Rainfall is the single most common natural trigger for landslides, responsible for roughly 61% of all recorded events worldwide. But it’s far from the only one. Earthquakes, volcanic eruptions, wildfires, rapid snowmelt, and coastal wave action can all destabilize slopes and send earth moving downhill.
Rainfall and Soil Saturation
Water is the dominant force behind landslides globally. Across three major global landslide databases, rainfall triggered between 51% and 92% of all recorded events, depending on the dataset. The mechanism is straightforward: water seeps into soil and fills the tiny spaces between particles, increasing weight while reducing the friction that holds a slope together. Once enough water accumulates, gravity wins.
Not all rain is equally dangerous. Short, intense bursts of rainfall (lasting under 60 minutes) are actually better predictors of debris flows than longer, gentler storms, particularly on slopes that have recently burned. The reason is that intense rain overwhelms the soil’s ability to absorb water, generating surface runoff that erodes loose material and channels it downhill. Prolonged rain over days or weeks matters too, though, because it gradually raises the water table and saturates deeper layers of soil that may anchor an entire hillside.
Soil type plays a major role. Clay-rich soils hold water and swell, creating slippery surfaces between layers. Sandy or gravelly soils drain faster but can liquefy under sudden saturation. Slopes that have been stable for decades can fail within hours during an unusually heavy storm if the soil reaches a critical saturation point.
Earthquakes
Seismic shaking is the second most recognized landslide trigger, though it accounts for a much smaller share of global events (around 1% by some estimates). That low percentage is misleading, because earthquake-triggered landslides tend to be catastrophic, affecting enormous areas all at once.
The relationship between earthquake size and landslide reach is dramatic. A magnitude 4.0 earthquake is roughly the minimum needed to trigger any landslides at all. At magnitude 9.2, the zone of potential landslides expands to approximately 500,000 square kilometers. The weakest shaking can still dislodge the most vulnerable materials: rockfalls, rock slides, soil falls, and disrupted soil slides are the first types of landslides to occur because they involve already-loose or fractured material sitting on steep slopes.
Distance from the earthquake’s epicenter matters enormously. Closer slopes experience stronger shaking and are far more likely to fail. But in very large earthquakes, landslides can occur hundreds of kilometers from the source, especially in mountainous terrain with steep, weathered slopes.
Volcanic Eruptions
Volcanoes are, in a sense, custom-built for landslides. They combine steep terrain, loose rocky rubble, and ready sources of water from rain, snow, and glacial ice. When an eruption occurs, several things can go wrong at once.
The heat from eruptions can rapidly melt snow and ice on a volcano’s flanks, generating massive flows of rock, mud, and water called lahars. These flows travel rapidly downhill and downstream under gravity, picking up debris as they go and growing in volume. Lahars can also form when volcanic crater lakes break through their walls, or when heavy rains erode fresh layers of loose volcanic ash.
Sometimes the volcano itself collapses. About 500 years ago, weakened rock on Mount Rainier gave way and produced a large lahar that traveled dozens of miles into surrounding valleys. The USGS considers lahars the most threatening volcanic hazard in the Cascade Range precisely because they can occur with little warning, travel far from the volcano, and bury everything in their path.
Wildfires
Wildfires don’t cause landslides directly, but they reshape the landscape in ways that make slopes extremely vulnerable to the next rainstorm. When fire burns through a forest, it heats organic matter in the topsoil and creates a waxy, water-repellent layer just below the surface. This coating prevents rainwater from soaking into the ground the way it normally would.
Instead of absorbing into soil, rainfall sheets across the surface, picking up loose ash, dirt, and debris. The result is often a debris flow: a fast-moving slurry of mud, rocks, and burned material that funnels into channels and valleys. Burned slopes can remain water-repellent for months or even years after a fire, meaning the landslide risk persists long after the flames are out. Post-fire debris flows are particularly dangerous because they can be triggered by relatively modest rainstorms that would be harmless on unburned terrain, sometimes from bursts of rain lasting less than an hour.
Rapid Snowmelt
In colder climates, the spring thaw is a well-known landslide season. Heavy winter snowfall stores enormous quantities of water on hillsides. When temperatures rise in spring, that water is released into the soil over a period of days to weeks, saturating the ground much like prolonged rainfall would.
Monitoring of a loess (fine, wind-deposited soil) landslide in one study illustrated the process in detail. Following a winter with 376.7 millimeters of cumulative snowfall, temperatures began climbing in mid-February. Soil moisture surged as surface snow melted, and by early March, when temperatures crossed above freezing, a second rapid spike in soil moisture triggered measurable slope movement. The entire process from the start of snowmelt to visible ground displacement took roughly two weeks.
Freeze-thaw cycles make things worse. When temperatures swing above and below freezing each day, water in the soil repeatedly freezes and expands, then thaws and contracts. This breaks apart the soil’s internal structure, weakening it from the inside. Combined with the added weight of meltwater, these cycles can push a slope past its breaking point. The final stage involves a rapid increase in pore water pressure and saturation, adding weight to the slope while simultaneously reducing the strength holding it in place.
Coastal Wave Erosion
Along coastlines, ocean waves act as a slow but relentless landslide trigger. Waves crashing against cliffs erode the base of the slope, carving an inward notch over time. Once that notch grows deep enough, the unsupported rock or soil above it collapses under its own weight.
A second mechanism involves air compression. In rocky coastlines, waves force air into existing cracks in the rock. As the tide rises and waves continue striking, that trapped air is rapidly compressed and decompressed. Over time, this cycling pressure widens cracks and breaks off pieces of rock, gradually undermining the cliff face. Storms with large waves accelerate both processes significantly, and a single powerful storm can trigger a collapse that normal wave action might have taken years to produce.
How These Triggers Combine
In practice, landslides rarely result from a single cause acting alone. A hillside weakened by an earthquake may not fail until a heavy rainstorm arrives weeks later. A slope burned by wildfire becomes vulnerable specifically because the next rainfall behaves differently on the altered soil. Snowmelt saturates the ground, and then a spring rainstorm provides the final push.
Climate trends are compounding these interactions. Regions at the intersection of different climate zones are seeing increasingly intense rainfall patterns, and research from Central Europe has documented a corresponding rise in shallow landslides over the past decade. More extreme precipitation events mean that slopes which were stable under historical weather patterns are now being pushed past their thresholds more frequently.