Tectonic control on landsliding in the nepal himalaya revealed by the 2015 gorkha earthquake

Paul Quackenbush, Josh West, Marin Clark, Dimitrios Zekkos, & Chamlagain Deepak

Submitted August 14, 2017, SCEC Contribution #7615, 2017 SCEC Annual Meeting Poster #133

The Mw 7.8 Gorkha earthquake in 2015 triggered more than 25,000 landslides, providing a rare opportunity to gain new general understanding of the links between faulting, tectonics, and landsliding. While landslides from many other seismic events are generally coincident with the location of high peak ground acceleration (PGA), the offset between high-density landsliding and high PGA in the Gorkha event indicates that in this case, additional factors were at work in determining the landslide distribution. In this study, we focus on one river valley, the Melamchi Khola, to investigate the controls on the distribution of landslides. We find that landsliding increased progressively from the Lesser Himalaya to the High Himalaya, a trend that is common across fluvial valleys in the earthquake-affected region. Landslide density for catchments in the Melamchi Valley is well correlated with mean catchment slope, relief, and normalized channel steepness (ksn), indicating a strong topographic control on landsliding, with little evidence for gradients in PGA controlling landslide occurrence in this setting.

To complement the topographic analyses, nine detrital samples and two bedrock samples from the Melamchi Khola were collected and analysed for (U-Th)/He thermochronology in apatite and zircon. Mean apatite-helium and zircon-helium ages range from 2.1-5.8 Ma and 2.6-5.2 Ma respectively and decrease from south to north, indicating a gradient in long-term exhumation rate that is coincident with the observed gradient in landslide density. Together with the observation that landslides across the earthquake-affected region are concentrated along fluvial reaches with high normalized channel steepness (ksn), these patterns point to a connection between landsliding and tectonic uplift. Our results highlight the role of tectonically controlled topography, rather than shaking, in determining the distribution of landslides from the Gorkha event and suggest the possibility that topography mediates long-term coupling between seismically driven erosion and tectonics in the Nepal Himalaya.

Quackenbush, P., West, J., Clark, M., Zekkos, D., & Deepak, C. (2017, 08). Tectonic control on landsliding in the nepal himalaya revealed by the 2015 gorkha earthquake. Poster Presentation at 2017 SCEC Annual Meeting.

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Earthquake Geology