FIGURE 5.7 Landslide susceptibility map (D) produced using geographic information systems methods of a part of San Mateo County, California, near the town of La Honda. A landslide inventory map (B) was compared with a geological map (A) and a slope map (C) to determine the percentage of each geological unit that has failed by landsliding in the past, and the slope important for the failure. These data formed the matrix for the susceptibility analysis (D). The higher the roman numeral, the more susceptible the slope is to landsliding in the future. Landslide deposits (L) are shown as a separate category (highest). This map was used by San Mateo County to reduce potential development in landslide-prone areas and to require detailed geological studies to determine the safety of building sites. Figure from Earl Brabb, U.S. Geological Survey.

Human intervention can reduce landslide risk by influencing some contributory causes. Projects that undermine slopes in marginal equilibrium or destabilize susceptible areas by quick drawdown of reservoirs can be avoided. Among projects that can lay the groundwork for disastrous landslides are road building, mining, fluid injection, and building construction that entails clearing vegetation. Planning and designing such projects with the local landslide potential in mind is absolutely essential. While these activities may not individually cause a landslide, they can increase the likelihood of slope failure as preconditions to which cloudbursts or earthquakes are added. Wherever hillsides receive precipitation over days and weeks, the pore-water pressure can build in rock fractures and decrease bulk shear strength, which can then induce displacements under less force than would be needed to shear a drier