Irshalwadi landslide: Geologist draws links between tragedy, past quakes

The second incident was an earthquake of magnitude 3.7 that occurred in Palghar district on June 24, 2021. While in both these cases no casualties were reported, cracks developed in houses, trees fell and many abandoned the village.

“It can be said that the Khardi earthquake of 1983 and the recent earthquake in Palghar in 2021 weakened the slopes and made Irshalwadi prone to landslides,” the former head of the geology department at Savitribai Phule Pune University, Thigale told The Indian Express.

“The seismic activity, mountainous terrain, incessant rain, adverse geological setup coupled with levelling of land, haphazard development and clustered housing, led to the Irshalwadi tragedy last month,” added the geologist who has been studying landslides in the Western Ghats, especially Maharashtra, for over 40 years.

“Khardi is around 80km away from Irshalwadi and Palghar around 100km. Irshalwadi is also merely 60km away from places such as Malin and Bhimashankar, which are susceptible to landslides,” Dr Thigale told The Indian Express. However, as pointed out by Deputy Chief Minister Devendra Fadnavis, the village was not named in the list of landslide-prone villages and had no history of landslides.

The Western Ghats cover at least 70 talukas from 12 districts of Maharashtra including Palghar, Thane, Raigad, Mumbai, Ratnagiri, Sindhudurg, Kolhapur, Sangli, Satara, Pune, Ahmednagar and Nashik. These mountains are prone to mass-wasting phenomenon which refers to the movement of rock and soil down mountain slopes due to gravity.

Landslides are a type of “mass wasting”. A report by the Council on Energy, Environment and Water, 2021 mapping climate vulnerability made a correlation between landscape attributes and extreme events. Landscape attribute changes include changes in land use and land cover, soil moisture, groundwater, slope, and elevation, among others.

The report pointed out that India has seen more than 478 extreme events since 1970 and there has been a spurt in their frequency after 2005. “Various studies have confirmed that landscape attribute changes have substantially contributed to this spurt,” it said.

When an earthquake occurs, cracks can develop due to shattering of regolith by secondary waves even in far off areas. These cracks make the mountain slopes unstable, Thigale explained.

Hydrogeologist and faculty member at Savitribai Phule Pune University Dr Bhavana Umrikar said, “When there is a crack, more water percolates into the pervious upper layers of soil and weathered rock. At some point the water reaches the hard, impervious layer of rock at the bottom. Because the water cannot go further down, it starts moving horizontally along the slope instead.”

This is how rain acts as an immediate trigger for landslides causing the majority of them in the monsoon season, especially during July. Apart from the one at Irshalwadi, experts have said that landslides in Western Maharashtra could be traced to some of the major earthquakes around the Koyna reservoir that took place as long ago as 1967.

The Koyna earthquake of 1967 had a magnitude of 6.3 and was one of the most severe earthquakes recorded in the area. With a death toll of around 200 and tremors that were felt upto 700km, it shattered the belief of scientists that Western Ghats were not prone to earthquakes. “A study in the Ratnagiri district showed that a majority of the 56 landslides that occurred in June 1983 were precisely along the hill slopes where the Koyna earthquake had created cracks,” said Thigale.

“Interviews of surviving residents of Malin, which saw a deadly landslide in July 2014, revealed they experienced tremors and aftershocks of the 1967 Koyna and 1993 Latur earthquakes.”

“This connection is likely to be missed when a district-wise assessment to list landslide-prone-villages is done,” he observed. “Nature does not follow man-made geographical boundaries.”

In an effort to promote a more holistic understanding of the likelihood of disasters and vulnerability of the Western Ghats, the Madhav Gadgil report of 2011 had recommended the establishment of a Western Ghats Ecology Authority under the Environment (Protection) Act. The recommendations of the Gadgil report were discarded by the Ministry of Environment and Forestry as basis for action in 2014.

Seismic activity, however, is only one of the factors that contribute to weakening of slopes. “Blasting of mountains for tunnels can create cracks even in far off places,” said Dr Bhavana Umrikar, hydrogeologist and faculty member at Savitribai Phule Pune University. “Reducing vegetation cover, changing patterns of rainfall due to climate change, expansion and clustered settlements as well as big development projects are other factors that increase the likelihood of landslides,” she explained.

“Landslides being slow moving processes in the Western Ghats, there is a wide scope to mitigate the disastrous situation,” said Thigale. “You cannot prevent the natural calamities but certainly minimise the death toll by taking appropriate measures,” he said.