SAR and optical images correlation illuminates post-seismic landslide motion after the Mw 7.8 Gorkha earthquake (Nepal)

In the days to weeks following an earthquake, landslides can display specific post-seismic motions, including delayed initiations and post-seismic relaxations. These motions have an uncertain origin, sometimes attributed to specificities of the landslide basal interface or to fluid transports in the landslide basal shear zone. Here we address this question, by documenting the co- and post-seismic motions of slow-moving landslides accelerated by the Gorkha earthquake (Mw 7.8, 25/04/2015, Nepal). We detect 11 slow-moving landslides over an area of 750 km(2) in the near field of the earthquake, and monitor their motions thanks to a time-series of Pleiades optical satellite images and SAR Sentinel-1 images. The post-seismic landslide motions are much larger than the co-seismic ones, reaching up to 34 +/- 0.6 m accommodated over 2 months. A delayed initiation of several days (> 4 days) is also measured for at least two of the landslides. We analyze our findings in regards with all the previous observations on slow-moving landslides accelerated by earthquakes, and propose that the post-seismic motions are caused by diffusion of groundwater from co-seismic material contraction up to the landslide basal shear zone or from internal landslide reconfiguration. Our observations strongly suggest the main control of the hydrology in the landslide processes under seismic forcings.

Automated summary

In the days to weeks following an earthquake, landslides can exhibit specific post-seismic motions, such as delayed initiation and post-seismic relaxation. This study investigates the co- and post-seismic motions of slow-moving landslides accelerated by the Gorkha earthquake in Nepal. The study identifies 11 slow-moving landslides over an area of 750 km(2) and monitors their motions using satellite images. The post-seismic motions of the landslides are found to be much larger than the co-seismic ones, with a delayed initiation of several days observed in some landslides. The study suggests that the post-seismic motions may be caused by diffusion of groundwater or internal reconfiguration of the landslides.