Stress triggering in a rain-induced earthquake swarm in the Palghar region, western India

Himangshu Paul , M Shekar

Rain-triggered seismicity has been reported only in a few regions globally and is typically short-lived. However, an earthquake swarm, inferred to be rain-induced in previous studies, persisted with intense activity for over two years in Palghar, western India. Between January 2019 and November 2020, ~8,300 well-located earthquakes with horizontal and depth uncertainties ≤ 1.5 km were recorded at depths ≤ 10 km. Local magnitudes ranged from -0.1 to 4.6, with a magnitude of completeness of 1.4. Statistical analysis of the swarm revealed new insights into its underlying mechanisms. The swarm could be divided into phases based on the rise and fall of seismicity. Phases coinciding with the end of monsoon seasons showed high b-values, low seismic moment, and a gradual rise and fall of seismicity frequency, resembling cataclastic deformation. During other seasons, low b-values, high seismic moment, and a gradual rise followed by a rapid decline in seismicity frequency indicated stick-slip motion. Mainshock-aftershock sequences caused by stress triggering were found to co-exist within the swarm for most phases. The interplay of multiple causal mechanisms possibly sustained the swarm’s longevity. This study is the first to report the presence of multiple causal mechanisms in a rain-induced swarm.