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Gravity and magnetic studies have been carried out to map the different depth formations of alluvium, Gondwana, Vindhyan, Mahakoshal, and the crystalline basement in the Narmada-Son lineament (NSL) near the Sahdol-Katni area, India. Higher elevations in the northern part of the study area have lower gravity; the southern part of the study area, however, is moderately elevated and also has a higher gravity anomaly, which justifies the isostatic adjustment. This indicates the presence of high-density material, for example Mahakoshal rocks, in the upper crust which causes the higher anomaly value. The Mahakoshal rocks are widely exposed at Mau, Chanaura, Nadawar, Khamaria, Umria, and near the TalaBarhi area and also extend from Sidhi to Agoni village further east of the area. The Mahakoshal rocks are thinner between the river Son and Tikwa village which causes a different gravity anomaly pattern changing from the NNE-SSW direction to the N-S direction. However, the trend of magnetic anomaly follows the same pattern toward the NNE-SSW direction, mainly because of the effect of the crystalline basement. Two magnetic highs are prominent in the Tikwa and Amarpur regions, 800 and 400 nTesla, respectively, because of the presence of the crystalline basement. The depth of the crystalline basement obtained by 2 D gravity-magnetic modelling varies from 2.7 to 2.9 km. From spectral analysis the average depth of the crystalline basement varies from 2.83 to 3.05 km. The different crustal depth sections obtained by 2 D simultaneous gravitymagnetic modelling correlate well with other constrained data.

Seismic hazard assessment of any region depends on the attenuation relation which relates the seismological data with parameters of engineering interest. In the absence of sufficient strong motion data for northwest Himalayan region, isoseismal maps of previous earthquakes were the only source of information for computing the attenuation characteristics of that region. The northwest Himalayan region had experienced major and moderate earthquakes in the past and isoseismal maps had been developed by various authors for each of these earthquakes representing intensity pattern and decay of intensity in different directions. With this information, attenuation relations of intensity with distance can be obtained. The present study includes 10 moderate and major earthquakes (Ms ≥ 4.9) that had occurred during the last 100 years.