Abstract:
Geomagnetic variations, recorded through a two-phase magnetovariational study carried out along the Ganga-Yamuna valley of the Garhwal Himalaya, northwest India, are reduced to a set of induction arrows spanning a period range of 12-128 minutes. The spatial behaviour of induction response indicates that the Main Frontal Thrust is a major electrical discontinuity with enhanced conductivity to the south, beneath the Indo-Gangetic plains. Simple two-dimensional (2-D) geoelectrical models with geophysical constraints on the thickness and resistivity of the sedimentary sequences indicate that the induced currents in the Indo-Gangetic plains contribute little to the observed induction response. A full 2-D electrical resistivity model which reproduces the observed electromagnetic response, essentially requires a highly conducting layer at mid-crustal depth beneath the Indo-Gangetic Plains, becoming less conducting on underthrusting beneath the frontal Himalayan belt. This layer coincides with the brittle-to-ductile transition zone along which lie the foci of most moderate earthquakes.
