Three-dimensional resistivity structure in MCT zone around Chamoli region, Garhwal Himalaya and its seismogenic implication

Abstract

Broadband Magnetotelluric (MT) data were recorded at 28 sites in and around Chamoli region of Uttarakhand, India. This dataset was processed and inverted to image the subsurface electrical resistivity of the region and to study its correlation with the seismic velocity model. The dimensionality analysis of the MT data reveals a complex strike direction hinting at the general 3D nature of the impedance tensor. Hence, 3D inversion of MT data was performed using the code ‘AP3DMT’. The final 3D resistivity model was obtained after several 3D inversion experiments. The inverted resistivity model shows broad intra-crustal low resistivity (<10 Ωm) feature distributed in the depth range 4–20 km. Width of this low resistivity feature varies with depth and it is maximum in the depth 8–16 km. Further, there are few high resistivity blocks intervening this broad low resistivity feature. The low resistivity features are interpreted to be a result of the fluids generated by metamorphic dehydration in deeper depths and pushed upward through the fractured zones. The deeper low resistivity zone is associated with the source area of the main shock of Chamoli earthquake of March 29, 1999 (Mb 6.3). The low resistivity features at the shallow depth (2–11 km) are interpreted as the fluid saturated fracture zones associated with swarms of aftershocks of the Chamoli 1999 earthquake. At shallow depths, hypocenters of the swarms and aftershocks are mainly concentrated in the high resistivity zone adjacent to these low resistivity features. However, at the deeper depths hypocenters of the main shock and its aftershocks are in the low resistivity zone, coinciding with the seismogenic source area of Chamoli Earthquake. Out of 423-recorded aftershocks, 336 are located in and around low resistivity zone while remaining 87 aftershocks are located in resistive zone, mainly at shallow depths (<4 km). The low resistivity fluid-filled zone is consistent with the low velocity zone deciphered through earthquake tomography study of the region.