Role of fluid on seismicity of an intra-plate earthquake zone in Western India: an electrical fingerprint from magnetotelluric study

The magnetotelluric (MT) investigation carried out in Koyna Seismogenic Zone (KSZ), an intra-plate earthquake region in Western India, along an E-W profile brings out moderately conductive (similar to 700-1000 Omega m) near vertical features within the very high resistive (> 20,000 Omega m) granite/granite-gneiss basement. Occurrences of these anomalous moderate conductors are corroborated with sensitivity analysis. The alignment of earthquake hypocenters along the resistive-conductive boundary signifies the moderate conductor as basement fault. The conversion of resistivity values to the ratio of seismic P- to S-wave velocity (vp/vs) suggests that the moderate conductivity of the fault zone (as compared to the surrounding basement) appears due to the presence of fluid in the fault zone. Geophysical evidences reveal similar to 2.5-3.6 vol% fluid in the fault zone with similar to 1.8-2.6% interconnected porosity, which migrates along the structural boundary and invades the mechanically strong basement to nucleate the brittle failure within it. The present study proposes two mechanisms for the seismicity in the Koyna region. First: the meteoric water circulation due to the loading-unloading of nearby Koyna reservoir acts as potential fluid source for this triggered seismicity, which has also been suggested by previous studies. Second: the fluid circulation due to a deep-seated source. The present MT study brings out a conductive feature below 20 km depth which is thought to be emerged due to the dehydration of amphibole bearing rocks. The fluid generated from dehydration might act as a probable source to the triggered seismicity; since the conductive feature has a linkage to the upper crust.