Most of these slides occur on slopes, in response to the natural pull of gravity. Ultimately, gravity and friction determine the extent of a landslide. When g p is greater than that of g t , the particle remains anchored in place. It is only when g t exceeds g p that the particle will move. The shaking that earthquakes produce disables the friction that restrains particle movement thereby separating particles.
Excessive precipitation produces a similar effect to that of earthquakes.
Precipitation decreases the amount of friction between particles and creates pore pressure that can work to push the particle apart. This effect is compounded by the presence of a week ionic bond between clay particles, which is easily broken when wet. In addition the free flow of water forces grain apart. When the flat clay particles separate, they slide easily past each other. Wave action on shorelines can take advantage of any of the destructive methods mentioned for the three other causes of landslides.