Delhi, the age old capital of India is always under the scanner of the security experts. The fear of terrorist attack is supreme in their minds, which is quite understandable, because of the geo-political conditions. In this process natural hazards like an earthquake have a much lower priority in their minds because the frequency of occurrence of earthquakes is much less. However, the toll taken by them is much more than any other hazard in terms of loss of life and property.
 
Just to remind the readers about the vulnerability of Delhi here I will quote a few lines from an earlier article, ‘Do we have a roadmap to seismic safety?’:

‘Examples of such earthquake generated tragedies are well recorded in the annuls of the Geological Survey of India (GSI). The first semi-scientific record in India is that of Delhi-Agra earthquake of July 15, 1505. A major earthquake again rocked Delhi on July 15, 1720. It was 22nd Ramzan and people had assembled in mosques to offer prayers. Many people lost their lives in Shahjenabad (New Delhi) and Old Delhi. After this Kaifikhan recorded in 'Muntakhabul-Ul-Lulab' that similar shocks continued to terrorize Delhi for 40 days.

On September 1, 1803, the Mathura-Delhi area was rocked at 3AM. Mathura suffered the maximum damage. Open fissures were formed on the ground and water gushed out with force. Delhi was also severely affected and top portion of famous Kutubminar tumbled down.’

These were the historical records when Delhi was rocked. But now with the science of ‘tell tale’ marks of past earthquakes having well developed, evidences are there that nearly 1000 years ago, while Prithviraj Chauhan fought with Mohammad Ghori in 1192 AD, Delhi must have been rocked severely! Evidences of this earthquake are recorded in Paonta Saheb, on the bank of Yamuna in Himachal Pradesh. So powerful was the earthquake that sub-surface sand got melted and was injected like a toothpaste across the rocks as ‘sand dykes’ and even the river course was partially obstructed to form a lake for a few years.
 
From 1505 to 2010 Delhi has evolved from a Capital of invaders to the Capital of a mighty democratic nation. And yet it has many more miles to go. In all these years the science of seismic studies has also evolved quite a bit. Forecasting an earthquake has not yet become a possibility, but at least vulnerable and highly vulnerable areas can be demarcated for taking precautionary measures. Delhi falls under the earthquake zone IV as per the Indian seismic zoning map of the Indian Standards Institution. With a jungle of skyscrapers and dense population, an earthquake can spell doom for Delhi. That is why many teams of seismologists have surveyed Delhi from time to time to find out the gravity of threat of an earthquake.
 
The modern science of seismology requires a constant and widespread monitoring of the pulse of the earth. If we take Delhi as a human body then a seismologists doctor would love to place it under an MRI scanner to know what is going on inside.  This monitoring is essential because firstly Delhi is barely 250 km from the Himalayan arc which is notorious for several thrusts (widespread faults) which cause severe quakes and also exposed to moderate/large local/regional earthquakes say S.K. Singh from Instituto de Geofisica and Instituto de Ingenieria, Mexico and his co-workers from Department of Earthquake Engineering IIT, Roorkee, Indian Meteorological Department, New Delhi and Ministry of Earth Sciences, New Delhi.
 
The findings of these researchers are based on an earthquake which occurred on 25th November, 2007. This earthquake Mw 4.1 is the best ever recorded earthquake event they say. Here it is pertinent to point out that Mw or the Moment Magnitude Scale is to measure an earthquake in terms of energy released. The seismologists analyze the waveforms on a number of seismographs, which see the earthquakes at different angles. Armed with this information the seismologists are able to reconstruct what happened where the earthquake occurred, such as the direction and tilt of the fault and the direction the rock moved. From this information a quantity called the seismic moment is worked out. The Mw is based on the seismic moment of an earthquake.
 
To a lay man all this sounds complicated but it is like the 3-D Eco of the heart which gives the cardiologist a picture of what is going on inside. Likewise the seismologists with such kind of studies are armed with a better picture to predict the future shocks in the event of an earthquake.
 
Based on the studies of 2007 earthquake and two smaller earthquakes studied earlier, S.K. Singh and his co-workers conclude that the earthquakes in the Delhi region have a lot to do with the faults nearby. Delhi is bounded by two major strike-slip faults namely the Mahendergarh-Dehradun subsurface fault and the Great Boundary fault. Peak ground acceleration at seven sites in the epicentral zone ranged between 7 and 118 Gal. An analysis of data shows that the earthquake occurred at a depth of 30 km and involved strike-slip faulting. Such types of faults are common at plate boundaries. It is common knowledge now that the plates of the earth are constantly on the move due to tectonic forces. Relative movement in such faults, between the two blocks is horizontal. The information on peak ground acceleration, however, Singh et al conclude is inconsistent with data from well-recorded Himalayan arc earthquakes.
 
These studies show that there is a variable site effect of earthquakes in Delhi. Though the knowledge about the possible impacts of higher magnitude earthquakes on Delhi is still hazy, but it is amply clear that due to variation in the site effect some of the sky-scrappers in Delhi may behave like pack of cards, while others may remain standing unfazed. In other words a detailed array of instrument stations need to be created in Delhi so that every tremor in the depths is recorded and analyzed before it is too late. Good news is that a 20 station strong-motion network is being established in Delhi by IIT Roorkee and a Broadband network by the IMD. This will certainly give better clues to the researchers for an accurate diagnosis and identification of unsafe areas.
 
The historical records of Delhi and the present developments in seismology though all have lose ends to tie up. But all of them point to on fact that Delhi is certainly vulnerable and we have to act before a disaster strikes. Identification of areas which are likely to face severe shaking and implementation of building codes to save the residents from being crushed under their own roof is of utmost urgency.