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The Palghar district in northern Maharastra was in a state of Panic on February 1, 2019 as it was rocked by a series of minor earthquakes. Media reported as many 6 tremors on the day, with magnitudes ranging between 3 and 4.1 on the Richter scale. According to a report by the National Centre for Seismology (NCS), many aftershocks of lesser magnitude have also been observed in the area. In the ensuing chaos and stampede, a two-year-old girl fell and died because of serious injuries in one of the villages close to Talasari city in the district.
Palghar has been witnessing an unusual frequency of earthquakes since November, 2018. This had led to the establishment of three temporary field stations of the NCS at Dhundalwadi, Dongripada and Talasari to observe and record the seismic activity.
NCS has categorised the unusual tremors as an ‘earthquake swarm’ which is a series of low magnitude earthquakes that occur in a localised region and over a period of time ranging from days, weeks to even months. “When seismic energy piles up inside the Earth and is released in small amounts from certain points, such a series of earthquakes can occur,” says Pankaj Kumar, who works on the crossroads of seismology and oceanography at the National Institute of Oceanography in Goa.
Sometimes, these rumblings of the Earth are also accompanied by acoustic or sound emissions. A Geological Survey of India (GSI) communication from 2003 says that such activity has been observed at 30 places in the region even though the Deccan Plateau is not an earthquake-prone zone because of its hard rock crust. “Seismic waves travel faster in hard rocks which helps the tremors dissipate faster, but there is also loose soil there which makes the waves stay longer, release more energy and cause more damage,” says Kumar.
The GSI communication also studied the occurrence of an earthquake swarm near Bamhori village of Seoni district in Madhya Pradesh between February and May, 2000. The swarm consisted of as many as 350 tremors but lack of data prevented GSI from reaching a conclusion on the cause of the phenomenon.
But earthquake swarms are not limited to the Peninsula. On August 27, 2016, a series of 58 earthquakes were recorded in the Rampur area of Himachal Pradesh. This Himalayan swarm was later attributed to low strength of the earth’s crust in the area which could not hold the tectonic energy.
The situation cannot be taken lightly as many major earthquakes in the region have been preceded by earthquake swarm activity. They come as foreshocks to the main earthquake which could be much greater in magnitude. Two such instances were the Latur and Koyna earthquakes in 1993 and 1967 respectively. While the Latur quake killed almost 10,000 people, close to 200 people died in the Koyna quake. There is, in fact, an ongoing debate about the fact that the Koyna earthquake was human-made and was caused by the huge Koyna dam completed in 1962. Such an earthquake can occur through the process of Reservoir Induced Seismicity (RIS) when large amount of seismic energy gets concentrated in a small area due the weight of the large structure and the water that it holds. This weakens the earth below and releases the energy in the form of an Earthquake. The Ministry of Earth Sciences is conducting a study in Koyna to ascertain if RIS was indeed responsible for the Koyna disaster and to understand how such events can be predicted.