slope movements and delineation of the degree of hazard are now being successfully pursued in many parts of the world and the techniques used can be widely applied at various levels of detail and sophistication. Figure 2, taken from a paper by Oyagi (1978), gives a schematic outline of the policy- making and investigational procedures which should be undertaken in planning to prevent or to mitigate landslide disasters. 19 6.3.2 Information required to assess risk Devastating landslides rarely occur where there have been no previous failures in adjoining or nearby areas, unless, of course, the activities of man have produced a new and dangerous setting. Therefore, the requirements in delineating the hazard are: (i) Knowledge of where past failures have occurred, Such knowledge is generally available from local records. For evaluation of the hazard in broad areas, the interpretation of aerial photo- graphs by an expert in landslide recognition is effective, quick, and not very expensive. This technique should, however, be validated by examination on the ground. (iiJ In order to make an adequate assessment of the stability of areas that have not yet failed, it is necessary to determine how and why the existing failures occurred. A geologist trained in photo-interpretation can often make reasonable inferences regarding the mechanism of failure and, in favorable circumstances, tell something about the geological conditions that contributed to instability. However, actual ground examination is generally necessary a6 well. (iii) For many purposes it is necessary also to determine, if possible, when failure occurred. This condition requires, for historic failure, correlation with all available information concerning possible triggering events, such as earthquakes, heavy rains, erosion, or man- made excavations. The kinds of information required will range in detail from areal surveys to physical borings, sampling, testing of materials, and stability analyses, depending on the appropriate scale of the 
**Analysis**
The text discusses landslide disasters, hazard assessment, and the necessary information to determine the stability of areas prone to landslides. The focus is on geological conditions, past failures, and potential triggering events. The language used is technical and scientific, suggesting that the text is related to a specific field of study or profession.

**Category**
Natural resources and environment, Science and technology