and (c) an earthquake near Moradabad on 15 August 1966 that killed 14 persons in Delhi. Iyengar (2000) also mentions about damage to one of the minarets of Delhi's Jama Masjid during the M4.0 earquakes on 28 July 1994.

Most recently, the 1999 Chamoli earthquake (M6.5) took place about 280 km from Delhi. Such a moderate earthquake does not normally cause damage at such large distance. And yet, several buildings in Delhi sustained non-structural damage possibility due to peculiar geological and geotechnical features if this area. Fig. 1 shows damage to the gound storey partion walls of a multistory apartment building in the Patparganj area. Collapse of a few architectural fins at the Shastri Bhawan during this earthquake is shown in Figs.2 (a,b). In 1985, an earthquake about 400 km from Mexico city caused very considerable damage and deaths in Mexici city, primarily due to the peculiar site conditions there. The Chamoli earthquake effects in Delhi indicate that there is real possibility of a large earthquake in the Himalaya causing considerable damage to Delhi.

It is therefore seen that Delhi is prone to severe earthquake damage both by nearby earthquakes and by large earthquakes occurring in the Himalayas. The scientists and engineers need to urgently take up detailed investigations to develop a more quantitative understanding of the seismic hazard faced by Delhi. Unfortunately, not many such studies have been carried out so far. For instance, paleoseismics studies to locate major earthquake events of the past, e.g., the 1720 and the 1803 events, would add significantly to the hazard evaluation. Due to its complex geological setting, some areas of Delhi are likely to sustain much higher levels of damages than the others and to evaluate this, detailed microzonation studies are needed. (e.g., Iyengar,2000).

Current status of building stock
The first code of practice for earthquake resistant design was developed in India as early as 1930's after the 1935 Quetta earthquake (e.g., Jain and Nigam, 200). The Bureau of Indian Standards developed its first code on aseismic design in 1962 (IS:1893-1962). However, till date there is no legal framework to require that all constructions in Delhi must implement seismic code provisions. The results is that most buildings in Delhi may not meet codal requirements on seismic resistance. Moreover, even if from now on we somehow ensure that all new construction will be earthquake resistant, there still will remain a very large inventory of old buildings that will be deficient for seismic safety. We need to develop a rational seismic retrofitting policy, first for the government- owned buildings and later for the private constructions.

As per Vulnerability Atlas of India (1997), for shaking intensity VIII, 6.5% houses in Delhi have high damage risk , and85.5% houses have moderate damage risk. These estimates are based on very simplistic assumptions. Systematic studies are needed on vulnerability of different types of constructions in the area. This will require experimental studies to evaluate strength, stiffness and ductility of different types of constructions as well as as analytical studies such as the Push Over Analysis. Experiences of past earthquakes both in India abroad have clearly outlined the vulnerability of multistorey reinforced concrete buildings if not designed and constructed correctly. Huge number of multistorey reinforced concrete buildings in Delhi, particularly those with open ground storey to accommodate vehicle parking, could also pose a major challenge in the event of a strong earthquake.

Infrastructure and other implications
Delhi is currently passing through a major infrastructure development phase with a large number of bridges, flyovers and the metro project under construction. After a severe earthquake, the transport infrastructure is earthquake resistant and the old one is seismically retrofitted. Indian seismic code (IS:1893-1984) is not applicable for major projects which require special studies on seismic design criteria. Moreover, the Indian seismic codal provisions on bridges as these exist today are obsolete and inadequate (e.g Jain and Murty, 1998).

Earthquake disaster in Delhi has the potential to go well beyond the statistics of deaths and injuries. Such a disaster in the country's capital, which also happens to be a major commercial and industrial centre, will have huge economic and political implications which will affect the entire country and not just the population of Delhi. This adds an extra dimension to the earthquakes problem for Delhi.

Plan of action
A valid question at this stage will be: should one be concerned about an earthquake which has a very low probability of occurrence, when Delhi faces so many day-to-day problems of environment, noise, traffic, water and power shortage, etc? The consequences of a severe earthquake to not seriously address the problem. Put it differently, considering the potential for a mega disaster, we cannot afford ignore the earthquake problem.

As a first step towards earthquake disaster mitigation the problem must first be recognized and quantified. Herein lies the first challenge: to discuss and debate the problem of this kind on a rational basis and yet not cause panic. Once the problem is identified and an action plan agreed upon, the need will arise for a political and administrative will to implement the action plan. It must be emphasized that the problem requires huge efforts and is well beyond a few individuals or a few organizations. Numerous scientific and engineering activities will have to be initiated simultaneously before we can even quantify the size of the problem by way of seismic risk scenarios.

As of now, there are too few experts in this subject for a large country like India. We must focus our attention to the institutionally and manpower development at all levels. Extensive studies are needed for seismic hazard evaluation for different parts of Delhi and vulnerability assessment for different kinds of constructions; using these, seismic risk evaluation for Delhi must be carried out. Manuals need to be developed outlining methodologies for new constructions and retrofitting of old ones. A strong legal and enforcement framework with appropriate incentives and punitive measures is required together with awareness programmes for general public. All these components must be taken up simultaneously; ignoring one aspect for the other could be counterproductive.

Delhi has had many damaging earthquakes in the past and is placed in a high seismic zone (zone IV). Delhi is prone not only to damaging earthquakes in or near Delhi, but due to its peculiar geological setting it could also sustain strong shaking due to a large earthquake in the Himalaya. Unfortunately, most buildings in Delhi may not meet Indian standards on aseismic constructions and may be considered deficient from seismic safety view point. Thus, there is a real potential for a great earthquake disaster in Delhi, the implications of which go well beyond casualties because of its political and commercial significance.

There is an urgent need for healthy debates on seismic risk aspects of Delhi and for reasonable assessment of the problem. Studies are needed on seismic hazard evaluation for different types of construction. Using these, seismic risk scenarios must be developed and implementation strategies chalked out for new and old constructions. The efforts required are truly multidisciplinary and should include components on technical training, institutional development, development of technical manuals, legal and enforcement aspects, and public awareness programmes. Most importantly, we need the political will to handle this problem and the biggest challenge perhaps lies in drawing the attention of political leadership to this problem when the city faces many other urgent problems.