Geoelectric characterisation of the junction of seismically active Delhi Hardwar Ridge and Delhi Sargodha Ridge

A magnetotelluric (MT) geophysical survey for the first time has been conducted for the geoelectric characterization of the junction of the contact zone of NNE-SSW striking Delhi Hardwar Ridge (DHR) and NW-SE trending Delhi Sargodha Ridge (DSR) in the Rohtak area, Haryana which has experienced 15 earthquakes of M2.0-M4.4 from April to August 2020. A total of 08 MT sites are acquired along a NW-SE profile of length 50 km. From the 2D MT data inversion, the DHR and DSR are for the first time characterized by equal values of moderate resistivity of 100 Ohm m at two depths. The resistivity variation for DHR corresponds to 100 Ohm m from the surface to the depth of 20 km, whilst DSR is found associated with the same value of resistivity extending in the NW direction. The DHR has been found striking NE-SW with a very shallow central axis (less than 400 m) having a width of 12-15 km forming half grabens on both limbs supported by shallow faults. The DSR has been found bifurcated from DHR at a depth of 12-13 km and extended in the NW direction. The DSR has been generated due to flexure bulging caused by collision and anticlockwise rotation of the Indian plate in the Eocene period. A NE striking steep dipping reverse fault (F1) has also been identified about 15 km west of the DHR. It is inferred that the DSR got upthrusted along this fault and became shallower in the NW region. The seismicity in the Rohtak and surroundings is located at the bifurcation points of DHR and DSR and the contact zone of DSR and reverse fault F1. The reverse fault F1 is also active and has generated microseismicity in the past.