A landslide is defined as the mass movement of rock, debris, or earth down a slope under the influence of gravity. This geological hazard is common in mountainous and hilly regions worldwide, posing a significant threat to life and infrastructure. Determining the exact global rate of occurrence is difficult because many small or remote slides go unrecorded. Scientists rely on advanced technologies and historical documentation to estimate the true frequency of these destructive events.
Measuring Landslide Frequency
Dedicated databases, such as the NASA Global Landslide Catalog (GLC), compile information from media reports, academic articles, and existing inventories to create a historical record of events. These records often show a reporting bias, with more events recorded in English-speaking countries and those with higher economic status, suggesting an underestimation in less developed regions.
Remote sensing technologies provide a broader, more objective view of landslide activity, especially in inaccessible areas. Satellite imagery and Light Detection and Ranging (LiDAR) are used to map terrain features and detect changes in the landscape before and after a slide occurs. On a local scale, scientists deploy in-situ monitoring tools to track subtle ground movements and measure the angle of slope change in real-time, providing precursors to potential failure.
Primary Natural Drivers of Occurrence
The frequency of landslides is governed by the interaction between the inherent susceptibility of the terrain and an external triggering event. Susceptibility is determined by local geography, including the steepness of slopes, the presence of weak or fractured geological material, and soil saturation levels.
The most frequent trigger globally is hydrological factors, particularly intense or prolonged rainfall. Heavy precipitation saturates the ground, increasing the soil’s weight and significantly elevating the pore water pressure within the slope material. This elevated pressure reduces the internal shear strength of the soil and rock, leading to slope failure. Rainfall triggers approximately 79% of non-seismic fatal landslide events.
Seismic activity is the other major natural trigger, often causing fewer but more catastrophic events. Earthquake shaking instantly increases the stress on a slope, leading to immediate liquefaction and failure, especially in saturated soils. While rainfall-triggered landslides are seasonal, seismic events occur randomly and often affect larger, deeper rock masses. A strong earthquake can destabilize slopes that may fail months or years later due to ordinary rainfall.
Human Influence on Occurrence Rates
Human activities frequently act as accelerators, increasing the likelihood and frequency of landslides by altering the natural balance of a slope. Deforestation is a significant modifier, as the removal of trees eliminates the stabilizing effect of their root systems, which anchor the soil and rock masses. This loss of vegetative cover also affects local hydrology, reducing the amount of water absorbed and transpired, which leads to increased soil saturation and weight.
Improper land use planning, particularly in mountainous terrain, contributes heavily to increased occurrence rates. Excavating the toe of a slope to create space for infrastructure like roads or housing removes natural support, immediately destabilizing the hillside. Urban expansion and mining operations can also alter natural drainage patterns, concentrating surface water flow onto vulnerable areas and accelerating erosion. Developing steeper, marginal land means more people are settling in areas already prone to sliding, compounding the overall risk and reported frequency.
Trends in Global Landslide Occurrence
Analyses of global disaster databases indicate a general increasing trend in the number of reported landslide events over the past six decades. This trend reflects both improved detection methods and a genuine rise in physical frequency in certain areas due to two interconnected factors: climate change and human development.
Climate change is driving an increase in the intensity and frequency of extreme weather events, which relates directly to the most common natural trigger. More frequent, high-intensity rainfall and prolonged periods of saturation translate into more hydrological-triggered landslides worldwide. Simultaneously, the development of unstable slopes for housing and infrastructure places more human assets in harm’s way, leading to a greater number of reported events. For instance, a recent study projected that the border region of China and Nepal could see a 30 to 70% increase in landslide activity due to more extreme precipitation events.