Himalayas,one of the most activegeodynamic domains of the world,is highly vulnerable to landslides and associated hazards(Larsen and Montgomery 2012; Kahlonet al.
2014). The study of landslides, its triggering mechanism and associated phenomena is an emerging interdisciplinary field of study in the Himalayanterrain, and a relatively limited study has been accomplished so far on this aspect. Landslides such as rock-fall and rockslides are most commonnatural hazardsin the Kullu valley of the Himachal Himalayas, causing major damages to economy (Varnes 1978; Brabb and Harrod 1989;Sah and Mazari 1998; Gardner2002; Kahlon et al. 2014; Banshtuand Versain 2015). They are of great concern to the public administrators as well as geoscientists.Landslides caused due to slope failure are dominantlycontrolled by interaction of multiple factors of endogenic (i.e.
geodynamics & tectonics) and exogeic (e.g. climaticmicro-parameters and site-specific external and internalslope geometry) geomorphic processes. The significant factors of slope failure are geo-mechanical attributes such as gradient of the slope, nature of the overburden, thelithological and structural attributes of rock stratum exposed or covered under regolith, and the ground water conditions (Dramisand Sorriso-Valvo 1994; Agliardi et al. 2001; Agliardi et al. 2012). In geo-dynamically active domainof the Himalayas, tectonicforcing has resultedin a high reliefterrain, rendering it vulnerable to slope failure (Summerfield and Nulton1994; Hurtrez et al. 1999; Montgomery et al.
2001).Hence, the tectonicprocesses are recognized as the main causative factors for the triggering of gravitational slope failurescoupled with landslide activities (Crudenand Varnes 1996; Gonzales-Diez et al. 1999; Galadini 2006; Larsen and Montgomery 2012). Long-term denudation and regional rock uplift along thrust planeslead to topographic up-heaval and the consequent formation of steep slopes. Tectonically inducedrock-deformation creates fracturing and shearingof the bedrock, rendering the terrain prone to slope failure(Hutchinson 1995; Cruden and Varnes 1996; Hermannset al.
2001; Kellogg2001). The bedding planes, joints and fault planes may also act as favorable sliding surfaces dependingupon their geometric characteristics (Bommerand Rodriguez 2002;Tibaldi et al. 1995; Crosta 1996; Galadini2006). Geometric relationships between topography (external geometry) and geologicstructure/ discontinuities, geo-mechanical properties of rock units (internal slope geometry), along with groundwater conditions, are the primarycontrolling factorsfor the slope failures(Freeze and Cherry 1979; Selby 1993). However,deformation(s) of the criticalslopes may primarilyget triggered by sudden changes in the pore pressure, natural geodynamics processes (ground movementand earthquakes), wind drivenvibrations and tree jacking,lightning (thunder) and some anthropogenic events (machine/vehicular vibrations).
Physiographically, the Kullu valley constitutes a part of the Beas river catchment area. The river originates from the Pir-Panjal range, near Rohtang crest (4038m) and flows transversally through two parallel ranges of Pir -Panjal andDhauladhar (Fig.-1). Kullu valleyhosts the sub-basins of tributaries like River Parvati, Hurla and Sainj. The upper reachesand high altitude areas are covered with periglacial and glacialdeposits. The terraces, fans and hill slopes haveprovided an ideal geo-environment for human activities including agriculture, dense settlements and other civil establishments (Sah and Mazari 2007).
The valley is known for (i) a vibrant cultural heritage attractinginternational tourism, (ii) major construction activities linked with a series of hydroelectric projects (viz. Parvativalley and Sainj valley Hydel Projects)and (iii) being a corridor of strategic importance to upper reaches of Himalayas. In September 1995, adisastrous landslide occurredat Luggar Bhatti near Kullu town in the valleywhich killed sixty-five people (Gardner 2002).Such hazards have been accelerated in the recent past (Sah and Mazari 1998).
In the last two decades, no significant study has been carried out on Kullu valley to look into the correlation between the litho-tectonic characters of the region and the geo-hazard events. In the bed rock river domains such as the Kullu valley, the topographic reliefmaintains the thresholdangle (maximum slope) by valley incision(Schmidt and Montgomery, 1995; Burbanket al., 1996; Hoek and Brown, 1980). However,the rate of erosiondue to active tectonicuplift increases nonlinearly with respectto the slope of topography until a thresholdslope angle is reached (Schmidt and Montgomery, 1995;Burbank et al, 1996).Until the topographic slope reachesthe threshold, the slope angles and erosion rates increaseaccording to the rate of regional uplift. Further,the threshold slope angle in responseto the tectonics-driven incision ismainly achieved by slope failuresof the over-steepened river banks (Burbanket al. 1996; Larsenand Montgomery 2012).Moreover the active rate of landslide also generatemore undulating surface.
Hence, the quantitative analysis and interpretation of terrain roughness has become increasingly important in characterizing the terrain unevenness dominated by old-landslides (Frankeland Dolan, 2007; Boothet al. 2009). The presentstudy is directed towards the analysis of landslide phenomena in the Kullu-Bhuntar- Manikaran region with a specialemphasis on their relationship with the morpho-tectonic environment of the terrain.This study callsfor a detailed investigation on the dynamics of slope instabilities leading to high vulnerability and risk of the Kullu valley in terms of landslide hazards. An attempthas been made to delineatethe relationship of topographic slope with existing deformational structures and spatio-temporal correlation of landslides with seismicevents in the region.
