Factor of safety, slope stability, rainfall and seismic triggering
The Factor of Safety (FS) is the ratio of resisting to driving forces on a potential failure surface. For an infinite slope, FS = (c' + (γz cos²β − u) tan φ') / (γz sin β cos β), where c' is cohesion, φ' is friction angle, γ is unit weight, z is depth, β is slope angle, and u is pore water pressure. FS > 1 means stable; FS < 1 means failure. Rainfall raises pore pressure (reducing effective stress), while earthquakes add pseudo-static horizontal forces — both can push FS below 1. The 1970 Huascarán rockslide (Peru) reached 280 km/h; seismic triggering is estimated to mobilize ~250 km³ of material globally per century.