Recognising Landslide Warning Signs

Landslides kill thousands of people each year worldwide and cause billions in damage. Unlike some natural hazards, landslides often give warning signs — if you know what to look for. Recognising these signs early and knowing when to leave can mean the difference between life and death. This knowledge is especially critical for people living in hilly or mountainous terrain, particularly after heavy rainfall, wildfires, or earthquakes.

What Causes Landslides

A landslide occurs when the forces driving slope movement — primarily gravity and the weight of water-saturated soil — exceed the forces keeping the slope stable. Common triggers include:

• Prolonged or intense rainfall — the most common trigger globally
• Rapid snowmelt — can saturate soils suddenly
• Wildfires — destroy root systems that hold soil; scorched slopes can fail within months of a fire
• Earthquakes — even small earthquakes can destabilise already-saturated slopes
• Erosion at the slope base — from rivers undercutting, wave action, or construction
• Human activity — improper grading, leaking water pipes, removal of vegetation, overloaded terraces

Understanding the trigger conditions in your area helps you know when to be most alert.

Physical Warning Signs on Your Property

Ground Movement

1. New cracks in the ground — especially those running across a slope rather than along it. Cracks perpendicular to the slope direction are a critical warning.
2. Bulging or swelling at the base of a slope — the slope material is being pushed forward.
3. Ground that is subsiding or sinking unevenly — particularly in areas that were previously flat.
4. Tilting or leaning trees, fences, telephone poles, or retaining walls — indicates downslope movement has begun.
5. Distorted or bowed retaining walls — these are designed to hold soil; if they are deforming, the load behind them has increased significantly.

⚠️ Diagonal cracks in a concrete or masonry wall that were not there before are a serious warning sign. These indicate differential ground movement — part of your foundation is moving relative to another part.

Structural Signs in Buildings

Sign	What It Indicates
Doors and windows that suddenly stick or no longer close flush	Foundation movement
New cracks in walls, ceilings, or floor tiles	Differential settlement
Gaps appearing between walls and ceilings/floors	Structure pulling apart
Floors tilting or feeling uneven	Foundation subsidence
Basement walls cracking or bowing inward	Lateral soil pressure increasing

Water-Related Signs

• Springs or seeps appearing where they didn't exist before — indicates water is saturating the slope
• Streams and creeks running muddy after rain — may indicate upslope erosion and instability
• Water bubbling up from the ground during rainfall
• Increased turbidity in well water after rain — groundwater patterns may be changing

Environmental Warning Signs

Sounds

Listen carefully during and after heavy rain near steep terrain:

1. Rumbling or cracking sounds from a slope — can indicate trees snapping as a mass begins to move
2. Unusually loud water sounds — a debris-filled flow sounds different from a clear stream; it may sound like rocks tumbling
3. Creaking or groaning from trees or soil — slope material under tension
4. Sudden silence — sometimes wildlife goes quiet before ground movement

Visual Environmental Changes

• Trees leaning downslope in an area that was previously upright — indicates progressive creep
• Changes in drainage patterns — water flowing in new directions
• Bare scars visible on hillsides where vegetation has been stripped, possibly by earlier, smaller slides
• Hummocky terrain — uneven, lumpy ground on what should be a smooth slope indicates past landslide activity. This terrain is prone to reactivation.

Warning Signs Specific to Debris Flows

Debris flows are rapidly moving, water-saturated landslides. They are especially dangerous because they can travel faster than a person can run.

Look for:

• Heavy rainfall of any intensity on a recently burned hillside — debris flows can initiate within minutes of rain beginning on fire-scarred slopes
• Sudden change in the colour or character of stream water — clear to muddy to extremely thick mud indicates debris mobilisation upstream
• Floating debris in a normally clear stream
• The smell of fresh soil or disturbed vegetation from a slope above you
• Water level in a stream rising rapidly even though rainfall at your location seems moderate (debris flows can dam and then release)

⚠️ If you hear a debris flow approaching — a sound often described as a freight train or thunder coming from uphill — do not wait. Move to high ground immediately, perpendicular to the flow path. A debris flow can arrive in seconds with no further warning.

When Are You Most at Risk?

Condition	Risk Level	Action
Dry season, stable slopes	Low	Normal awareness
Heavy rainfall >10mm/hr	Elevated	Monitor for signs
Prolonged rain (>100mm over days)	High	Prepare to evacuate; watch signs
Rain after wildfire on burned slope	Very High	Pre-emptively evacuate if ordered
Earthquake + saturated soil	Very High	Immediate evacuation from slopes
Steepness >30° with recent disturbance	High	Consult geologist; know exit routes

What to Do When You See Warning Signs

Immediate Steps

1. Do not go near the slope to investigate. A slope in motion can accelerate suddenly.
2. Alert all household members. Establish a verbal code word or alarm for "landslide alert — leave now."
3. Call emergency services and report the signs you're observing — this helps authorities activate warnings for the broader community.
4. Evacuate uphill or perpendicular to the slope. Move to the highest ground possible, away from the flow direction.
5. Do not shelter in your vehicle if it is in a likely path — a debris flow can engulf and carry vehicles.
6. Alert neighbours — landslides often affect multiple properties simultaneously.

Leave Even Without an Official Warning

You do not need to wait for an official evacuation order if you observe multiple warning signs during or after heavy rain or an earthquake. Local authorities may not have real-time visibility of slope conditions on your property. Your direct observation is valid grounds to leave.

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Quick Reference

Warning Sign	Urgency
Ground cracks across slope, doors sticking	Prepare to evacuate — call emergency services
Bulging at base of slope, tilting trees	Leave now
Rumbling or freight-train sound from slope	Move to high ground immediately
Stream suddenly muddy and rising fast	Move to high ground immediately
Rain on recently burned hillside	Pre-emptive evacuation recommended
Springs appearing, new seeps	Notify authorities; increase alertness
Hummocky ground near your home	Know your evacuation routes; consult geologist