A new hypothesis for the deep subsurface structures near the Bhuj 2001 earthquake (Mw 7.6) hypocentre zone and its tectonic implications

We provide a new hypothesis for the deep subsurface structures near the Bhuj 2001 earthquake region based on magnetotelluric (MT) investigations carried out close to the epicentre zone. 2-D inversion of broad-band MT data of two profiles of lengths 32km (AA′) and 52km (BB′) revealed a thick (∼3km) highly conductive (1–4 Ω-m) surface layer of fluvio-marine Mesozoic–Cenozoic sediments. The models delineate the hypocentre zone located at ∼20–25km depth that manifests the high resistivity–conductivity transition zone. The accumulation of compressive stresses post-rifting along this weak zone has resulted in the reverse slip of Bhuj 2001 earthquake. The reverse fault (F1) associated with the earthquake is believed to be an ancient normal fault formed during the rifting phase. Contrary to earlier suggested theories, we suggest that F1 got initiated along the high resistivity–conductivity transition zone causing the Bhuj 2001 event. The geoelectric models revealed a laterally extending partially resistive zone at 20–30km depth range showing a tendency to extend further deep. Model calculations using synthetic data also support this observation. Therefore, we hypothesize the presence of a basal detachment, marking the transition zone between the continental crust and the lithospheric upper mantle at ∼40km depth, intersected by the F1. The geoelectric models suggest that the crustal thinning caused the asthenospheric upwelling and/or serpentinization leading to the ascent of volatiles and melts. The subsurface geometry in Kachchh basin suggests the thick-skinned deformation.