Amit Sharma
Land subsidence is the progressive sinking or levelling of the Earth’s surface caused by a variety of events, which has a wide range of implications and after effects. This behaviour may have serious consequences for both natural landscapes and infrastructure.
Understanding the origins, consequences, and after effects of land subsidence is critical for reducing its effects and devising sustainable land use policies. Sangaldhan-Parnote (J&K) covers an aerial distance of approx. 13 kms which lies in the Ramban district of J&K. The mean sea elevation of this area is about 1400 m. This area possesses huge vulnerability to landslides, floods etc. There area are multiple factors involved in land subsidence in this area which includes geogenic, anthropogenic etc. These factors greatly influenced the topography and land use land cover patterns of the area. In this article, major focus is on the geological factors.
Different researchers have performed multiple experiments and detailed survey over this area and found the basic geological cause of this disaster. Tectonically, the Sangaldhan-Parnote area lies between the Main Central Thrust (MCT) and the Main Boundary Fault (MBF) of Western Jammu, Lesser Himalayan region. This area is traversed by a regional reverse fault referred to as Murree Thrust (MT) and Panjal Thrust (PT). The MT is exposed from Dhamkund (confluence of Chinji Nala and Chenab River) to Duskar Dalba village in the western side and upto Parnote village on the eastern side. In this stretch, there is presence of thrust and faults in the sedimentary rocks types. River Chenab is cutting the Sangaldhan and Parnote site on two major extremes by fault line/ Reverse Fault. On the left side of the Chenab River, the Sangaldhan area possess the Murree Formation i.e., fine to medium Grey, Green sandstone, alternative beds mudstone & clay etc. Above the Murree formation there is presence of Murree Thrust over which Ramban Formation lies and below the Murree formation there is presence of Sirban formation. On the right side of the Chenab River, the Parnote site possess the Subathu Formation (Shale, clay, limestone) above which there is presence of Panjal thrust and Ramban Formation and below the Parnote Formation there is Sirban Formation. In Sangaldhan and Parnote area continues percolation and seepage of water due to the weak/porous rock formation are the main causes of Landslides & subsidence of land. As the Chenab River coming from the West and East sides confluences near the Dhamkund, there is presence of reverse fault which shows the neo-tectonics activity (tectonic mountain building) in the area. In addition to this, about 100 million years ago, there was place called Tethys ocean between the Indian and Eurasian plate and there was third plate at the base of this plate called as Kshiroda plate. As soon as Indian plate broke from Gondwana and collided with Kshiroda plate below Tethys Ocean, the Kshiroda plate began to collapse between Indian and Eurasian plates. Due to this, base of Tethys ocean began to rise above the Kshiroda plate and Kshiroda plate itself began to collapse below Eurasian plate. This explain the new concept of formation of Himalayas due collision of three plate boundaries.
In May 2023, the minister of Earth Sciences said that, Northern India and Nepal experienced 97 earthquakes of Magnitudes 3 to 3.9 from January to November against 41 quakes each in 2022 and 2021 and 42 in 2020. But the earthquake on the western end i.e. Karachi and the eastern end i.e. Myanmar boundary of Himalayas did not experience much earthquakes. As per study conducted by geologist Rachel Headly, University of Wisconsin, shows that Himalayas are rising 4mm per year. Geologists of Stanford University of US, studied the whole Himalayan boundary based on Helium isotopes and found that 3He concentration is more in Ladakh, Uttarakhand and Nepal Himalayas which indicates that crust is thin and mean depth of the elastic layer of Indian plate is about 80-85km beneath the surface in this region. As per seismic waves study done by Geologists of Ocean University of China on the northern edge of Indian tectonic plate, found that there is variation in the seismic pattern of the waves and gave the interpretation that Magma is concentrated between the Crust.
Others factors includes prolonged drawing of groundwater. When groundwater is extracted at a pace that is quicker than it can be supplied naturally, the aquifers below the surface shrink, causing land subsidence. Soil contraction and expansion, referred to as soil subsidence, happen as clay-rich soils alter quantity in accordance with variations in moisture level. Clay minerals may soak up and discharge fluid, enabling elements of soil to widen and shrink as needed. Soils contract as moisture vanishes during periods of drought, resulting in sinking and earth cracks. Soils expand in response to moisture absorption during wet seasons, generating rise and rumbling. Soil Compression- Urbanization, the development of heavy infrastructure such as buildings and roads, and the positioning of large weights on the soil’s surface can all compress soil layers. This compression decreases the size of the pore spaces in the soil, allowing it to compact and sink.
Land subsidence can harm or destroy infrastructure, including buildings, roads, bridges, and pipelines. This can pose serious dangers to public safety and necessitate expensive servicing and repairs. Subsidence has a detrimental influence on agricultural output because it alters soil drainage trends, reduces the productivity of the soil, and increases the likelihood of flooding. This can have a financial effect on farmers and raise questions regarding availability of food in society. Land subsidence can worsen the penetration of saltwater into freshwater aquifers. As the ground falls, the freshwater-saltwater boundary shifts inwards, polluting water for consumption resources and limiting the supply of freshwater for agricultural and domestic usage. The subsidence of land can increase groundwater contamination concerns. As the ground sinks, contaminants can reach aquifers, poisoning drinking water and compromising the safety of people. Subsidence-induced alterations to groundwater circulation routes can potentially aid in the dispersion of pollutants providing over time environmental problems. Subsidence can change the terrain of a region, changing naturally occurring drainage systems and raising the danger of floods, especially in low-lying regions. This can lead to property destruction, fatalities, and community dislocation. Land subsidence can cause the depletion of wetland habitat ecosystems, and areas of high biodiversity. It can also destabilize ecosystems, altering numbers of animals and biological cycles. Land subsidence can be caused by mining activities such as extracting coal, collecting minerals, and quarrying, which remove rock from subterranean or surface reserves. Mining-induced subsidence happens frequently when holes or caverns are generated below ground, causing above rock strata to buckle or subside. This can lead to surface crashes, craters, and ground turbulence, putting communities, structures, and habitats under threat. In some circumstances, unused mines may remain to erode gradually, posing potential hazards to adjacent neighbourhoods and environments.
After effects of Subsidence
Despite the fact that the subsidence has stopped, structural harm to infrastructure and buildings can endure. This may involve continual tracking, servicing, and modification to ensure that impacted structures remain safe and stable. Subsidence can cause ground unsteadiness, making it vulnerable to additional development or demise particularly during tremors such as earthquakes. This adds new hazards to property and safety for people. Land subsidence can have long-term financial ramifications, influencing real estate prices, insurance rates, and city infrastructure upkeep and repair expenditures. In severe circumstances, land subsidence can cause community relocation as livable terrain becomes unusable owing to rising threat of flooding, infrastructural damage, or environmental deterioration. Land subsidence may trigger enforcement actions with the goal of limiting future hazards, such as exploitation of groundwater boundaries, land-use land cover rules, and the establishment of construction norms and standards to improve architectural durability.
In summary, land subsidence is a multifaceted process with numerous origins, and repercussions. Addressing subsidence’s difficulties necessitates a multifaceted strategy that includes scientific study, technical remedies, policy modifications, and community involvement. Identifying the variables that cause subsidence and applying preventive actions allows us to limit its effects and encourage sensible land-use decisions for the betterment of both present and future generations.
(The author is Research Scholar Dept. of Geology University of Jammu)
Follow our WhatsApp channel
