Understanding landscape mass-wasting in response to geophysical forcings to constrain long-term erosional budgets

Landsliding is the dominant process of sediment production in active mountain range. Various geophysical forcing are triggering widespread landsliding over different timescales.

Though less continuous than climatic forcing, shallow earthquakes are an important trigger of mass wasting. PGA not only controls the spatial density of landslides (Meunier 2007) but also their size (Valagussa 2019) and their location in the landscape (Milledge 2019), with implications still to be explored. Combining simple seismological scalings with landscape characterization allows to relate EQ sequences to landslide flux, over various timescales (Marc et al., 2015, 2016, 2017).

Intense rainfall is another recurrent trigger. Landslide catalogue associated with rainfall events are multiplying and showing specific relations with rainfall and topography (Marc et al 2018), in particular, landslide scar appear unrelated to saturation index (Emberson et al., 2021), and in well constrained cases they seem to relate better to rainfall anomaly (ie relative to local extreme rainfall) than to total event rainfall (Marc et al 2019a, in Review).

These triggers can be combined for a given mountain range, such as the Nepalese Himalayas (Marc et al., 2019b). Here we combined a fine characterization of the landslide frequency size distribution for EQ and monsoon rainfall to derive a long-term erosional flux. Accounting for a distribution of EQ properties and the frequency of giant paleolandslides we obtain an erosion rate consistent with the thermochronometric exhumation rate. Recent work has shown that the monsoon strength variations could also modulate the landslide response and that landsliding caused by extreme daily rainfall could also be isolated (Jones 2021), which could allow to refine and better understand how this budget has changed in the past.

Last, we remind that, large, deep seated landslides remain an important and understudied challenge. Indeed, they are poorly correlated to rainfall properties (either the monsoon (Marc et al 2019b, Jones 2021) or the rainfall anomaly from large typhoons (Marc et al., 2019a). However, they may be a dominant contributor to erosion and sediment volumes, and their control, likely related to mechanical strength degradation, should be investigated urgently.