Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

K. C. Joshi ¹, Y. P. Sharda ², Jaya Singh ¹, S. K. Gupta ¹, P. Pande ¹

Affiliations
1 Geological Survey of India, Lucknow - 226 024, IN
2 Geological Survey of India, Delhi -110 062, IN

Abstract

Seismic microzonation studies in Delhi have been carried out solely on the strength of the geotechnical data available with the Geological Survey of India. The main derivative products of the exercise include the shear wave velocity and predominant frequency maps on 1,50,000 scale. These maps are based on the N value data of subsurface soil columns, mostly down to a depth of 30 m, as observed at 121 Bore-Hole sites, in addition to the depth to bedrock, and other geological and geomorphological information.

The different ground response parameters, e g shear wave velocity and predominant frequency have been estimated by using empirical relationships developed elsewhere, especially m California, USA Although the tenacity of the qualitative/ semi quantitative results is yet to be affirmed, the maps, by and large conform to the effects of the past near and far events, such as the 1905 Kangra, 1960 Delhi and 1999 Chamoli earthquakes.

The amplification of the ground motion being the inverse function of impedance contrast between the underlying hard rock and the overlying soft sediments in Delhi, where the underlying rock is represented by quartzite, practically, the relative values of shear wave velocity of the soft sediments indicate the likely amplification - the low velocity promoting higher amplification A maximum ground motion amplification of 12 5 times and a minimum of 5 times, when assuming the damping as zero, has been estimated However, the actual amplifications are bound to be less because the soil does dampen the ground motions The predominant frequency map prepared on the assumption of simplified subsurface configuration, provides a generalized picture of the range of frequencies expected to get amplified -the amplification governed by the shear wave velocity of the soil cover there.

Keywords

Seismic Microzonation, Geotechnical Studies, Site Amphfication Effects, Delhi Area.

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G. C. Kandpal ¹, K. C. Joshi ¹, D. D. Joshi ¹, B. K. Singh ¹, Jaya Singh ¹

Affiliations
1 Geological Survey of India, Lucknow - 226 024, IN

Abstract

Signatures of active tectonics have been observed in the eastern part of Doon valley in proximity to a lineament (fault?) cutting across the Himalayan grain. A normal fault has been observed over the Doon sediments represented by a sequence of silty clay, gravel and clay exposed in a quarry section. The fault displaces the litho sequence with a vertical throw of 1 5 m. This suggests tectonic movement along the fault during Quaternary period.

Keywords

Neotectonics, Quaternary, Doon Valley, Uttaranchal.

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K. C. Joshi ¹, S. Sengupta ², G. C. Kandpal ¹

Affiliations
1 Geological Survey of India, Northern Region, Lucknow-226 024, IN
2 Geological Survey of India, Eastern Region, Gangtok - 737 101, IN

Abstract

An earthquake of magnitude 5.0 with its epicenter in the West Sikkim district occurred on the evening of 20^th May 2007. The macroseismic study revealed an ENE-WSW isoseismal pattern indicating a trend conforming to that of the major tectonic discontinuities in this part of the Himalaya.

The relatively moderate event, felt even at far away places like, Siliguri, Darjeeling, etc induced a maximum intensity of VI (MSK) in the form of development of cracks in the walls, fall of thick chunks of plaster in Type A or even Type BC structures at localities like Tashiding, Ralang, Rabang, Keozing, Sakyong, etc.

The study also revealed that the NW-SE trending Tista lineament is playing an important role in the release of the tectonics strain in this region and that the rupture initiated under the influence of the northward directed tectonic stresses at an asperity formed at one of the decollements within the crust by the intersection of this tectonic discontinuity was interpreted to be responsible for this seismic event.

Keywords

Seismology, Earthquake, Sikkim.

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