7.24 Mass-Movement Style, Activity State, and Distribution

The collection and analysis of mass-movement data and the broader dissemination of results requires the adoption of standard methods for describing mass-movement style or type and activity state. Main mass-movement styles, such as falling, toppling, sliding, flowing, spreading, and creeping, are defined herein. Well-known substyles are explained and different activity states of mass movements are described. Activity states may be related to the immediate behavior (prefailure, failure, and reactivation), or to the morphological status (active, suspended, dormant, and inactive). Finally, the global distribution of landslides is discussed, including the prevalence of mass movements along convergent plate boundaries, climatologically wet regions, and mountainous highlands. The global distribution of mass-movement disasters is shown, concentrated in the Asia Pacific, and in South America. 7.24.1 Mass-Movement Style The collection and analysis of mass-movement data and the broader dissemination of results require the adoption of standard methods for describing mass-movement style or type and activity state. Mass movements have been classified in several ways (Shroder et al., 2005), including rheology (i.e., Pierson and Costa, 1987), engineering properties (Hoek and Bray, 1981), shear type and grain size (i.e., Sassa, 1989), and velocity (Nemčok, 1977). The most common type of classification is a combination of mass-movement style and material type. Sharpe (1938) proposed a mass-movement classification system based on four major mass-movement styles: slowflowage types, rapid-flowage types, landslides, and subsidence. Varnes (1978) developed what has become perhaps the most widely used classification system based on five movement styles (falls, topples, slides, spreads, and flows; see Chapters 7.13, 7.14, 7.15, 7.16, and 7.17) (Figure 1) and on material type (rock debris and earth). Varnes’ (1978) classification has been since modified by several authors (i.e., Hutchinson, 1988; EPOCH, 1993; Cruden and Varnes, 1996); however, in general, changes have been limited in nature. One potential drawback of Varnes’ (1978) classification is the lack of distinction for creep as a mass-movement style. Cruden and Varnes (1996), however, suggested that creep can be incorporated into Varnes’ (1978) classification by a description of velocity behavior. Others, Sidle and Ochiai (2006), for instance, consider creep and similar behaviors (such as solifluction) to be definitive styles in their own right. Several distinct mass-movement styles are described herein: falling, sliding, flowing, spreading, and creeping.