13. Perturbation of Earth Surface Process by Geophysical and Meteorological Process in the Nepal Himalaya

Nepal Himalaya is one of the seismically active mountain belts in the world with several kilometers of relief and very prone to catastrophic mass failure. The collision between Indian and Eurasian plates resulted in numerous tectonic faults and highly deformed rocks, which are responsible for triggering many earthquakes of different scales. High grade of rock weathering and subsequent torrential rainfall are directly related to increase the numerous geo-hazard problems, i.e., landslides, debris flow, floods, etc. The M_w 7.8 Gorkha earthquake 2015 has ruptured a 150-km-long section of the Himalayan décollement and triggered many co-seismic landslides in the central Nepal. These landslides are carrying large volume of sediment to the rivers and deposited in valleys and foothill of the Himalaya. Several catastrophic valley infills are quite interesting in the Himalayas and urgently needed for gauging and predicting the recovery times of seismically perturbed mountain landscapes. In this context, research on Pokhara Valley has suggested that this valley was formed due to catastrophic events in the medieval period. Therefore, tectonic and geomorphic adjustment to several catastrophic aggradation pulses has been ongoing for many centuries in this region. In addition to this, reoccurring earthquake and cloud bursting have negative impacts the people’s livelihood in the mountainous region. Moreover, most of the human settlements in the mountains lie within the large-scale landslides where people are constructing village roads without proper investigation of these large-scale geomorphic landforms. Mountain slopes are unstable due to reoccurring earthquakes and intense rainfalls. For example, a high-intensity rainstorm in July 1993 hit the central Nepal and destroyed many lives and properties. Similarly, Asian Monsoon in 2020 has triggered many landslides and debris flows in the Nepal Himalaya killing more than 303 people in the different parts of Nepal.