Seasonal variation of earthquake source characteristics of the long-duration Palghar swarm, western India

Himangshu Paul , Sunilkumar T. C., M Shekar

A swarm sequence initiated in November 2018 in the Palghar region near the western coast of India, and continued with high intensity for two years. About 8300 earthquakes were found with horizontal and depth uncertainties of ≤ 1.5 km after relocation. The earthquakes were distributed along two narrow N-S oriented clusters, separated by ~5 km. We performed time-domain regional waveform full moment tensor inversion to compute focal mechanisms of the 33 ML ≥ 3.5 earthquakes, followed by stress inversion. Source parameters for 28 of these events were estimated from frequency spectra analysis. The focal mechanisms indicate dominant normal faulting along steeply dipping, N-S-oriented nodal planes, with high non-DC components (~42%). Stress inversion also reveals a normal-faulting regime, in contrast to the regional India-Eurasia compressive stress regime. The Palghar swarm has been previously linked to rainfall, and our results also show contrasting earthquake source properties during the post-monsoon period, when cumulative rainfall is highest, compared to summer. During the post-monsoon period, the strikes are strictly N-S, slips are ~-100°, and a pure normal-faulting regime is observed, accompanied by increased non-DC components and reduced static stress drops. In contrast, during the summer season, oblique-normal faulting occurs along NNE-SSW-oriented planes, with slip angles reduced to about -65°. We infer that pure normal faulting induced by vertical rainwater load during the post-monsoon transitions to a combination of regional convergent stress regime and vertical loading due to remnant water during summer, resulting in oblique normal faulting. This interplay possibly sustains the rain-induced swarm.