Extreme Rainfall-Induced Landslides on 1 June 2025 in Southern Assam, Northeast India

ABSTRACT In the early hours of 1 June 2025, a series of landslides occurred in the Rajpur Village of southern Assam, Northeast India, following incessant rainfall due to early onset of the Indian Summer Monsoon (ISM). The landslides occurred almost simultaneously along the partially weathered hillslopes composed primarily of matrix supported gravels, between 03:00 and 03:30 IST. These were preceded by an extreme rainfall pulse of 230 mm recorded at 02:30 IST which is part of a cumulative total of 458.8 mm over the previous three days in the area. The landsides caused grievous injuries to a few people and damaged number of dwelling units with consequent displacements. Post disaster site investigation and first-hand accounts suggest an extreme rainfall triggered shallow landslide mechanism in play when saturation in the porous rock and soil mass exceeded threshold. In this report we present a preliminary assessment of the rainfall event and physical site factors leading to the landslides on this particular date. The good correlation observed here between rainfall record and occurrence of landslides provides an important case study to work out a regional threshold for rainfall triggered landslides in the foreland fold belt region of Northeast India. This will further contribute towards developing a region-specific landslide early warning system in a scenario of changing monsoonal regime.