A long duration non-volcanic earthquake sequence in the stable continental region of India: The Palghar swarm

Abstract

In November 2018 a low magnitude earthquake swarm started on the west coast of central India which is continuing into 2020. The swarm started much after the monsoon season and was on decline in May–June 2019 but the earthquake frequency again increased during monsoonal rainfall which started in June 2019. So far it has produced more than 16,000 earthquakes of magnitude −0.5 to 3.8 with an equivalent single earthquake magnitude of 4.5. All the earthquakes are tightly clustered in a region of 10 × 6 km2 and occur through normal slip on north-south oriented east dipping steep fault(s) which extends up to a depth of 6–7 km only. The InSAR analysis reveals a subsidence of ~3 cm between November 2018 to May 2019 in the swarm region and confirms that the resulting deformation occurred through normal slip. The occurrence of tightly clustered shallow focus earthquakes causing subsidence due to normal slip in the overall compressive regime of stable continental region of the Indian plate, imply some shallow subsurface process of precipitated water migration leading to collapse of subsurface cavities and may not be linked with the tectonics of the region. We propose that aseismic slip driven by the fluid migration at shallow depth is responsible for the swarm.