Delineation of crustal and lithospheric structure below the 2019 Palghar swarm activity region, Maharashtra, India

Here, we present crustal and lithospheric structure at six broadband stations below the Palghar swarm activity region, Maharashtra, India, which has produced 7786 events of M L 0.4–4.7 during November 2018–March 2020. The crustal thicknesses are modelled through the Differential Evolution (DE) waveform inversion of radial P -receiver functions (PRFs), while lithospheric thicknesses are modelled using the joint inversion of PRFs and fundamental model surface wave group velocity dispersion data (SWD). This swarm activity has produced 7786 events of M L 0.4–4.7 during November 2018–March 2020. On an average, our modelled crustal velocities from the PRF modelling through the DE waveform inversion delineate a crustal seismic velocity model similar to an oceanic crustal model near the Western Ghat continental margin. Our modelled Moho depths range from 39.5 to 42 km, with an average crustal thickness of (40.5 ± 1) km, while lithospheric thicknesses vary from 108 to 120 km, with an average lithospheric thickness of (114 ± 5) km. Our modelling reveals on an average a gently north dipping Moho and a sharply SSE dipping lithosphere over an area of 44 × 44 km underlying the Palghar region where the presence of pockets of CO 2 -rich carbonatite partial melts at 108–120 km depths is also inferred. The average crustal model as derived from the joint inversion of PRFs and SWD suggests a low Vs-layer at 5–20 km depth correlating well with the crustal moderately conductive zones as derived from the modelling of magneto-telluric data wherein most of the micro-earthquakes have occurred below the Palghar region. Thus, we propose that the occurrences of earthquakes at 0–5 km depths might have been triggered by rain or meteoric water while the occurrences of deeper earthquakes (at 5–20 km depths) could be triggered by the presence of aqueous fluids. Some earthquakes are also occurring between 20 and 32 km depths, which might have been triggered by the presence of aqueous fluids or entrapped volatile CO 2 emanating from the presence of pockets of CO 2 -rich carbonatite partial melts beneath the Palghar swarm activity region.