The collapse of Tsergo Ri took place in Nepal's Langtang valley,[1] perpendicular to the Himalaya[2] and about 60 kilometres (37 mi) north of the Nepalese capital Kathmandu.
[6] Rocks formed by deformation, intrusions of granite, and layers of pyrrhotite ore, which are unstable under mechanical load and neotectonic faults, may have been weak structures that facilitated the collapse.
[9] The Tsergo Ri landslide was probably triggered by seismic activity,[10] perhaps on the Himalayan Main Central Thrust;[11] a water level drop in the Paleo Kathmandu Lake took place at the same time and may have been caused by the same earthquake.
[13] Based on reconstructions of the pre-landslide topography, there may have been a 7,500–8,500 metres (24,600–27,900 ft) high[14][15] trilateral mountain in the area,[16] or a set of ridges.
[24] The landslide debris consists of individual compact blocks on top of a basal breccia[25] and originally may have reached a thickness of 600–800 metres (2,000–2,600 ft).
[32] Slow mass movements into valleys[19] and weather/monsoon-controlled mudflows also occur,[33] and there is evidence that the debris from the Tsergo Ri landslide is especially unstable.