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Monsoon behind low magnitude earthquakes in India’s west coast

K. S. Jayaraman

doi:10.1038/nindia.2020.39 Published online 3 March 2020

A series of low intensity earthquakes have been shaking up the ground below India's Western Ghats for the last 17 months. Geophysicists now say that the earthquakes, which continue to occur in the country's western coast even today, are a localised geological phenomena likely triggered by the monsoons1.

There is no reason to panic due to these low magnitude temblors, they assure.

The first earthquake in the series, with a rumbling sound, was detected in November 2018 near the village Dhundhalwadi in the Palghar district of Maharashtra. Since then more than 16,000 earthquakes of magnitude −0.5 to 3.8 have occurred. "It is causing panic among people but our analyses suggest that it is non-tectonic and non-volcanic in nature and may not lead to a big earthquake in the region," Vineet Gahalaut, the corresponding author at the National Geophysical Research Institute (NGRI) in Hyderabad told Nature India.

Earthquakes develop as fore shocks, main shocks and aftershocks. Sometimes, earthquakes do not have a clear main shock and they are clustered in space and time. Such sequences are referred as earthquake "swarms" that lasts a few hours to a few months.

The report says that the "Palghar swarm" in Maharashtra started after intense rainfall during the 2018 monsoon. After a short decline, the earthquake frequency increased again during the monsoon rainfall starting June 2019 suggesting that the swarm was probably triggered by the monsoon. All the 16000 earthquakes are clustered in a small region of 10 km × 6 km with their focal depths between 2 and 6 km. Analysis of satellite-based Interferometric Synthetic Aperture Radar (InSAR) images, a powerful technique to observe surface deformation, has revealed surface subsidence of ~3 cm between November 2018 to May 2019 in the swarm region.

Location of the Palghar swarm in India's Western Ghats. © Sharma, V. et al The authors say the swarm characteristics — tightly clustered and shallow focus earthquakes —and the occurrence of subsidence imply that this swarm might not have a tectonic origin. "It could actually be a localised non-tectonic activity probably caused by the collapse of subsurface cavities due to migration of precipitated water.

"Interestingly, magneto-telluric studies taken up in the region in 2019 to understand the genesis of these earthquakes clearly indicate presence of fluids exactly coinciding with the cluster of earthquakes," the authors report. This confirms the presence of fluids in the region, they say.

Noting that the earthquakes swarm observed is very close to India's west coast, geophysicist Ramesh Singh at the Chapman University in California, US, says that water intruding into the coastal region due to tidal waves may lead to change in the hydrological regime.

"But the rainfall and earthquake activities clearly show that the earthquakes are associated with the rainfall," he says. Singh says it is very important to have information about the subsurface geology and hydrological regime. "There is also need to monitor water level changes and temperature changes in water wells," he adds.