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Earthquakes in Delhi


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.

Areas affected by earthquakes of M-6.5

Earthquake M
Sq km
I-VIII
Sq km
I-VIIS
q km
Total under
VII & VIII
Koyna 1967 6.5 130 430 560
Uttarkashi 1991 6.6 700 1300 2000
Killari 1993 6.3 420 930 1350


Recent earthquakes in Delhi
(location and magnitude)

Date Latitude oN Longitude oE Magnitude
June 6, 1992 28.65 76.69 2.8
Feb 16, 1993 28.63 76.35 2.6
Mar 27, 1993 28.63 77.20 3.6
Aug 6, 1993 28.64 77.14 2.5
Dec 3, 1993 28.60 77.40 3.5
July 28, 1994 28.51 77.25 2.8
Oct 15, 1994 28.59 79.92 2.8
Nov 16, 1994 28.50 76.95 2.9

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:
  • Considering areas affected during past earthquakes of M - 6.5, it can be expected that such an earthquake occurring in Delhi could adversly affect the whole of it with damaging intensities and more than 50% of the Delhi Metropolitan Area - in terms of probable damage scenario, earthquake would be the worst natural disaster for Delhi.
  • <> An issue of concern to development managers and citizens of the city today is that many groups of Indian and American scientists have forecast a major seismic upheaval of a probable magnitude * or 9 on the Richter scale in North India. The forecast is based on a detailed studey of past Indian earthquakes and analysis of plate tectonics of the region.
  • As such, management measures in earthquake-related disasters mainly cover actions to minimize structural damage, and preparedness for rescue and relief. In India, where 90% of the population lives in buildings built with out proper guidance from qualified engineers and architects, occurrence of an earthquake of even a medium scale spells disaster.
  • The Latur earthquake of September, 1993, measuring 6.4 on the Richter scale and inflicting 10,000 casualties, took the scientific community by surprise as it occurred in Seismic of all zones in the country, and has thus compelled them to rethink its impact on Architecture and Planning in the country.
  • Delhi, which is lies in Seismatic Zone IV, is currently experiencing mild seismicity. An earthquake of magnitude 7.0 on the Richter scale, that was once considered hypothetical, is today a very real possibility. Keeping in view the forecast of a major earthquake resistant design consideration, it has become imperative to size up the earthquake scenario of the city and increase awareness of earthquake resistant techniques.
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. 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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