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

C. L. Singh ¹, G. K. Ghosh ¹

Affiliations
1 Department of Geophysics, Banaras Hindu University, Varanasi - 221 005, IN

Abstract

Magnetic and electrical investigations were carried out in an area of about 3000 sq. km in the Northeastern part of the Vindhyan basin in Varanasi-Mirzapur area of U.P. Major portion of the area surveyed is concealed under the thick blanket of Gangetic alluvium. The study has helped in delineating the topographic features of the crystalline basement as well as the trend of the Vindhyan and Bijawar sedimentation. Presence of the fault in the northern part of the area with NE-SW orientation is correlated with wrench fault mapped by Raina (1972) about 300 km in the westsouthwest direction. Its presence is also reflected in the Vindhyan sediments as revealed by magnetic and electrical sounding data. A prominent uplift in the basement is mapped to the north of Mirzamurad followed by rather steep slope further south with thicker sediments of the Vindhyan and Bijawars.

Keywords

Magnetic Methods, Electrical Methods, Wrench Fault, Crystalline Basement, Vindhyans, Varanasi, Uttar Pradesh.

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G. K. Ghosh ¹, A. K. Mahajan ²

Affiliations
1 Oil India Limited, Duliajan, Assam - 786 602, IN
2 Wadia Institute of Himalayan Geology, 33, GMS Road, Dehradun - 248 001, IN

Abstract

Seismic intensity information of historical earthquake of NW Himalayas has been utilized for evaluating the attenuation of the intensity with the epicentral distance. Purposefully, the observed intensity of big historical earthquake may implicitly incorporate some site effects arising due to the near surface geology or deposited soil. The isoseismal map of the 1905 Kangra earthquake yields an area of perceptibility defined by the intensity VII (RF Scale), which is mainly elongated in the NNW-SSE direction. However, the higher isoseismals of intensity VIII and IX are more symmetrical and elongated in the SE direction. These isoseismals are more compressed in the northeast and northwest direction indicating fast attenuation characteristics, which may be either due to the presence of any transverse geotectonic feature or change in lithology. In the present study the regression relation has been derived to incorporate the attenuation of intensity with variable magnitudes at different epicentral distances. The derived attenuation relation is useful for assessing damage of a potential future earthquake (earthquake scenario-based planning purposes) for the Kangra region, Northwest Himalaya. The derived relation is mentioned below:

               I(R, Ms) = 2.856 + 1.31^* Ms - 0.0017 R^* - 0.9598 ^*ln(R)

The decrease in intensity with distance has been used to calculate the PGA using the relation of Atkinson and Sonley (2000). The calculated values are quite in agreement with the values obtained by others around its epicentral zone. Another derived relation is used to calculate the size of the Kangra earthquake (Mw = 8.4) using intensity attenuation characteristic.

Keywords

Intensity, Attenuation, Seismic Hazard, Isoseismal, Meizoseismal, Magnitude and Epicenter.

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