Landslides triggered during the 2015 Gorkha earthquake in Nepal reveal non-linearity of topographic controls

Seismically triggered landslides, which can number in the tens of thousands during large continental thrust earthquakes, can cause immediate devastation and are a pernicious link in the chain of "cascading hazards" that follow these events. Understanding what determines the spatial distribution of earthquake-triggered landslides is a fundamental requirement for predicting associated hazard and for building resilience, as well as for identifying the role of landslides in topographic evolution. Landslides triggered during the 2015 Gorkha earthquake provide new insight into the role of topography in regulating landslide occurrence. Canonical models suggest landslide occurrence depends on seismic ground motion, rock properties, and hillslope angle, often linearly parameterizing the dependency on slope angle. We have analyzed in detail the Melamchi Khola valley in central Nepal, where the best available data suggest that variations in rock properties and ground motion during the Gorkha event were relatively minor, but where there is a pronounced south-north increase in landscape steepness. We find a correlative increase in landslide activity, suggesting a topographic control on Gorkha-associated landsliding. However, we observe a strong non-linearity in the relationship between landsliding and hillslope angle; from south to north, slopes approach a threshold angle 35° even as landslide areal density continues to increase. The relationship between landslide density and the normalized steepness of river channels is also non-linear, but channel steepness is a more sensitive recorder than hillslope angle of landslide occurrence during the earthquake. Thus, we are able to document for the first time a threshold-regulated behavior for seismicallytriggered landslides, as previously found for their non-seismic cousins. Apatite and zircon (U-Th)/He ages reveal that the south-north gradient in both topography and landslide occurrence in the Melamchi Valley is related to gradients in exhumation associated with tectonic forcing. Our observations show that the topographic control on landsliding can be highly non-linear and suggest that the fluvial response to tectonics may aid in the prediction of landslide hazard during earthquakes.