Integrating geomorphic indices with patterns of tectonic influence in the Mandakini river basin

Seismic hazard assessment in active regions necessitates precise evaluation of tectonic activity. Although geomorphic indices are widely applied to evaluate relative tectonic activity, the influence of thrusting and seismicity driven by plate movements on geomorphic indices remains insufficiently explored. Focused on the Mandakini Basin, Garhwal Himalaya, Uttarakhand, India, this study investigates this influence by analysing four key geomorphic parameters-Hypsometric Integral (HI), Elongation Ratio (Re), Transverse Topographic Symmetry (T) Factor, and Valley-Floor Width to Height (Vf) ratio, computed from SRTM 30 m data. We analysed ten watersheds, categorized based on the presence of thrust, seismic activity, both, or neither. High hypsometric values (0.47–0.56) are typically observed in watersheds influenced by thrust and earthquake epicenters, while low values (0.36–0.37) are observed in areas dominated by thrust-free areas. In the studied watersheds, elongation ratio results revealed that the basin is more elongated (0.56–0.66) when a thrust intersects the trunk channel; otherwise, it assumes a less elongate or oval shape (0.71–0.86). Significant channel deflection (T = 0.58 and T = 0.47) was observed because of the Bhilangana and Main Central Thrust. However, exceptions were observed in watersheds located between thrusts or near earthquake epicenters, suggesting localized tectonic influence. Kruskal-Wallis H-test confirm that HI and Vf significantly vary with tectonic setting. Additionally, optimized hot spot analysis revealed spatial clustering of seismicity along major thrusts. This study highlights the importance of integrating spatial thrust and seismic data with geomorphic analysis for a more comprehensive understanding of active tectonic controls on the basin.