Reading the slope: five landslide warning signs

A landslide is rock, earth and debris moving down a slope. Some slides happen with almost no warning, which is why the warning signs matter. The United States Geological Survey (USGS) lists the changes to watch for, and most can be spotted from a safe distance.

First, new cracks: fresh cracks in the ground, driveways, pavements, walls, ceilings or foundations. A crack that was not there last week, or one that is widening, deserves attention, as does soil pulling away from a foundation.

Second, leaning: trees, fences, retaining walls and utility poles that tilt or move on a hillside can show the ground beneath them is shifting.

Third, changes inside the house: doors or windows that stick or jam for the first time can mean the frame has shifted. So can new cracks in plaster, tile or brick. Report anything unusual to your local authorities so the area can be assessed.

Fourth, water where it should not be: springs, seeps or saturated ground in normally dry places, water ponding on a slope, or a broken or leaking utility line. Water adds weight and reduces the strength holding soil and rock in place.

Fifth, sound and stream behaviour: a faint rumbling that grows louder as a slide nears, trees cracking, or boulders knocking together. A stream that suddenly rises, falls or turns muddy during or after heavy rain can mean debris has entered the channel, or a natural dam has formed. Move away from the watercourse and to higher ground.

Why October keeps reminding us

In Colombia, a landslide at a gold mine in Santa Bárbara de Iscuandé, Nariño department, killed at least 14 people and injured seven others, according to regional officials reported by Reuters and AFP. Mines, quarries and other human-modified slopes carry their own risks, because digging changes the shape and support of the ground.

In Kenya, a quarry collapse at Mutonga, in Igoji, Meru County, on 21 October killed at least one worker, with several others feared trapped and rescue efforts ongoing, according to People Daily and Turkana Daily News. Reports have differed on the exact number trapped, so this guide does not quote a trapped figure.

Earthquakes are another trigger. The 9 October magnitude 7.7 earthquake on the Azuero Peninsula in Panama triggered hundreds of shallow landslides, according to satellite analysis reported on 21 October. After the 22 October magnitude 7.0 earthquake in Tibet, rockfalls blocked roads, according to reports citing Chinese state media and the BBC. After any strong earthquake in hilly country, treat slopes as unstable for days.

Then there is the case that breaks the usual rule. The flood on the Budhi Gandaki river in Nepal on 11 October was first blamed on a landslide, but Nepal's disaster authority later found it was triggered by a rock-and-ice avalanche at about 6,000 metres, with permafrost thaw making such collapses more frequent, according to reporting on 14 October. Five people were reported missing. High-mountain slopes can therefore fail without any rain at all.

What to do in heavy rain on or near a slope

Heavy or prolonged rain is the most common landslide trigger in most places. Stay alert to the signs above, know your high ground in advance, and follow any warning or evacuation order promptly. Do not wait to see whether the slope moves first.

If you are outside and a slope is moving, move sideways, away from the path of the slide, and get to higher ground fast. Do not try to outrun debris moving downhill, and do not cross a debris flow or a flooded channel, which can carry boulders and trees far faster than a person on foot.

If you are driving, watch for collapsed pavement, mud and fallen rock, especially beside road cuts and embankments. Never drive through floodwater or a debris flow; turn around and find another route.

If you cannot get out of a building in time, USGS advice is to move to an upper floor if there is one, towards the downhill side of the house and away from windows on the uphill side. In a single-storey house, climb onto a sturdy table or counter and hold on. Cover your head and stay put until all movement has stopped.

The no-rain rule and the quake rule

The no-rain rule is that a slope can fail without rain. Thawing permafrost, a rock-and-ice collapse high in the mountains, a leaking water line, or excavation at the base of a slope can all destabilise ground on a dry day. That is why a fresh crack or a leaning tree deserves attention in any season.

The quake rule is that shaking changes slopes. A strong earthquake can crack ground, loosen rock and redirect water, leaving slopes ready to fail in the next storm, or on their own. After a quake, avoid travelling beneath steep slopes where you can, expect closures, and turn back if the road looks damaged.

After a slide: stay away, and report it

The danger does not end when the first slide stops. Slopes often fail again, and material above or beside the scar can still be loose. Stay away from the slide area, do not walk across fresh debris or climb the scar to look, and do not enter a damaged building to recover belongings.

Report the slide to your local authorities, with the location and anything you observed, so professionals can assess further movement. Watch for after-signs too: cracks at the top of a scar, new springs of water, and material tumbling down the slope.

Living on a hillside: choices that lower the risk

No guide can replace a professional assessment of a property. But several choices lower risk, and they are worth discussing with local authorities or a qualified professional before you build, buy or alter a property on a slope.

  • Route water away from slopes: drain runoff, downspouts and driveways well away from unstable slopes, and repair leaking lines promptly.
  • Keep ground cover on slopes, since vegetation helps hold surface soil.
  • Do not disturb a slope without advice: avoid excavating at its base or adding heavy fill at the top.
  • Ask about hazard maps: places that have slid before are more likely to slide again.
  • Plan the escape in advance: know the high ground, safe routes and local warning channels.
  • Use retaining walls and drainage works only with professional design.

This guide does not claim landslides are predictable. Recognise the signs, respect warnings, keep your distance, and give professionals the information they need.

What our records show, and what they miss

Our incident records show the same pattern across very different places: slopes fail in heavy rain, in earthquakes, in mines and quarries, and sometimes without any obvious trigger. They also show that the most dangerous slides are often the fast ones, with little time to react, which is why advance knowledge of high ground and escape routes matters. What the records miss matters too: slow movement, small cracks and leaning trees are rarely reported, so a quiet slope can look safe in the data while it is already changing. The absence of a record is never proof that a slope is stable.

The short version

  • Learn the five signs: new cracks, leaning trees and poles, sticking doors and windows, water where it should not be, and rumbling or sudden stream changes.
  • In heavy rain near a slope, know your high ground, follow official warnings, and never cross a debris flow or drive through floodwater.
  • After a strong earthquake in hilly country, treat slopes as unstable for days and expect road closures and further slides.
  • If a slide is moving, get sideways and uphill fast; if you cannot get out, go upstairs, away from the slope side, and protect your head.
  • After a slide, stay away, watch for further movement, and report it to local authorities for professional assessment.

Sources: USGS landslide guidance; Reuters and AFP on Colombia; People Daily and Turkana Daily News on Meru County; satellite reporting on Panama; Nepal's disaster authority, reported 14 October; and BBC and Xinhua on Tibet. Where sources disagreed, the lower verified figure is used.