Abstract
In mountainous environments, quantifying the drivers of mass-wasting is fundamental for understanding landscape evolution and improving hazard management. Here, we quantify the magnitudes of mass-wasting caused by the Asia Summer Monsoon, extreme rainfall, and earthquakes in the Nepal Himalaya. Using a newly compiled 30-year mass-wasting inventory, we establish empirical relationships between monsoon-triggered mass-wasting and monsoon precipitation, before quantifying how other mass-wasting drivers perturb this relationship. We find that perturbations up to 5 times greater than that expected from the monsoon alone are caused by rainfall events with 5-to-30-year return periods and short-term (< 2 year) earthquake-induced landscape preconditioning. In 2015, the landscape preconditioning is strongly controlled by the topographic signature of the Gorkha earthquake, whereby high Peak Ground Accelerations coincident with high excess topography (rock volume above a landscape threshold angle) amplifies landscape damage. Furthermore, earlier earthquakes in 1934, 1988 and 2011 are not found to influence 2015 mass-wasting.
Highlights
In mountainous environments, quantifying the drivers of mass-wasting is fundamental for understanding landscape evolution and improving hazard management
High rates of tectonic uplift and the Asia Summer Monsoon (ASM) drive high background rates of mass-wasting[15,18–20], which are perturbed by events including floods[21], extreme rainfall[22,23] and earthquakes[3,24,25]
Until empirical relationships between ASM strength and mass-wasting volume are defined, our ability to quantify mass-wasting perturbations due to extreme events across central-eastern Nepal is limited, impeding efforts to account for extreme events in mass-wasting forecasts and time-dependent landslide susceptibility models
Summary
In mountainous environments, quantifying the drivers of mass-wasting is fundamental for understanding landscape evolution and improving hazard management. Compilation and comparison of erosion rates measured over different timescales can isolate the roles of different masswasting drivers Such approaches typically utilise proxies, including cosmogenic nuclides or suspended sediment flux, to establish long-term background erosion rates against which shorter term perturbations captured by field sampling or remote sensing can be measured[2,12,13,15]. Whilst relationships between precipitation intensity and short-term suspended fluvial sediment flux in the Himalaya are well described[15,19,26], an empirical relationship between ASM strength and ASM-triggered mass-wasting in central-eastern Nepal remains elusive[3]. This is problematic, as demonstrated by the 2015 Mw 7.8 Gorkha earthquake. As well as providing insight into the processes controlling landscape evolution, this permits further investigation into the characteristics and processes of earthquake preconditioning in the Himalaya
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