The story so far:
There has been a spate of small earthquakes in Maharashtra over the last fortnight, according to local reports. Of the 29 tremors during this period, 24 were in and around Nashik, four were in Palghar and one was in Nagpur. They weren’t particularly strong — the strongest was in Nagpur on August 8, of magnitude 4.3 — although they were also shallow, around 5-8 km deep. Seismologists call this pattern of quakes an earthquake swarm, and Maharashtra has experienced them often before.
Also read | Magnitude 3.7 tremor hits Palghar in Maharashtra
What is an earthquake?
An earthquake is a sudden movement of rocks along a fault, i.e. a fracture or a region of weakness in the earth’s crust. In some places, friction can prevent two rock blocks from sliding past each other.
So when forces act on these rocks, they accumulate stress. As they get more and more stressed over time, at some point they will snap and the portion of that fault will slip, releasing the stored energy as seismic waves.
There are different kinds of seismic waves. P waves compress and stretch the rock in the direction in which they’re travelling. S waves shear rocks sideways. Surface waves travel along the earth’s surface. Together, these waves are responsible for the shaking that people aboveground experience during an earthquake.
Also read | Palghar felt two tremors of magnitude 3.4 in an hour
What is an earthquake swarm?
Often, what people call an earthquake is the mainshock, which is followed by aftershocks. But in an earthquake swarm, there is no clear mainshock. Instead there are several earthquakes of comparable magnitude and that magnitude can fluctuate over many days.
However, the occurrence of a swarm doesn’t mean a bigger earthquake is about to occur — nor do all swarms around the earth have the same causes. Palghar in Maharashtra experienced a major swarm from November 2018, recording thousands of earthquakes for more than a year, with the National Centre for Seismology recording more such events — just of shorter duration — in the years since.
Famously, in mid-2024, a volcanic area called Campi Flegrei near Naples, in Italy, experienced a major seismic swarm. On May 20 in particular, locals recorded more than 150 earthquakes within a few hours, including one magnitude 4.4 tremor — the strongest in the area in four decades. Authorities had to evict a women’s prison and several residential buildings even as a few hundred people spent the night in their cars.
Why does Maharashtra experience swarms?
For an earthquake to occur, the basic ingredient is a rock that is being stressed too much. And not all quakes require major faultlines, even if the most powerful ones occur there.
Maharashtra itself lies far from a tectonic plate boundary and experiences something called intraplate earthquakes. Most of the State straddles the Deccan Plateau, whose bedrock consists mostly of the basaltic lava flows known as the Deccan Traps. This feature formed around 66 million years ago when copious amounts of lava erupted through fissures in western-central India. Over time, thousands of individual lava flows accumulated, sometimes hundreds of metres deep. After they solidified, these thick layers of basalt hosted fractures and other physical weaknesses that could transmit stress the same way faults do.
The Indian plate is also still moving and deforming, and is accumulating stresses in its interior areas. And because the Deccan Plateau lies within the Indian plate, these stresses can trigger its own ‘faults’. Studies around Nashik have also found geological structures such as shear zones and fractures that contribute to local earthquake activity.
What does water have to do with quakes?
Water can move deeper into the ground through the soil, the fractures, and permeable rocks — and if it reaches a network of cracks connected to a fault, it can increase the pressure there and further stress the rocks. If this triggers an earthquake, the phenomenon is called rainfall-triggered seismicity.
Researchers who studied the Palghar swarm in 2018 using satellite data found evidence of land subsidence and suggested that rainwater moving into the subsurface region could have triggered the faults. A more detailed study that analysed data of more than 21,000 earthquakes between January 2019 and October 2020 in Palghar concluded that seismic activity had increased following rainfall and that water had migrated into cracks and fissures, increasing pore pressure and helping drive the swarm.
This said, other studies have also reached somewhat different conclusions about the importance of rainwater as a cause, and have instead pointed to normal faulting, a process called aseismic deformation (where the earth’s crust moves gradually, without producing a detectable earthquake), and tectonic stresses and structures in the crust as contributing factors.
In Koyna in Maharashtra, in fact, researchers have been following earthquakes that have been occurring for decades around the Koyna-Warna reservoir, in a famous case of reservoir-triggered seismicity. As scientists from the CSIR-National Geophysical Research Institute, Hyderabad, and the Academy of Scientific and Innovative Research, Ghaziabad, wrote in a 2025 paper in Scientific Reports:
“On 10th December 1967, a magnitude 6.3 earthquake occurred southwest of the Koyna Dam, which is located on the Koyna River in Maharashtra, India. It caused widespread damage in the nearby town of Koynanagar, with houses completely destroyed and over 200 people killed. Such a large earthquake was unprecedented in this region, with no record of seismicity prior to the start of the dam construction in 1956. After the associated reservoir was filled in 1962, seismicity started in the Koyna region, which could be recorded at the seismological station in Pune, 120 km away. The minor seismicity culminated in the occurrence of the Koyna earthquakes of 1967.”
Published – August 17, 2026 08:31 am IST
