A landslide occurs when gravity overcomes slope resistance. The most common triggers are prolonged or intense rainfall saturating the soil and reducing its cohesion. Earthquakes, deforestation, and excavation are also triggers.
In Spain
The most affected regions are the Cantabrian coast, Pyrenees, Betic ranges, and areas with expansive clays. Rainfall thresholds for landslides vary by terrain: in clay, 30–50 mm in a few hours may suffice; sound rock requires exceptional precipitation. Landslides can dam rivers, destroy roads and infrastructure, and cause fatalities.