3D crustal resistivity structure beneath the Wagad aftershock zone of the 2001 Bhuj earthquake, Kutch, India: Heterogeneous resistivity structure controlled by widespread fluid infiltration and clues to aftershocks pattern

Abstract

The epicentral region of the 26 January 2001 Bhuj earthquake (7.7 Mw) in the Kutch region of India has been experiencing a vigorous and unique intraplate aftershock activity, which shows a V-shaped pattern and N/NE migration along the western margin of the Wagad uplift located to the northeast of the mainshock. This study presents 3D resistivity structure of the Wagad aftershock zone, imaged using magnetotelluric (MT) data, to understand the crustal geoelectric structure and subsurface geometry of faults/lineaments, which could provide critical inputs to evaluate and strengthen the earthquake generation mechanism and aftershocks pattern. The obtained 3D conductivity structure showed a thick (~3 km) and homogeneous conductive (1–30 Ωm) layer of Tertiary/Mesozoic sedimentary cover in the area. A highly heterogeneous upper and middle crust, characterized by a mix of conductive (<30 Ωm) and resistive to moderately resistive (>300 Ωm) structures, is imaged below the sedimentary cover. Enhanced conductivity zones showed spatial correlation with the surface trace of various faults/lineaments and are interpreted as fluid enriched zones in the crust, which play a critical role in earthquake triggering. Significantly higher fluid enrichment noticed in the crust below two minor faults, Khadir Transverse Fault (KTF) and Manfara Fault (MF), which are hitherto considered to have an insignificant role in the seismogenesis of the area. The aftershock events seem to occur mostly on the resistive parts of the crust, and its pattern shows good alignment with the anomalous conductive zones associated to the above two faults. The results provide a strong clue to infer that the KTF and MF have a vital role in the observed aftershock activity and control the V-shaped aftershocks pattern.