What is a Landslide: Types, Causes, and Consequences

How does a landslide differ from a mudflow, and where does this natural phenomenon occur most often?

What is a Landslide

During a landslide, the earth “slides” downhill, taking trees and houses — everything on the surface — with it.

A landslide is a natural phenomenon in which a mass of soil and rock shifts down a slope under the influence of gravity. The process can occur slowly, almost imperceptibly, or rapidly, causing destruction. In emergencies, a landslide’s speed can reach 3–5 m/s, and the volume of displaced material can amount to hundreds of thousands of cubic meters. In simple terms, a landslide is when the earth “slides” downhill, taking trees and houses — in short, everything on the surface — with it.

Landslides can be caused by natural factors or human activity.

How a Landslide Differs from a Mudflow

A mudflow is a rapid stream of water mixed with stones, sand, clay, and tree debris that forms in the channels of mountain rivers or ravines after heavy rains or snowmelt.

Here is how it differs from a landslide.

• Speed of Movement. A landslide moves slowly or moderately fast. A mudflow is always a rapid torrent.

• Composition and Formation. A landslide consists mainly of soil and earth, occurring on slopes due to the loss of soil stability. A mudflow forms in river channels after heavy rainfall or melting ice and snow. Here, water is the primary component, with stones and mud merely supplementing it.

• Consequences. A landslide destroys objects by shifting them together with the ground, while a mudflow floods areas, creating blockages.

Classification of Landslides

Landslides can be classified according to various criteria.

• By Material Type: soil, rock, and sedimentary.

• By Speed of Movement: slow (millimeters per year), moderate (centimeters per day), and fast (meters per second).

• By Cause of Formation: due to excess water in the soil, earthquakes, or human activity.

• By Type of Movement: when soil slides, collapses from steep slopes, or flows like a liquid mass.

Landslide-Prone Regions

A landslide is a phenomenon that occurs in many corners of the world. However, we can highlight areas where landslides happen most frequently.

Worldwide

• The Himalayas (India and Nepal). The main causes of landslides in this region are: deforestation for agriculture and construction, frequent earthquakes in a seismically active zone, the instability of slope rocks in the Himalayan mountain system, and heavy rainfall during the monsoon season, which lasts from late May to September. Every year, Nepal loses hundreds of hectares of agricultural land due to landslides. In 2013, monsoon rains caused a series of floods and landslides in northern Indian states. The disaster claimed the lives of about a thousand people, destroyed over 600 residential buildings, 150 bridges, and the Kashang hydroelectric power plant in Himachal Pradesh.

• California (USA). Intense rains in spring and winter, the geological activity of the San Andreas tectonic fault, and large-scale frequent fires that destroy vegetation increase the risk of landslides in California. In 2018, heavy downpours in the Montecito area triggered a series of devastating landslides. They destroyed over 100 homes and damaged the utilities of nearly 200 more buildings, including administrative ones. Nineteen people died, 24 were seriously injured, and another 160 sought medical and psychological assistance.

• Indonesia and the Philippines. The humidity of the tropical climate, seasonal downpours and frequent typhoons, high seismic activity, and volcanic eruptions place the Philippines in a high-risk group for landslides. In 2021, landslides and floods in East Java claimed the lives of over 40 people. In the Philippines, during the winter of 2023, landslides and floods resulted in 38 deaths, damaged 1,200 houses, and destroyed 548. Over 900,000 people were evacuated from particularly dangerous settlements.

Causes of Landslides

Landslides occur when the force of gravity exceeds the cohesive strength of the soil that holds it on a slope. Let’s discuss the main causes.

• Over-saturation: heavy rains, snowmelt, or water leaks from pipelines reduce soil stability.

• Erosion: the destruction of a slope’s base by rivers, wind, or sea waves makes the slope unstable.

• Seismic Activity: earthquakes cause sharp vibrations that destroy the bonds between soil particles.

• Human Activity: road construction, deforestation, and mining disrupt the natural structure of slopes.

Consequences of Landslides

Landslides are one of the most destructive natural phenomena. Here is what they can lead to.

• destruction of infrastructure;

• floods;

• loss of agricultural land;

• loss of human life and injuries;

• destruction of ecosystems;

• worsening of the epidemiological situation.

Long-term consequences are also possible. For instance, new hills, pits, or blockages change the geography of an area. And this can increase the risk of repeated slope destruction and the occurrence of new landslides.

What to Do During a Landslide

In areas of increased landslide risk, the population is informed in advance about possible evacuation methods and procedures in case of an emergency. The notification is sent through local media and special alarm signals.

If the landslide is moving slowly, it is important to determine a safe place for possible evacuation in advance. Evacuation is necessary when the landslide moves rapidly — from 0.5 m per day.

Preparation and Actions During a Landslide

1. Stay calm and strictly follow the instructions of the authorities and the State Emergency Service (SES). Warn neighbors, and help children, the elderly, and people with disabilities evacuate.

2. Place documents, a radio, a hiking shovel, vital medications, and essential items in a “go-bag”. Create stocks of drinking water and long-life food products.

3. Prepare warm clothing, and move valuable and fragile items to the safest possible location, if one exists.

4. Disconnect electricity, gas, and central heating. Secure windows and doors with additional shields.

5. In case of evacuation, upon returning home, inspect the building and surrounding area for casualties. And be sure to check the condition of engineering structures (roof, walls, floors) and utilities. The consequence of a landslide can be damage to both external and internal walls, systems, and devices of electricity, gas, and water supply.

Measures to Combat Landslides

To reduce the risks of landslides and their consequences in potentially hazardous zones, a whole range of measures is taken. The main ones include the following:

• slope stabilization;

• restriction of deforestation, construction, and mining in hazardous areas;

• construction of protective dams;

• monitoring of moisture levels, instability, and erosion of soil;

• closure of hazardous zones, especially during and after periods of possible earthquakes and heavy rains;

• installation of early warning and public alert systems;

• educating the public on actions to take during a landslide.