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

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 5 m 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 ~0.1 km 2 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 (>1 km 3 ) 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 +/-0.75 mm.yr -1 in agreement with most thermochronological data. Independently of the specific total and relative erosion rates, the heavytailed 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 10 Be

Initiation and Runout of Post‐Seismic Debris Flows: Insights From the 2015 Gorkha Earthquake

“…Changes in Type‐2 activity in Nepal are consistent with other observations where rainfall‐induced landslides became more frequent after large earthquakes but tapered off in following years (Fan et al, ; Marc et al, , ). The approximately tenfold increase in Type‐2 flow activity in 2015, and the return to mean pre‐earthquake levels in 2016, matches landslide rate variations between 2010 and 2017 across Nepal reported by Marc et al (). Progressive reduction in Type‐2 debris flow activity in 2016 and 2017 could be due to rock “healing” after seismic weakening (Brantut, ; Lawrence et al, ; Marc et al, ) or failure of many of the most unstable hillslopes immediately after the earthquake, leaving fewer opportunities for failure later.…”

“…Recent studies have evaluated coseismic and post‐seismic landslide inventories, including those associated with the Gorkha earthquake (Marc et al, ; Martha et al, ; Roback et al, ). With some exceptions (Guo et al, ; Ni et al, ; Tang et al, ; Tang & Liang, ; Xu et al, ; Zhang & Zhang, ), this work has focused on hillslope failures occurring during shaking (Keefer, ; Li et al, ; Roback et al, ) and on weakened hillslopes over subsequent years (Fan et al, ; Marc et al, , ). Here we specifically seek understanding of the channelized, fluidized flows associated with heavy rainfall in the weeks to years after earthquakes.…”

“…In steep terrain, coseismic landslides deliver enormous quantities of debris to channels (Dadson et al, ; Jibson & Keefer, ; Keefer, ; Marc et al, ; Roback et al, ; Xu et al, ). In months and years following, heavy rainfall may mobilize this debris (Iverson et al, ; Zhang & Zhang, ), while new landslides form on weakened hillslopes (Fan et al, ; Marc et al, , ) and can generate additional flows. With communications, transportation, and supply chains already disrupted and structures weakened, post‐seismic debris flows may be particularly deadly and destructive (Zhang et al, ).…”

Landslide Scaling: A Review

“…For power scaling, the lower the scaling exponent, or less steep the power function, the greater the contribution of large events relative to small events (e.g., Hergarten, ). Second, landslide scaling can form the basis for probabilistic landslide hazard assessment for a region over a given time interval (e.g., Guzzetti et al, ; Marc et al, ). Third, the scaling observed in a natural system can be used to calibrate and assess the applicability of models.…”

“…To facilitate such comparisons, scales to quantify landslide event magnitude have been proposed by Malamud et al () and Tanyaş et al (). Fifth, landslide scaling provides a means of estimating erosion rates in a region due to landslides and, thus, the landslide contribution to a regional sediment budget (e.g., Jeandet et al, ; Marc et al, ). Finally, more complicated processes may be informed by the parameters of landslide scaling, such as the reworking of sediment following a landslide by river sediment export (Croissant et al, ) and the impact of landslides on organic carbon cycling in a montane forest (Hilton et al, ).…”