6.1.3 Study of Earthquake Swarms
Srinagesh et al. (2020) studied earthquake activity in the Palghar region, Maharashtra, India. Until 31 August 2019, a total of 4854 earthquakes have been located here, whose local magnitude (ML) varies from 0.1 to 4.1. Majority of the earthquakes ( \(\sim\) 94%) were located in the depth range of 4–16 km. The precise earthquake relocations reveal two clusters. The N–S trending cluster north of 20.04\(^{\circ}\)N extends to a depth of 10 km, whereas the NE–SW trending cluster to the south of 20.04\(^{\circ}\)N extends to 16 km depth. The shallow northern cluster is noticed to be sandwiched between two mapped mafic intrusions, whereas the deeper southern segment shows earthquakes clustering around the mafic intrusion. The modeled composite focal mechanism solutions for both the north and south clusters suggest normal faulting with a minor strike–slip component as the dominant deformation mode for the Palghar region. From relocated seismicity, they detected a deeper seismically active zone (with M > 3) at 4–16 km depth, occupying a crustal volume of 1440 km3.
Mandal et. al. (2021b), conducted a comprehensive analysis of swarm activities in two regions of the Indian shield -- (i) Palghar (Maharashtra) and (ii) Pulichintala (Andhra Pradesh). The 3-D mapping of b-value and fractal correlation dimension (D2) reveals that the Palghar sequence follows typical characteristics of swarm activity (the b and D2 values vary from 0.1 to 2.5, and 0.39 to 2.62, respectively). On the contrary, the Pulichintala sequence (with b and D2 values varying from 0.2 to 1.68, and 0.68 to 3.0, respectively) shows negative characteristics. The Palghar region is interpreted as a region of higher tectonic stresses.
Swarm activity similar to that at Palghar in Maharashtra (Mahesh et al., 2020), has been observed in Navsari and Jamnagar districts in South Gujarat and Saurashtra. At Navsari, a swarm activity was observed around the Keliya dam from Sep.2016 just after the Indian monsoon period that continued for about 4 months. Again, the swarm activity recurred in Aug.2017 and continued for about 5 months. A local network of four stations was installed by ISR to monitor the swarm activity, in addition to the Gujarat state seismic network (Sateesh, et al., 2019; Srijayanthi et al., 2022). A total of 1048 earthquakes were located around the Keliya dam and 229 events in the Dadra and Nagar Haveli (DNH) region from Sep.2016 to June 2018. The seismicity in both the regions followed a \(\sim\) NW–SE trend. It was confined to an area of 13 km × 2 km with a depth extent of 3 km at Navsari and 15 km × 2 km with depth of 6 km in DNH. In the Jamnagar district, ISR observed post-monsoon swarm activity in Sep.2019 with 76 clustered earthquakes having NW-SE trend that are in line with the strike of local lineaments and dykes.
Parija (2021) critically examined the 2011-09-18 Sikkim earthquake of M 6.9 and found it to be associated with episodes of precursory swarms, quiescence, mainshock and aftershocks. The precursory swarm and quiescence period consist of four earthquake swarms and one foreshock event of magnitude (mb \(\ge\)4.5) in the epicenter preparatory zone of the 2011 Sikkim earthquake. The 2011 Sikkim earthquake had about five aftershocks of magnitude (mb) \(\ge\) 4.5 between 2011 and 2014 for the same region.
Parvin et al. (2021) analyzed two swarm activities in the Hyderabad region and found small absolute stress drop values (< 1 MPa) and a positive correlation between the static stress drop and the magnitude of the earthquake with seismic moment varying between -0.09 < ML< 1.52. They observed a clear correlation of earthquakes associated with the fractures and faults in the vicinity of water bodies which are more sensitive to variations in hydrostatic pressures caused by vertical flow recharge from rainfall and deeper pore-fluid pressure diffusion.
Rekapalli and Gupta (2021) tried to understand the foreshock-aftershock patterns, main shock to the largest aftershock magnitude ratio, and difference in magnitude of two moderate injection-induced seismicity (IIS) earthquake sequences from Oklahoma, USA, namely, Prague (M 5.7, 2011) and Pawnee (M 5.8, 2016), and comprehend the shallow crustal heterogeneity. The analysis of temporal variation of “b” value from 2002 to 2018 suggests an increase in b-value after 2009. A reduction in b-value after 2016, in response to the reduced injection volumes is noted. A sharp fall in b-value usually precedes the main shocks of magnitude M \(\ge\) 3.5. The foreshock b-values are lower than the regional b-value and aftershock b-values are higher than the regional b-value within the error limits. The investigated earthquake sequences fall under Type 2 of Mogi’s model. The characteristics of ISS observed at Oklahoma are similar to the observations for the reservoir-triggered seismicity (RTS). However, with multiple injection wells operating from time to time in the region with varying amount of fluids injected, the entire IIS at Oklahoma has an appearance of a swarm.
Wadhawan et. al. (2021) studied a highly clustered shallow (<0.4 km) earthquake activity of low magnitude with accompanying rumbling sound in Sadrabadi and Zilphi villages in Dharni Taluka of the Amravati district, Maharashtra during the monsoon period of 2018 and found it to have the characteristics of a swarm. They found a strong correlation between rainfall and swarm activity and categorized it as hydro-seismicity, resulting from hydro-fracturing of the soil/weathered basalt and collapsing and caving of the rocks. In the past, no such activity has been reported from the region during or after the monsoon, despite the fact that there was more rainfall in 2019. Therefore, they suggest that the low magnitude earthquake swarm at a very shallow depth might have been induced by the percolation of monsoonal rainwater through the weathered and fractured rock-mass associated with the fault system of the Narmada Son failed rift region.