Significance of VP/VS ratio in locating earthquakes of a long-duration swarm in the western coast of India

Abstract The ongoing earthquake swarm in Palghar, western India, since late 2018, is a natural laboratory to understand long-duration seismogenesis. However, to understand the seismogenesis mechanism, accurate estimation of the key parameters like precise earthquake locations and velocity model of the region is important. In 2020, two separate studies undertaken by two organisations in India, viz., CSIR-National Geophysical Research Institute (NGRI) and National Center for Seismology (NCS) reported significantly differing results from the 8–12 months of earthquake monitoring in Palghar. CSIR-NGRI reported the depth of earthquakes in the range of 4–15 km, whereas NCS reported a much shallower depth down to 7 km only. We investigated the reason for this depth discrepancy using datasets from the networks of both these organisations. We found that for the available velocity models of the region, the earthquake locations using the two datasets are different. The selection criteria for velocity models, viz., travel-time residuals and location errors did not work for these velocity models. The network geometry and source-station separation are found to be some of the important factors for the observed inconsistency. However, the main reason is the inaccuracy of the velocity models in correctly locating the earthquakes. We estimated a velocity model by grid-searching V P and V P /V S ratio to minimise the location difference between the two datasets. It consists of a V P lying between 5.55–5.85 km/s and a V P /V S ratio between 1.77–1.81 of the shallow crust. For this velocity model, about 80% of the earthquakes of the CSIR-NGRI network were located within ~ 7 km depth. There is a greater overlap of the two seismicities such that the earlier depth discrepancy is resolved. We found strong correspondence between rainfall and seismicity. In the absence of other sources like the reservoir, volcanism, tectonics, fluid injection or other anthropogenic activity, we infer that the high V P /V S ratio is caused by the rainfall saturation of cracks in the Palghar seismicity zone.