1. Introduction

As the failure mechanism of sloping earth materials driven by gravity, landslides are one of the most widespread and effective agents in shaping the Earth’s surface (Eckel 1958, p. 1), through redistributing earth material from uplands into river systems. They occur in all mountainous regions of the Earth, in response to a wide variety of natural conditions and triggering processes (see for example Figs. 1.1 and 1.2). These triggers include windstorms and rainstorms, earthquakes, and the numerous consequences of human activity, such as the rapid increase in size of areas that have been rendered vulnerable to large-scale soil erosion. Landslides in the USA, particularly in southern California, are severe hazards that cause substantial human and economic losses, estimated at an annual average of 25–50 deaths, and a financial cost of approximately $1 billion to $3 billion per year (NRC 2008). In addition, globally, they cause both significant environmental damage and societal disruption (Dai, personal communication 2012), and affect developing countries disproportionately.