Shear Stress
Mass movements are driven by gravity, and while shear strength represents those factors which oppose this force, shear stress represents those factors which help to drive downslope movement. One very important component of shear stress is slope angle. As slope angle increases, the force vector components parallel to the slope, known as the shear force (fs), increase, eventually reaching a point known as the angle of repose, which is the steepest angle at which the slope material is stable. This increase of shear stress with slope angle is why steep slopes are particularly unstable and prone to mass movements.  |
| A graphic demonstrating shear force
(fs) increasing as slope angle increases. Image source: http://opengeology.org/textbook/10-mass-wasting/ |
Another particularly important factor in increasing shear stress is the increase of weight on a slope. This often occurs as a result of rapid rainfall, though this can also occur in other ways, such as the construction of buildings on slopes.