Application of Multivariate Techniques to Assess Active Tectonics: Implications for Geohazard Risk in the Siang Valley, Arunachal Pradesh, India

The Siang River Basin of Arunachal Pradesh, located within the tectonically active Eastern Syntaxial Bend (ESB) of the Himalaya, is highly vulnerable to landslides and erosion. The basin is traversed by major thrust zones, the Bomdila Thrust (BT), Main Central Thrust (MCT), Tidding Thrust (TT), and Lohit Thrust (LT) that strongly shape its geomorphic and tectonic framework. This study analyses 41 fourth-order sub-basins to evaluate how morphometric, lithological, and neo-tectonic factors drive mass-wasting processes. Multivariate techniques, including Principal Component Analysis (PCA) and Agglomerative Hierarchical Clustering (AHC), were applied to morphometric parameters. Results show consistent clustering between PCA and AHC, identifying dominant morphometric controls on basin development. The Varimax-rotated PCA factor loading matrix further clarified the contribution of individual variables to each component. The Index of Relative Active Tectonics (iRAT) indicates that about 67% of the basin falls within moderate to very high tectonic activity zones, with landslides most frequent in areas of moderate to low iRAT values. A total of 476 landslides were mapped, with 214 located in the Lesser Himalaya, 119 in the Higher Himalaya, and 143 in the Trans-Himalaya. Notably, 173 landslides were directly linked to road-cutting activities, highlighting the influence of anthropogenic disturbances. The findings highlight the combined role of tectonic activity, slope morphology, lithology, and anthropogenic disturbances in shaping hazard dynamics. This integrated geomorphic approach provides a framework for identifying active slope-failure zones and supports regional planning and landslide risk mitigation in the Siang River Basin.