Landslide causes and triggers

Hillslope (in)stability is governed by the balance of stability factors. If stability is lost, gradually or instantly, slope failure ensues. Assessing the causes of instability is useful for hazard analysis and mitigation, and for considering the role of landslides in landscape systems and evolution. Geological and geomorphological conditions (e.g. material type, strength and structure and hillslope geometry) predispose slopes to failure; knowledge of these conditions can help to predict the location, types and volumes of potential failures. The timing of failure, often by a specific trigger, can be anticipated by detecting and assessing movement patterns, establishing triggering thresholds or using probabilistic methods. However, predicting timing remains challenging due to the difficulty of measuring material strength degradation which can lead to failure with no readily observable trigger. This chapter describes concepts of stability and explores some of the major causes and triggers of hillslope failure and opportunities for further research.