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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 21 Sep 2018

Research article | 21 Sep 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Earth Surface Dynamics (ESurf).

Long-term erosion of the Nepal Himalayas by bedrock landsliding: the role of monsoons, earthquakes and giant landslides

Odin Marc1, Robert Behling2, Christoff Andermann2, Jens M. Turowski2, Luc Illien2,3, Sigrid Roessner2, and Niels Hovius2 Odin Marc et al.
  • 1École et Observatoire des Sciences de la Terre – Institut de Physique du Globe de Strasbourg, Centre National de la Recherche Scientifique UMR 7516, University of Strasbourg, 67084 Strasbourg Cedex, France
  • 2Helmholtz Centre Potsdam, German Research Center for Geosciences (GFZ), Telegrafenberg, 14473 Potsdam, Germany
  • 3Laboratoire de Géologie, Ecole Normale Supérieure, 24 Rue Lhomond, 75000, Paris, France

Abstract. In active mountain belts with steep terrain bedrock landsliding is a major erosional agent. In the Himalayas, landsliding is driven by annual hydro-meteorological forcing due to the summer monsoon and by rarer, exceptional events, such as earthquakes. Independent methods yield erosion rate estimates that appear to increase with sampling time, suggesting that rare, high magnitude erosion events dominate the erosional budget. Nevertheless, until now, neither the contribution of monsoon and earthquakes to landslide erosion, nor the proportion of erosion due to rare, giant landslides have been quantified in the Himalayas. We address these challenges by combining and analyzing earthquake and monsoon induced landslide inventories across different timescales. With time-series of 5m satellite images over four main valleys in Central Nepal, we comprehensively mapped landslides caused by the monsoon from 2010 to 2018. We found no clear correlation between monsoon properties and landsliding, and a similar mean landsliding rate for all valleys, except in 2015, where the valleys affected by the earthquake featured ~5–8 times more landsliding than the pre-earthquake mean rate. The long-term size-frequency distribution of monsoon induced landslides (MIL) was derived from these inventories and from an inventory of landslides larger than ~0.1km2 that occurred between 1972 and 2014. Using a published landslide inventory for the Gorkha 2015 earthquake, we derive the size-frequency distribution for earthquake-induced landslides (EQIL). These two distributions are dominated by infrequent, large and giant landslides, but underpredict an estimated Holocene frequency of giant landslides (>1km3) which we derived from a literature compilation. This discrepancy can be resolved when modelling the effect of a full distribution of earthquake of variable magnitude and considering that shallower earthquake may cause larger landslides. In this case, EQIL and MIL contribute about equally to a total long-term erosion of ~2±0.75mm.yr−1 in agreement with most thermochronological data. Independently of the specific total and relative erosion rates, the heavy-tailed size-frequency distribution from MIL and EQIL and the very large maximal landslide size in the Himalayas indicate that mean landslide erosion rates increase with sampling time, as has been observed for independent erosion estimates. Further, we find that the sampling time scale required for adequately capturing the frequency of the largest landslides, which is necessary for deriving long-term mean erosion rates, is often much longer than the averaging time of cosmogenic 10Be methods. This observation presents a strong caveat when interpreting spatial or temporal variability of erosion rates from this method.

Odin Marc et al.
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Odin Marc et al.
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Publications Copernicus
Short summary
We mapped landslides (> 100 m2) in the Nepal Himalayas due to 8 monsoons and large landslides (> 0.1 km2) since 1970. Adding inventories of Holocene, giant landslides (> 1 km3), and of landslides from the 2015 Gorkha earthquake, we constrain the size-frequency distribution of landslides induced by monsoons and earthquakes. They both contribute ~ 50 % to a long-term (> 10 kyr) total erosion of ~ 2 mm/yr, matching long-term exhumation rate. Large landslides, rarer than 10Be sampling time, drive erosion.
We mapped landslides ( 100 m2) in the Nepal Himalayas due to 8 monsoons and large landslides...