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Earthquakes in Delhi

Geological Setting of Delhi
Delhi, the capital of India is bounded by the Indo-Gangetic zalluvial 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.

The ridge may be said to enter Delhi from the SW. The eastern part of the ridge extends up to Okhla in the South and disappears below Yamuna alluvium in the NE on the right bank of the river. River Yamuna enters Delhi from the North and flows Southward withan Eastern bend near Okhla. The exposed rocks of Delhi are mainly quartzites with moderate folding. What is of interest in seiosmic hazard estimation is the depth of sediments over the rock layers.

Seismicity around Delhi appears to be associated with a major geological structure, which is known as theDelhi-Hardwar Ridge. This ridge constitutes an important tectonic block between 28o - 30 o N and 76o - 79o E with a NESW trend. It coincides with the extensionof the Aravali Mountain belt beneath the alluvial plains of the Ganga basin to the northeast of Delhi towards the Himalayan mountain (Jain,1996).

In engineering terminology this generally referred to as the depth of bedrock below ground level. It is quite well known that tall buildings founded on deep alluvial deposits can be vulnerable to even long-distance eazarthquakes due to resonance effects. Detailed and accurate information on the depth of bedrock in the Delhi region is not available. GSI reports mention that the bedrock depth is 60 m in the Patel Road area, 15 m in Connaught Place Central Park, 40-50 m near Rajghat and 150 m and beyond in the Yamuna river bed. Similarly, the depth is reported to be 80-100 m in the Aurobindo marg-Hauz Khas area. Large number of borehole data are available with various construction agencies in Delhi. As a first effort, nearly 100 borehole have been compiled at the Central Building Research Institute (CBRI), Roorkee for further work. The bedrock profile as estimated from this data is shown in the fighre above. Admittedly this description is only a first attempt. Most probably the bedrock in the trans-Yamuna region is very deep.

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.

Although significant contribution have been made to decipher the seismacity pattern of the Himalayas in the recent past, earthquake pattern in other parts on North India is hardly understood. Delhi region happens to be one such area of great significance due to its dense population density and documented seismicity in the region.

Generalized map of important earthquake events reveals their correlation with major fault zones with the Delhi-Hardwar Ridge. Another study of local small earthquake events (micro earthquakes) shows a good correlation of Seismicity with major fault zones. It is likely that seismicity near Rohtak is caused by a fault zone/gravity high of the Aravalli. Another fault zone crosses Delhi and appears to be active. The major fault zone passing through Mathura and Moradabad is likely to be active and might have caused a few earthquakes.

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

However, various recent earthquake events felt in and around Delhi recorded by various observations located in various parts Delhi are listed below. Since 1994 there has been a marked cessation of seismic activity. It is shown that the stresses, which earlier were being released periodically, are now stored in rocks like a spring. If the trend continues then one day these stresses will be released resulting in an earthquakes.

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