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. Tectonic Setting It is seen that the Delhi region has a long seismic history being affected by earthquakes of local origin as well as these on Himalayan origin. Based on the tectonic map of the region prepared by Srivastav and Roy, this region is characterized by several dominant features such as the Delhi - Hardwar ridge, the Aravalli - Delhi fold, the Sdohna fault, the Mathura fault and the Moradabad fault. Verma et al and Chouhan et al., who have have studied more than 100 events recorded in the region have shown that the epicentres have a pattern of clustering in two belts, namely Rohtak and Delhi. They also opine that the local activity shows a switching between these two places, with Rohtak being more active than the Delhi area. The distribution of the epicentres appears to have a NE-SW trend correlated with the direction of major tectonic features of the region. According to these authors it is not possible to associate the seismicity of Delhi with any particular tectonic unit. On the other hand, a number of lineaments appear to be active to various degrees. Extract from Seismatic status of Delhi mega city , RN Iyengar.
(location and magnitude)
Rigorous quantifications of seismic hazard in detail for the Delhi region has nit been carried out so far. Chouhan estimated the focal depth of earthquake around Delhi to be about 8 km. His frequency-magnitude analysis lead to the conclusion that the maximum size of an earthquake that may occur in this region would be 7.6. The work ofd Srivastav and Roy indicates that in a period of 50 years a magnitude 6 earthquakes is almost certain and that there is 80% probability of a 7 magnitude event visiting the region. Khattri has carried out an exercise estimating the seismic hazard for the northern region of the country. According to this study in a fifty year window of 1983 - 2033, the peak ground accelerations around Delhi would be 0.2 g with 10% probability of excedance. Delhi and its environs have not yet experienced this level of ground vibration in the above tiome period. Even if these estimates need further refinement they indicate the nature of events expected for Delhi city. Extract from Seismic status of Delhi mega city, RN Iyengar. Soil considerations Coupled with the settlement pattern, the geological characteristics, such as depth of alluvial soil, play a crucial role in determining the magnitude of risk. The Narain-Patel Road section and the Yamuna River - bed section, extending till NOIDA and Faridabad, are more vulnerable to damage even by a moderate earthquake because they are on alluvial soil upto 200 m deep. These regions face a very grave problem of soil liquefication during an earthquake. Moreover, earthquakes are amplified by alluvial soils. The Ridge is comparatively safe as it has a rocky base. Concerns Areas subjected to damaging intensities MSK VIII and VII in 3 earthquakes of Richer M-65 are seen to be as follows:
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. The ground acceleration recorded in Delhi city has been of the order of 10cm/s2 on soft soil. Three stations on soft soil recorded the event. Stations on hard rock did not record the event at the trigger level of 0.001g acceleration. This implies that the base rock level motions have been amplified by deep soil deposits to the order of 10 cm/s2 at surface level. The narrow band Fourier spectrum indicates strong filtering characteristic of the site which responds mostly near its natural frequency of 1-2 Hz. For engineering evaluation of seismic risk to man-made structures, estimation of the hazard in terms of probable ground acceleration due to future events is essential. Since almost all parameters such as return periods, magnitudes, epicentral locations, and site characteristics are uncertain or random variable, seismic hazard estimation by its very nature is probabilistic. Extract from Seismic status of Delhi mega city, RN Iyengar | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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