Researchers say the region's geology, combined with monsoon and reservoir water seeping 20 km deep into crustal faults, is driving the swarm of small, repeated quakes that began in early October.

Five tremors in 17 hours

The sequence began on Friday, 23 October, at 9:57 pm with a 2.2 magnitude tremor, followed at 10:18 pm by a 1.8 magnitude jolt. On Saturday afternoon, three tremors struck within ten minutes: 2.4 at 1:59 pm, the strongest, 2.9, at 2:02 pm, and 1.7 at 2:10 pm, leaving residents frightened as the ground shook repeatedly within 17 hours.

People in the area felt the ground shake again and again through the night and into the afternoon, and fear spread as the tremors continued.

A pattern since early October

The 17-hour sequence is part of a longer swarm that began in early October in the Talala-Sasan belt. A 3.2 magnitude earthquake struck on 1 October along with several smaller tremors, and just three days before the latest sequence, on Wednesday 21 October, a 2.8 magnitude quake was recorded about 3 km from the area, again with light tremors.

Seismologists call such a series a swarm earthquake: a run of small to moderate tremors without one single dominant mainshock, spread over days or weeks.

What the studies say

The Talala-Sasan region sits on the Girnar fault line, running east-north-east to west-south-west, with faults of the Narmada and Aravali trends close by. Studies by Gujarat's Institute of Seismological Research and Oxford researchers have examined the area closely, and their conclusion is that the fault lines themselves cause the quakes, while water can increase the sequence.

After the monsoon, rainwater and reservoir water seep into the ground and reach faults 20 km deep in the earth's crust, raising pressure inside the cracks, which can trigger tremors. In short, the region's geology sets the stage, and water pressure can multiply the shocks.

Dams and a river close by

The epicentre area is ringed by water bodies: the Kamleshwar dam on one side, Hiran-2 on another and the Malanka dam on the third, with the Hiran river flowing nearby. Scientists say that pattern of dams and a river close to the fault zone is part of why water reaches deep into the crust here, feeding the swarm.