# Rainfall Induced Landslide Studies in Indian Himalayan Region: A Critical Review
## 2 Materials and Methods  
### Study Region  
The Indian Himalayan region is one of the most diverse and heterogeneous area in terms of geology, lithology, rainfall distribution, land use/land cover, soil properties, and road and stream networks, which makes it highly prone to landslides. The region covers 16.2% (~500,000 km\({}^{2}\)) of India's landmass as well as 10 of its states (Ministry of Environment, Forest and Climate Change, Government of India). The region extends 26\({}^{\circ}\)20\({}^{\prime}\)-35\({}^{\circ}\)40\({}^{\prime}\) N and 74\({}^{\circ}\)50\({}^{\prime}\)-95\({}^{\circ}\)40\({}^{\prime}\) E and covers the states Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, Arunachal Pradesh, Meghalaya, Nagaland, Manipur, and Mizoram, as well as the hill stations of Assam and West Bengal. The region has been divided both horizontally and vertically into four and three divisions, respectively. In horizontal context, it has been categorized as Jammu and Kashmir Himalaya, Himachal Himalaya, Uttarakhand Himalaya, and Eastern Himalaya (Figure 1). Uttarakhand Himalaya is further divided into the Garhwal and Kumaon Himalayas. In terms of vertical divisions, it has been divided into Great Himalaya, Middle Himalaya, and Lesser Himalaya or Shivalik Ranges [5]. Geologically, the Himalayas are broadly divided into four areas across its length: (i) Foothill or Outer Himalaya; (ii) Lesser Himalaya; (iii) Higher Himalaya; and (iv) Tethyan or Trans-Himalaya [6]. The major rivers that originate from the great Himalayan Mountain Ranges are the Indus, Ganges, and their various tributaries. The region exposes diverse geology with different rock types representing the complete spectrum ranging in age from Archaean metamorphites/granitoids to the youngest Quaternary alluvium. The region is tectonically and physiologically divided into three broad domains: the Peninsula India, the Extra-Peninsular India, and the Indo-Gangetic Plain [7]. The Indo-Gangetic plain is sandwiched between the shield area of the Peninsula India and the highly deformed suites of the Himalaya of Extra-Peninsular India, comprising essentially the younger meta-sediments. The tectonic trough (foreland basin) sandwiched between peninsular shield in the south and Himalayan Mountains in the north formed due to upliftment of the latter has been filled up by sediments derived from both sides, especially from the Himalayas. Structurally, the Himalayan Mountain chain occurring all along northern part of India can be divided into four contrasting longitudinal litho-cum-morphotectonic belts from south to north: (i) foothill belt; (ii) Main Himalayan belt; (iii) Indus-Shyok belt; and (iv) Karakoram belt. The foothill Himalaya is a 10-50 km wide Miocene to Lower Pleistocene Molasse sequence represented by Siwalik, Murree, and Subathu Group of rocks. The belt is a domain of active tectonics, having participated in the terminal phase of the Himalayan Orogeny. This is followed to the north by the Lesser and Higher Himalayas, represented by geological sequences of Proterozoic age with a Phanerozoic cover of varying thicknesses in different parts. The foothill Himalaya is overlain by alluvium and separated from the Lesser Himalaya by the north-dipping fault commonly known as the Main Boundary Fault (MBF) or the Main Boundary Thrust (MBT) in Garhwal, Kumaon, Darjeeling, and Arunachal Pradesh Himalaya. The Main Frontal Thrust (MFT) limits the margins of the Siwalik Zone against the Ganga Plains [8]. The Lesser Himalaya is 60-80 km wide and is a discontinuous belt stretching between the MBT in the south and the Main Central Thrust (MCT) in the north [9]. It consists of autochthonous Late Proterozoic sediments, thrust over by three vast nappes that are built up successively of Palaeozoic sediments, Precambrian epimetamorphics, and mesograde metasediments [10]. The epi-metamorphic and meso-metamorphic nappes throughout their extent are characterized by Early Proterozoic (= 1900 Ma) and Early Palaeozoic granitic bodies of large dimension.