Dynamic processes of 2018 Sedongpu landslide in Namcha Barwa–Gyala Peri massif revealed by broadband seismic records

At 22:48 pm, October 16, 2018, a large glacial mudslide occurred upstream of the Sedongpu gully on the southern mountainside of Gyala Peri in Mainling County, Tibet. To investigate its dynamical features, the force-time history of it was inverted using the broadband recording of six stations in the Namcha Barwa network. Preliminary results of the landslide force-time history indicated that the mass of the Sedongpu landslide is about one million tonne. The velocity and displacement curves from integration implied that the mass center of the Sedongpu event slipped down for a distance of 5100 m with an average velocity of 10 m/s (peaked at about 25 m/s). The fitted displacement curve (using the motion equation) revealed that the movement of the Sedongpu landslide experienced four processes. In the first 150 s, the stability at the upper part of Sedongpu glacier broke down, and the detached glacial deposits gradually moved downward. The subsequent 70 s is the stage of acceleration for the debris flow with the velocity increased and the acceleration peaked. After encountering the leading edge of the glacial deposits, the debris flow decelerated for about 80 s as the slope of the path declined and the canyon narrowed. In the final 180 s, the debris flow might have been blocked by the leading edge of the deposit or a large topographical bend; hence, it stopped eventually. The entire process lasted for 480 s. We proposed that the surface wave generated by an earlier earthquake (about 10 min before the landslide event) in Nagqu, Tibet, played a decisive role in triggering the severe event on the basis of the prolonged impact from rainfall and regional fault activity.