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

J. R. Kayal ¹, Sagina Ram ², O. P. Singh ¹, P. K. Chakraborty ¹, G. Karunakar ³

Affiliations
1 Gelogical Survey of India, Central Geophysics Division, 27, J.N. Road, Kolkata - 70001 6, IN
2 Geological Survey of India, Northern Region, Lucknow - 226 024, IN
3 Geological Survey of India, Eastern Region, Salt Lake, Kolkata - 700 094, IN

Abstract

The aftershock sequence of the Chamoli earthquake (m_b 6.3) of March 28, 1999 provides an opportunity to study the aftershock characteristics and the seisrnotectonic structure in the Garhwal Himalaya. Detailed analysis of the spatial variation, the frequency-magnitude distribution (b-value), the activity decay-rate (p-value) and fault-plane solutions of the aftershocks throw new light on the aftershock characteristics and seismotectonics of the region. The observations suggest that the aftershock generating process and the b- and p- values changed with time, space and magnitude-ranges. The main shock occurred on the Basement Thrust by a thrust-fault mechanism. The aftershocks, on the other hand, are triggered by the seismogenic faults to the south of the Main Central Thrust (MCT) by thrust as well as by strike-slip faulting, and all the events occurred above the plane of detachment.

Keywords

Main shock, Aftershocks, Seismicity, Fault-Plane Solution, Seismotectonics, Chamoli, Garhwal Himalaya.

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D. K. Sawa ¹, K. Choudhury ¹, P. K. Chakraborty ¹

Affiliations
1 Central Geophysics Division, Geological Survey of India, 27, J L Nehru Road, Kolkata - 700 016, IN

Abstract

A number of saline/brackish water zones are present in the subsurface around the Kolkata metropolis. Mixing of fresh and brackish groundwater has created environmental problems in certain areas. Vertical Electrical Soundings (VES) employing Schlumberger configuration have been deployed in the eastern and southern Kolkata metropolis for delineating the subsurface saline water zones. Interpretation of VES data has brought out disposition of saline/brackish and fresh water zones at different depth level which would be useful in the proper drawal of groundwater with minimum risk of saline contamination. Resistivity surveys have also delineated clay formations which act as barriers for saline water percolation or transmission. Aquifer zone at some depths south of Bhangar canal is vulnerable for saline water contaminations large part of this area is occupied by brackish/saline water in the subsurface. It has been found that the area north of the canal does not contain any appreciable saline water in the subsurface and may be considered environment friendly it is further inferred that the saline water was trapped inland in the Holocene times during marine transgressions causing salinity of groundwater at several places.

Keywords

Saline Water Zones, Vertical Electrical Sounding, Kolkata Megacity, West Bengal.

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