Interplay between aseismic and seismic slip in an earthquake swarm in Western India

The Palghar Swarm in Western India is unique given its occurrence within the stable continental interior, its unusually long duration (having started in November 2018 it continues unabated), and extremely high seismicity rate (up to a few hundreds of earthquakes a day). Given the small spatial extent (around 100 km²) of the swarm and the dense seismic network deployed by Indian agencies to monitor it, the swarm offers a unique opportunity to understand the processes driving swarms within the stable interior of the Indian plate which, compared to continental interiors elsewhere in the world, is unusually seismically active. The swarm clusters along two lineaments not expressed on the earth surface. Our InSAR analysis, assuming the lineaments to be subsurface faults, reveals predominantly normal dip-slip motion along both faults during several time windows between March 2019 and January 2020. We find the geodetically inferred moment to be an order-of-magnitude larger than the cumulative seismic moment throughout this time window indicating the presence of substantial aseismic slip. The aseismically slipping patches on the two faults migrate spatially and seem well correlated with the migration of seismicity. We explore the interaction between aseismic slip and the swarm seismicity by calculating resolved Coulomb Stress changes due to migrating aseismic slip on each fault and at the hypocentres of earthquakes large enough for a reliable moment tensor to be inferred. Preliminary results suggest a complex relationship between aseismic and seismic slip and a possible involvement of fluids. These results raise the question whether aseismic slip is commonly associated with earthquake swarms within the Indian continental interior and if these might be associated with deep fluid sources within the Indian continental crust.