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Subsurface map of Delhi
Importance of subsurface features
During an earthquake, actual ground shaking at a particular location mainly depends on three factors; Source mechanism, propagation path and local soil condition. The previous two parameters may be the same in a city, but different soil conditions may create different degree of damage in some locations. Though few meters of the top surface of the ground may be composed of different layers of soil compositions, but in the present study we have taken the sedimentary and filled up overburden above the bedrock as an individual factor. It has been repeatedly observed that some localities behave differently in terms of damage to the engineered structures in the same city during an earthquake. In recent years, 1985 Michoacan, Mexico earthquake (Singh et.al., 1988), 1988 Armenian earthquake (Borcherdt et.al., 1989), 1989 Loma Prieta earthquake (Hough et.al., 1990, Borcherdt et.al., 1992) and some other devastating earthquakes have clearly proved this phenomena if one goes through their damage patterns. Iyengar (1999) refers some of the well known facts of 1934 Bihar earthquake, 1964 Nigata Japan earthquake, 1964 Alaska USA earthquake, 1971 San Fernando earthquake, 1991 Uttarkashi earthquake and 1993 Latur earthquake where structures located on loose granular or sandy soils has experienced major damages. Recent Chamoli earthquake (29th March, 1999), which was severely felt in Delhi, caused some damages in buildings standing on very thick layer of sedimentary deposit. It had already been established about a century ago that ground motions are substantially amplified near the ground surface with thick sedimentary cover (Milne, 1898). Since then numerous studies have been undertaken by different research workers to substantiate the fact, in different parts of the globe. Studies on the subject are still being persued with the sole intention of enriching and authenticating the database. Review of the published literature on the subject has been presented by Aki (1993) and Bard (1995) in near past. Aki (1988) concluded in his review paper, among other things, that soil sites show higher amplification than rock sites by a factor of 2 - 3.  Delhi has some heavily populated colonies on deep deposits of alluvium in various locations, particularly in the trans Yamuna area. These areas are seismically vulnerable as large ground motion amplifications are expected due to their typical ground compositions. On the basis of past earthquake records Khatri (2000) makes observation that Delhi region is likely to experience a peak ground acceleration of 0.21g with the shaking duration of about 30 seconds, with a 10% of probability of excedence in a time window of 50 years. Recent Chamoli earthquake, originated about 400 kms away from Delhi could generate peak ground acceleration of the order of 0.01g in Delhi (CBRI Report 1998). Given the greatly increased level of damage that can be produced, it is of practical importance to develop methods for assessing the nature of, and potential for, sediment amplification and design of critical and strategically essential infrastructurd facilities. Subsurface map of Delhi Generally the bedrock topography in Delhi region is undulating with several humps and depressions. The depth of bedrock varies from near surface to beyond 200 meters deep from place to place, which is caused mainly due to the complex tectonic features of the area and thick sediments deposited on the bedrock. Several agencies like GSI, RITES, CGWB and others have carried out geotechincal investigations in Delhi for various purposes. The investigations carried out by the different agencies mainly include geophysical surveys, geotechincal investigations and drilling of shallow as well as intermediate depth drill holes with a view to either determine the nature of material lying below the surface, locate the possible source of ground water and delineate the bedrock profile.  With the sole purpose of generating a three-dimensional subsurface map of Delhi, an attempt has been made to collect the subsurface data available with various agencies. About 100 subsurface details spread all over Delhi have been collected and analyzed. As the informations collected originated from different sources, there was no technical uniformity in them. The information collected by a drinking water locating body has to be different in terminology from the information gathered by a geotechincal/construction agency. Authors tried their best to take up the cases of uniform nature to the maximum possible extent. Out of these, actual thicknesses of sedimentary deposit at about 50 well spread out locations were evaluated for the preparation of subsurface map. A three-dimensional map showing the thickness of sedimentary cover from ground surface covering an area of 18/20km has been generated with the help of Surfer software package. Fig.1 and 2 give the details of the topography and sedimentary deposit thickness variations from place to place in the studied area of Delhi. |