New Delhi: Thirty people have died in a landslide in Pune district on Wednesday. As many as 150 people buried in the debris are feared dead by the disaster management authority. The landslide was triggered by intense rainfall. Here are a few FAQs about landslides in India.

What are landslides?

Landslide can describe a range of processes that result in the downward and outward movement of slope forming materials, including rock, soil, artificial fill or a combination of these. The material may move by falling, toppling, spreading or flowing.

When do landslides take place?

In India, increases in pre-monsoon (April and May) rain and monsoon rainfall could lead to an increase in landslides in the Himalayas and Western Ghats. If there is increased pre-monsoon rain in April and May, it can cause soil moisture to build sooner and elongate the landslide-prone risk period. In southern India, landslide incidence is greatest during retreating monsoon, between October and December.

How many people have died due to landslides this year?

The government has not collected records of deaths caused by landslides this year.

What causes landslides?

Landslides can be caused by human and natural factors. Natural factors can include lithology, structure, slope, land cover, seismicity and rainfall precipitation, while human factors can include deforestation, improper land use and unplanned activities. For the construction of 1km road in the Himalayas, 30,000 to 40,000 cubic metre of soil or rock material is excavated. Strong earthquakes also trigger landslides, particularly in regions marked by critically disposed and unstable slopes.

How is rainfall related to landslides?

Most of the landslides are triggered by rain. Intense rainfall, along with many other factors can cause slope saturation, which is a primary cause of landslides. High amount of rainfall and long duration are responsible for landslides, and are also related to debris flow. Landslides produce 550 cubic metre of debris per year.

Is the frequency of landslides increasing?

Experts believe that the frequency of landslides is indeed increasing. The main reasons for this are primarily unplanned development in landslide prone areas which can change the geomorphology of that area.

Because of accelerated deforestation, rampant urbanization, high frequency of earthquakes, fragile geological structures, steep topography and intense rainfall in the mountainous regions of the South Asia, the number of fatal landslides, casualties and economic loss is increasing year by year, according to the Saarc Disaster Management Centre.

Increasing extreme events due to climate change are also responsible for this phenomenon. Rainfall is becoming extreme in India, either heavy or scanty. If there is a lot of rainfall in a short span of time, then the soil is not able to absorb the moisture, which makes it vulnerable to erosion and slope instability, eventually leading to landslides.

Which areas in India are most vulnerable to landslides?

The Geological Survey of India estimates that 15% of India’s landmass or 0.49 million sq. km area is prone to landslide hazard. While 20 states of India are affected by different degrees of landslides, Sikkim and Mizoram are in the severe hazard category.

The Himalayas and the north-eastern region are highly vulnerable to slope instability due to the immature and rugged topography, fragile rock conditions, vulnerability to earthquakes resulting from proximity to the plate margins, and high rainfall. Human developmental activities also contribute to increasing the hazard of landslide in the Himalayan and north-eastern regions.