Post-seismic Evolution Of The Landslides Triggered By The 2015 Nepal Earthquakes

On April 25,2015,an Mw 7.8 earthquake rocked Gorkha,Nepal in the central Himalayan orogenic belt,affecting southern Tibet and other neighboring countries.About half of a month later,on May 12,a major aftershock of Mw 7.3 with the epicenter east to the mainshock,again shook this area.These two strong earthquakes both occurred before the rainy season(from June to September)in Nepal.The seismic area is located in the front where the Indian plate thrusts beneath the Eurasian plate,thus hosting the most tectonically active zones.Moreover,the area is a mountainous region with deep gorges and in the South-Asian monsoonal area with abundant precipitation especially in monsoon seasons.Because of its particularity of the tectonic setting,geographical location,topography,and climate in Nepal,the seismic area faces active landslide hazard even in aseismic years.As well,the remote sensing interpretation also shows that the main earthquake and the strong aftershock both triggered a large number of co-seismic landslides.The emergency investigation immediately taken after the earthquake in June 2015 also indicated that the heavy rainfall events in the following rainy season of 2015 resulted into more active landsliding in the seismic area,including the emergence of many new slope failures as well as the further expansion of pre-seismic and/or co-seismic landslides.To better understand the post-seismic evolution of the landslides in the Tibetan area and serve the disaster prevention and mitigation in the southern Tibet and the Belt and Road countries,this study focuses on the characteristics of the successive occurrence and post-seismic evolution of landslides triggered by the main earthquake,the strong aftershock and the post-seismic strong rainfalls in Nepal.Field investigations were taken in 2015,2017,and 2018 to collect in-situ photos and aerial photographs of landslides along the Araniko and Pasang Lhamu highways.A region with an area of?380 km~2 on the Araniko Highway,which is the important connection between Nepal and China and affected jointly by the mainshock,strong aftershock,and the rainfalls,was selected as the study area.Using the visual interpretation method,a multi-temporal data set was built for this area,including the landslides triggered by the pre-earthquake rainfalls,the mainshock,the strong aftershock,and the 2015–2019 monsoon rainfalls,based on the field data,the landslide map of the whole seismic area,and the time-series remote sensing images.Combined with the control factors including terrain,lithology,earthquake,roads,streams,and precipitation,the successive characteristics of spatial distribution,geometry and geomorphometry,and susceptibility mapping for landslides in the aforementioned 8 inventories were analyzed respectively to study the changes of the mechanisms related to landslides that occurred in different periods.Also,according to the multi-temporal landslide inventories,temporal changes of the NDVI,and the data collected from 3 landslide investigations,the post-seismic evolution of the landslides in the study area was explored.The results are summarized as follows:Generally,the mainshock and the strong aftershock triggered lots of co-seismic landslides.The total landslide number and area decreased since 2015.There were more small-scale landslides triggered by the pre-seismic rainfalls,the mainshock,the strong aftershock,and the 2015–2017 rainfalls,while more large-scale landslides were found after the 2018 and 2019 monsoon seasons.Nevertheless,the number of large-scale landslides in the inventory related to the mainshock is the largest comparing to other inventories and the inventories of the landslides triggered by the strong aftershock and the 2015 rainfalls include more small to medium landslides than others.Spatially,the effects of elevation,roads,and streams vary obviously for the landslides induced by different triggers in different years.The mainshock and strong aftershock tend to trigger landslides in similar geo-environments.The distribution of the rainfall-triggered landslides in 2015 has both the features of pre-seismic rainfall-and earthquake-triggered landslides.Temporally,for>90%of the factors,the new landslide area increased first and then decreased after the earthquakes,and locally peaked after the 2015 monsoon seasons.Moreover,the length,height,and length to width ratio of the seismic landslides are much larger than those of the rainfall-triggered landslides.There were more landslides with larger length-width ratios and height-length ratios in the inventories related to the seismic landslides and the 2015 rainfall-triggered landslides which are seriously affected by earthquakes.Besides,the landslides in these two inventories are typically longitudinal in planar shape.Elevation,slope,roads,and streams are four factors that have significant impacts on the landslides of different geometrical shapes that occurred in different years.The best susceptibility mapping results are obtained based on the landslides triggered by the mainshock and the strong aftershock which both have success and prediction rates larger than 87%.The high susceptibility regions moved from the slope surface to channels since 2015.After a series of attempts using susceptibility models to backward predict the landslides in the following years,the combination of the susceptibility results of pre-seismic and the mainshock-triggered landslides,which have a higher prediction accuracy,has prediction rates larger than 70%till 2018,which would aids in efforts of future seismic landslide mitigation and post-seismic reconstruction.The landslides triggered by the 2015 mainshock,the strong aftershock,and the2015 monsoon rainfalls are becoming less active with time going by and rapidly decreased within the first 3 years.Besides,the decreasing rate of the strong aftershock triggered landslides is the most obvious.The scale of the disappeared landslides is increasing.The vegetation cover varies with the rainfalls monthly in the whole study area.After obviously increasing in the first 2 years after the earthquake,the vegetation again decreased but slowly.The effects posed by the earthquakes on the vegetation along the highway were much significant and probably will last for a longer time.The results of the landslide evolution obtained based on the 3 field investigations verifies that the April 25 Mw 7.8 Gorkha,Nepal earthquake and its May 12 Mw 7.3 aftershock significantly increased landsliding along the Araniko and Pasang Lhamu highways.The landsliding was active within 3 years after the earthquake and the continuing rainfall after earthquakes,and strong aftershock shaking are key factors influencing the future landsliding in these earthquake-affected areas.