Magnetotelluric study of an intraplate seismic zone in the Palghar region of the Deccan Volcanic Province, India

摘要

The Deccan Volcanic Province in the western part of the peninsular India consists of a thick pile of flood basalts resting mainly on the Archaean and Proterozoic rocks forming the basement. This intraplate region experiences moderate seismic activity, the most recent one being a swarm-type activity in the Palghar region, about 120 km north of Mumbai, that started in November 2018 and has produced a few thousand micro-earthquakes and a 4.3 magnitude earthquake since then. We have carried out a magnetotelluric (MT) study along a 35-km long profile across the seismic zone to delineate the subsurface structure to understand the possible cause for the seismic activity. Broadband MT data were acquired at 18 sites with average station spacing of 2 km. Impedance tensors were analyzed for distortion and dimensionality, decomposed into TE- and TM-mode, and inverted by a 2D inversion algorithm. The geoelectric structure yields an assemblage of highly resistive and moderately conductive blocks in the uppermost crust resting on a major listric-type fault, that possibly reaches the surface at the West Coast Fault from a depth of about 15 km beneath the Panvel flexure zone. In conjunction with the regional Bouguer gravity anomaly and the seismicity distribution, we infer that the upper crustal heterogeneities coupled with the basement fault and low rheological strength of the fractured upper-to-middle crust might be leading to triggering of the seismicity in the region.