Landslides are dangerous natural hazards that occur when soil, rocks, mud, debris, or other earth materials move rapidly or gradually down a slope. They can occur on natural hillsides, mountains, road cuttings, riverbanks, construction sites and areas where slopes have been modified by human activity.
Some landslides develop slowly and provide warning signs, while others can happen suddenly with little or no warning. In severe cases, a landslide can bury homes, block roads, damage infrastructure, destroy vehicles and cause serious injuries or deaths.
Understanding what causes landslides, recognising early warning signs and knowing how to respond can significantly reduce the risk to people and property.
WHAT IS A LANDSLIDE?
A landslide is the downhill movement of rock, soil, earth or debris under the force of gravity.
Landslides can range from relatively slow movements of soil to extremely rapid flows of mud, rocks and debris. Some can move faster than a person can run, making early evacuation particularly important.
Landslides can include several types of movement, such as:
- Rockfalls
- Rockslides
- Mudflows
- Debris flows
- Earth slides
- Slope failures
- Large-scale ground movement
A landslide does not always occur as a single event. A slope may continue moving after the initial failure, meaning that an area affected by one landslide may remain dangerous.
WHY DO LANDSLIDES HAPPEN?
Landslides generally occur when the forces pushing material downhill become greater than the forces holding the slope together. Several natural and human factors can contribute to this process.
1. HEAVY AND PROLONGED RAINFALL
Heavy rainfall is one of the major triggers of landslides.
When large amounts of rain enter the ground, soil can become saturated and heavier. Water can also increase pressure within the soil, reducing the stability of the slope.
During periods of intense or prolonged rainfall, slopes that were previously stable can become unstable and collapse.
This is particularly important in tropical regions where intense rainfall can occur over relatively short periods.
2. STEEP SLOPES
Steep slopes naturally have a higher potential for slope instability because gravity constantly pulls soil and rock downhill.
The risk can become greater when steep slopes contain:
- Loose soil
- Fractured rock
- Weak geological layers
- Poor drainage
- Unstable fill material
- Previous landslide scars
USGS notes that areas containing weak or fractured materials on steep slopes can be particularly susceptible to landslides.
3. EARTHQUAKES
Earthquakes can trigger landslides by violently shaking the ground.
The shaking can weaken slopes and cause rocks, soil and debris to lose their stability.
A slope that is already weakened by rainfall or erosion may be particularly vulnerable when an earthquake occurs.
4. SOIL AND ROCK EROSION
Water from rivers, streams and heavy rainfall can gradually remove material from the base of a slope.
When the supporting material at the bottom of a slope is eroded, the upper portion may become unstable.
Coastal erosion can also contribute to slope instability in some locations.
5. CHANGES IN GROUNDWATER
Changes in groundwater conditions can influence slope stability.
Water entering cracks and spaces within soil and rock can weaken the material and increase pressure within the slope.
Poorly controlled water drainage can therefore become an important factor in slope instability.
6. DEFORESTATION AND LOSS OF VEGETATION
Vegetation can contribute to slope stability because plant roots help hold soil together.
When trees and vegetation are removed from a slope, the soil may become more vulnerable to erosion and movement.
However, vegetation alone cannot guarantee slope stability. The geology, drainage, slope angle and engineering condition of the site are also important.
USGS recommends retaining suitable native vegetation on slopes where appropriate and avoiding activities that unnecessarily weaken slopes.
7. CONSTRUCTION AND HUMAN ACTIVITIES
Human activities can alter the natural stability of slopes.
Potential risk factors include:
- Cutting into hillsides
- Excavating the bottom of slopes
- Poorly designed drainage
- Construction on unstable slopes
- Removing vegetation
- Adding excessive weight near the top of a slope
- Poorly managed earthworks
- Improper disposal of soil or debris
USGS identifies disturbance and changes to slopes caused by human activity as possible landslide triggers.
This does not mean construction automatically causes landslides. Proper geological assessment, engineering design and drainage management can substantially reduce risk.
THE DANGERS OF LANDSLIDES
Landslides can produce both direct and secondary hazards.
1. DEATH AND SERIOUS INJURY
Fast-moving soil, rocks and debris can strike or bury people with little time to escape.
2. HOMES CAN BE DAMAGED OR DESTROYED
A landslide can:
- Damage foundations
- Break walls
- Collapse structures
- Push buildings from their foundations
- Bury houses under soil and debris
USGS reports that landslides can bury homes and damage infrastructure.
3. ROADS CAN BE BLOCKED
Landslides can cover roads with:
- Mud
- Rocks
- Trees
- Soil
- Concrete
- Other debris
This can isolate communities and prevent emergency services from reaching affected areas.
4. BRIDGES AND INFRASTRUCTURE CAN BE DAMAGED
Landslides can damage:
- Bridges
- Railways
- Roads
- Drainage systems
- Water pipelines
- Electricity infrastructure
- Communication networks
This can cause prolonged disruption after the initial event.
5. POWER AND UTILITY HAZARDS
Moving ground can damage electrical poles, water pipes, gas systems and other utilities.
Leaning poles, exposed cables and broken pipelines can create additional hazards.
6. RIVERS AND DRAINS CAN BECOME BLOCKED
A landslide can enter a river or stream and create a temporary natural dam.
Water may accumulate behind the debris. If the blockage suddenly fails, a large amount of water and debris can rush downstream, potentially producing severe flooding.
WARNING SIGNS OF A POSSIBLE LANDSLIDE
Not every landslide provides warning signs, but recognising changes around a slope can help people react earlier.
Important warning signs include:
- New cracks appearing in the ground
- Cracks appearing on roads
- Bulges developing on the ground
- New cracks in walls or foundations
- Doors or windows suddenly becoming difficult to open
- Fences becoming distorted
- Trees beginning to lean
- Utility poles tilting
- Retaining walls moving or bulging
- Water suddenly appearing on a slope
- New springs developing
- Water pooling in unusual places
- Changes in drainage patterns
- Soil or rocks beginning to move downhill
- Increasing amounts of mud or debris in streams
These signs are documented by USGS as indicators of possible slope movement.
Malaysia's National Security Council (MKN) similarly highlights signs such as cracks on slopes, water emerging from slopes, leaning fences, walls, poles and trees, damaged water pipes, and cracks in soil or building foundations.
SOUNDS THAT MAY INDICATE A LANDSLIDE
A developing landslide may sometimes produce unusual sounds.
Pay attention to:
- Cracking or breaking trees
- Falling rocks
- Rumble or roaring sounds
- Ground vibrations
- Unusual rushing water
- The sound of debris moving downhill
USGS notes that rapidly moving landslides can produce a loud rumble and ground vibrations that may resemble a passing freight train.
However, the absence of these sounds does not mean a slope is safe.
CHANGES IN STREAMS CAN BE A WARNING SIGN
People living near hills, rivers and streams should pay attention to sudden changes in water.
For example:
Suddenly muddy water
A stream that suddenly becomes much muddier than normal may indicate that soil and debris have entered the water upstream.
Sudden changes in water level
An unexpected rise or fall in stream level can indicate unusual activity upstream.
A stream suddenly stops flowing
A landslide may have blocked a drainage channel or stream, creating a temporary natural dam. If that blockage fails, water and debris can travel rapidly downstream.
HOW TO STAY SAFE DURING A LANDSLIDE
1. LEAVE THE DANGER AREA IF IT IS SAFE TO DO SO
If you observe clear signs of an active landslide and have a safe escape route, leave the area immediately.
Do not wait to see how large the landslide becomes.
USGS advises people who observe signs of an active landslide to leave the area immediately if it is safe to do so.
2. MOVE AWAY FROM THE PATH OF THE LANDSLIDE
If a landslide is moving, do not remain directly below the slope.
Move away from the direction of movement using a safe route.
Avoid moving toward the landslide simply to observe it.
3. DO NOT TRY TO CROSS A LANDSLIDE AREA
A slope that has already collapsed may still be unstable.
Additional landslides can occur after the first event, sometimes in multiple pulses.
4. IF YOU ARE DRIVING, SLOW DOWN AND STOP IN A SAFE LOCATION
If you encounter rocks, mud or debris on a mountain road:
- Do not drive through the debris.
- Do not stop directly below an unstable slope.
- Avoid road sections showing cracks or deformation.
- Follow official road closure and evacuation instructions.
Road cuts and embankments can be particularly vulnerable during heavy rainfall.
WHAT IF A LANDSLIDE IS ALREADY HITTING YOUR BUILDING?
This situation can be extremely dangerous and the safest action depends on the building and the direction of movement.
If evacuation is unsafe because the landslide is already striking the building, USGS advises occupants to move to a higher part of the building where possible rather than attempting to run directly through an active landslide.
Do not blindly follow a generic escape route if the landslide is already moving around the building.
The priority is to avoid being caught in the moving debris and follow instructions from emergency authorities.
HOW TO PREVENT OR REDUCE LANDSLIDE RISK
Landslides cannot always be prevented, particularly when caused by major natural events. However, the risk can often be reduced through proper planning, drainage, engineering and land management.
1. CONTROL WATER DRAINAGE
Water management is extremely important.
Ensure that:
- Gutters are maintained
- Drainage channels remain clear
- Stormwater does not unnecessarily flow onto unstable slopes
- Leaking pipes are repaired
- Water is directed through appropriate drainage systems
USGS notes that stormwater concentrated onto a slope can rapidly saturate soil and increase the likelihood of landslide movement.
2. PROTECT VEGETATION ON SLOPES
Avoid unnecessary removal of trees and vegetation from unstable slopes.
Appropriate vegetation can contribute to soil stability and reduce erosion.
3. DO NOT ALTER A SLOPE WITHOUT PROPER ASSESSMENT
Major excavation, cutting or filling on a hillside should not be treated as an ordinary landscaping project.
A qualified geotechnical or engineering professional should assess potentially unstable slopes before significant modifications are made.
4. MAINTAIN RETAINING STRUCTURES
Retaining walls and other slope-support systems should be inspected for:
- Cracks
- Bulging
- Tilting
- Water leakage
- Movement
- Structural deterioration
A retaining wall that is moving may indicate a larger slope-stability problem.
5. MONITOR HIGH-RISK AREAS DURING HEAVY RAIN
People living near steep slopes should be especially alert during and after prolonged or intense rainfall.
Do not assume that a slope is safe simply because it has remained stable for many years.
HOW TO PREPARE YOUR FAMILY FOR A LANDSLIDE
Preparation should happen before an emergency.
Create an emergency plan
Your family should know:
- Where to evacuate
- Which roads are safe
- Where to meet if separated
- Who needs additional assistance
- How to contact one another
- Where emergency supplies are stored
Prepare an emergency kit
Useful items include:
- Drinking water
- Non-perishable food
- Flashlight
- Extra batteries
- Power bank
- First-aid supplies
- Essential medication
- Mobile phone
- Important documents
- Basic hygiene supplies
- Whistle
- Necessary items for children and elderly family members
USGS recommends having an emergency preparedness kit and household evacuation plan for people living on or below slopes.
WHAT TO DO AFTER A LANDSLIDE
The danger may continue even after the ground stops moving.
Do not immediately return home
Wait for authorities to determine that the area is safe.
A slope can remain unstable after the first landslide and additional movement can occur.
Stay away from damaged slopes
Do not climb onto or walk across fresh landslide debris.
Avoid damaged utilities
Stay away from:
- Fallen electrical cables
- Broken water pipes
- Damaged gas systems
- Unstable buildings
- Damaged bridges
Report warning signs
Report new cracks, slope movement, blocked drainage, unusual water flow or other signs of instability to the relevant local authorities.
In Malaysia, MKN advises the public to report early landslide warning signs to the authorities and the Public Works Department (JKR) for further action.
LANDSLIDES AND HEAVY RAINFALL
Heavy rainfall deserves particular attention because it can trigger both landslides and flash floods.
During prolonged rain:
Heavy rain → soil becomes saturated → slope becomes weaker → slope fails → soil, rocks and debris move downhill
At the same time:
Heavy rain → rapid runoff → drains/rivers become overwhelmed → flooding occurs
This means communities located below steep slopes may face more than one hazard at the same time.
For this reason, people should take official heavy-rainfall and flood warnings seriously, particularly when living or travelling near steep terrain.
WHAT NOT TO DO DURING A LANDSLIDE
❌ Do not stand near the edge of an unstable slope.
❌ Do not approach an active landslide to take photographs or videos.
❌ Do not walk across fresh landslide debris.
❌ Do not drive through mud or rock debris blocking a road.
❌ Do not ignore new cracks in the ground or buildings.
❌ Do not remove vegetation unnecessarily from unstable slopes.
❌ Do not redirect large amounts of water onto a slope.
❌ Do not return to an evacuated area without official clearance.
❌ Do not assume the first landslide is the only one that will occur.
LANDSLIDE SAFETY CHECKLIST
BEFORE A LANDSLIDE
✔ Know whether your home or workplace is near a steep slope.
✔ Learn the local evacuation routes.
✔ Keep drainage systems maintained.
✔ Watch for cracks and changes in the ground.
✔ Prepare an emergency kit.
✔ Have a family communication plan.
✔ Monitor official weather and emergency information.
DURING A LANDSLIDE
✔ Stay calm.
✔ Move away from the danger area if it is safe.
✔ Follow evacuation instructions.
✔ Avoid unstable slopes and debris.
✔ Stay away from rivers and drainage channels affected by debris.
✔ Do not attempt to cross an active landslide.
AFTER A LANDSLIDE
✔ Stay away from the affected slope.
✔ Expect possible secondary movement.
✔ Avoid damaged buildings and utilities.
✔ Follow official instructions.
✔ Report additional warning signs.
✔ Return home only when authorities say it is safe.
CONCLUSION
Landslides can happen naturally, but their impact can be made worse by heavy rainfall, unstable slopes, erosion, poor drainage and inappropriate changes to hillside terrain.
The most important protection is early recognition and early action.
Watch for ground cracks, bulging soil, leaning trees or poles, unusual water flow, damaged structures, muddy streams and unusual sounds. These signs do not guarantee that a landslide will occur, but they can indicate increasing slope instability and should not be ignored.
If a landslide is occurring, do not approach it, do not attempt to cross the debris and evacuate when it is safe to do so. Remember that additional movement can occur after the first landslide.
KEY SAFETY MESSAGE
IF YOU SEE THE WARNING SIGNS OF A LANDSLIDE, DO NOT WAIT TO FIND OUT WHAT HAPPENS NEXT. MOVE TO A SAFE LOCATION IF YOU CAN DO SO SAFELY, STAY AWAY FROM UNSTABLE SLOPES AND FOLLOW OFFICIAL EMERGENCY INSTRUCTIONS.
Sources: U.S. Geological Survey (USGS), Malaysia National Security Council (MKN), and Public Works Department (JKR) guidance referenced by MKN.
