Coseismic landsliding due to the M7.8 Gorkha earthquake sequence poses immediate and prolonged hazards to communities in the Nepalese Himalaya, as well as a rare opportunity to study the effect of large earthquakes on erosion and sediment budgets in mountain belts. We present real-time response models developed within hours of the event using a simplified Newmark analysis, which were used to prioritize early scientific efforts and to guide rescue and recovery efforts by US and international agencies. Analyses included prediction of regional landslide occurrence and identification of potential landslide dam locations, which could cause flooding upstream and downstream if the dam is catastrophically breached.
Subsequent investigations have included mapping of coseismic landslides using pre- and post- event satellite imagery and field observations, and inversion of mapped landslide distributions for estimates of near-surface rock strength. Compared to model predictions using regionally uniform rock strength, observed landslides are more concentrated north of the physiographic transition between the Lesser and Greater Himalaya where hillslope gradients suddenly steepen. Fewer landslides than predicted occurred in the high elevation, steep glaciated terrain and in areas of highest modeled PGA, just to the south of this physiographic transition. Discrepancies between model predictions and observations could arise from spatial variability in rock strength, in PGA or frequency content at specific site locations, or by pre-conditioning (topographic or otherwise) for landslide hazard.
Co-seismic landsliding produces a prolonged hazard for years to come. In the near term, more frequent landslides are expected to occur during the summer monsoon seasons by remobilization of debris and due to a dynamic increase in pore-pressure on hillsides or near ridge tops that were pervasively cracked during the main earthquake or aftershocks, but did not previously fail. Because larger seismic events are known to occur in the Himalaya, we anticipate that feedbacks between the seismic cycle and sediment transport influence long-term erosion patterns.