Thick sediments in the central Ganga basin: evidence from magnetotelluric investigations

English Abstract:

The collision of the Indian and the Eurasian plates around 55 million years ago and flexing of the Indian plate created a foreland basin south of the Himalaya, named as the Indo-Gangetic basin. The sedimentation in the basin by the giant Himalayan Rivers and their tributaries has led to the deposition of a thick sequence of sediments masking the underlying rocks and tectonic features of the Indian plate. Mapping of the thickness of these sediments is of immense importance for the assessment of earthquake hazard potential due to amplification of seismic waves. Since a large population lives in this basin, it is necessary to quantify the thickness of sediments in the basin and use this information in site amplification studies. We have carried out a magnetotelluric (MT) investigation along a 285 km long profile across the central Ganga basin to delineate the sedimentary thickness in this part of the basin. MT time series processing and transfer function analysis of the data of 39 sites in the frequency range of 0.001-1000s has been carried out for this purpose. The subsurface electrical resistivity section obtained after the inversion of MT data reveals a significant contrast in the geoelectric structure along the profile. The sedimentary thickness at the northern end of the profile, close to the Himalayan foothills, is estimated to be more than 9 km, which is considerably thicker than the average thickness of 4 to 5 km used in flexural modeling studies. The high conductivity of sediments together with the high Vp/Vs ratio obtained by a seismological study in the Ganga basin implies poorly consolidated nature of the sediments and, thus, high seismic hazard potential.