Landslide development within 3 years after the 2015 Mw 7.8 Gorkha earthquake, Nepal

The Araniko and Pasang Lhamu highways are two critical trading routes connecting Nepal and China that are experiencing and are threatened by landsliding. The April 25 Mw 7.8 Gorkha earthquake and its May 12 Mw 7.3 aftershock in 2015 significantly increased landsliding along these highways. Multi-temporal remote sensing images (pre-seismic to December 2017) on Google Earth, aerial photography acquired in 2018 using an unmanned aerial vehicle (UAV), and field investigations and surveys in June 2015, November–December 2017, and April–May 2018 quantify the development of selected landslides after the earthquake along the two highways. These include detailed studies of 39 and 30 landslide locations in 2017 and 2018, respectively, with 14 locations common to both the 2017 and 2018 studies. Only the landslides in five locations showed a slight decrease in activity after the earthquake, and most landslides continued to move and enlarge. More than five new landslides were triggered by monsoon rainfalls in the summers of 2017 and 2018. All the landslides pose significant challenges for infrastructure reconstruction and hazard reduction in the area affected by the Gorkha earthquake, especially along the Araniko and Pasang Lhamu highways. Comparison of landslide debris volume, pre- and post- earthquake rainfalls, and earthquakes that occurred before and after the 2015 Gorkha earthquake with the 1999 Chi-Chi, 2005 Kashmir, and 2008 Wenchuan earthquakes show that continuing rainfall after earthquakes and strong aftershock shaking are key factors affecting continued landsliding in these earthquake-effected areas.