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Topographic characteristics of landslides induced by the 2015 Gorkha earthquake, Nepal

The Gorkha earthquake occurred in central Nepal at 11:56 am local time (7:05 UTC) on April 25, 2015 (USGS, 2015) and ruptured the Main Himalayan Thrust (Duputel et al., 2016). The earthquake had a magnitude of Mw 7.8 and was followed by hundreds of aftershocks, including a Mw 7.3 major aftershock on May 12, 2015 (USGS, 2015). The earthquake had no clear geomorphic expression or surface rupture (Angster et al., 2015), but the mainshock and aftershocks caused many landslides (Kargel et al., 2015). The landslides caused > 9000 fatalities and damaged infrastructure (Kargel et al., 2015), making this the most devastating earthquake in Nepal since the 1934 Nepal–Bihar earthquake (Sapkota et al., 2016).


INTRODUCTION
The Gorkha earthquake occurred in central Nepal at 11:56 am local time (7:05 UTC) on April 25, 2015(USGS, 2015 and ruptured the Main Himalayan Thrust (Duputel et al., 2016). The earthquake had a magnitude of Mw 7.8 and was followed by hundreds of aftershocks, including a Mw 7.3 major aftershock on May 12, 2015(USGS, 2015. The earthquake had no clear geomorphic expression or surface rupture (Angster et al., 2015), but the mainshock and aftershocks caused many landslides (Kargel et al., 2015). The landslides caused > 9000 fatalities and damaged infrastructure (Kargel et al., 2015), making this the most devastating earthquake in Nepal since the 1934 Nepal-Bihar earthquake (Sapkota et al., 2016).
Several studies have interpreted the distribution of landslides associated with the Gorkha earthquake using field reconnaissance and satellite images, and analysed their controlling factors (Kargel et al., 2015;Martha et al., 2017;Roback et al., 2017;Tiwari et al., 2017). The landslides are mainly shallow-disrupted landslides and are distributed across the Lesser and Higher Himalayas (Roback et al., 2017), and developed in Proterozoic metamorphic units and intrusive complexes (Kargel et al., 2015). A high landslide density correlates with steep topography (>30°) and high peak ground acceleration during the earthquakes (>0.6 g) (Kargel et al., 2015).
Previous studies have provided digital maps of landslide distribution, but freely available landslide inventories are limited and are only available as GIS point data (Kargel et al., 2015) or polyline-based data (British Geological Survey, 2015). Furthermore, pre-existing and enlarged landslides have not been discriminated in the mapping (e.g., Tsou et al., 2017). In this paper, we develop two polygon-based inventories of landsides that were enlarged or newly induced by the 2015 Gorkha earthquake and its aftershocks. We report on and discuss the characteristics and distribution of the landslides and make recommendations for the mitigation of future landslide disasters. This paper presents some of the main results of the Japan-Nepal Urgent Collaborative Project, which focussed on the April 2015 Nepal earthquake as part of the J-Rapid Program of the Japan Science and Technology Agency (JST-PROJECT-15657342).

Study area
The study area of 7.8 × 10 3 km 2 is mountainous, at elevations from 360 to 7230 m (Fig. 1). The area covers six districts such as Gorkha, Rasuwa, Dhadin, Nuwakot, Sindhupalchouk, and Dolakha. The major rivers are the Daraundi Khola, Budhi Gandaki, Trishuli River, Sun Koshi, Bhote Koshi, and Tama Koshi, which all flow southwards. V-shaped fluvial gorges with steep hillslopes are distinct geomorphic features of these river catchments, with local increases in relief of 400 m over horizontal distances of < 1000 m. Longitudinal profiles of these rivers show distinct knickpoints distributed to the north of a macroscale physiographic transition, referred to as physiographic transition 2 (PT2; Harvey et al., 2015). PT2 is located just south of the Main Central Thrust (MCT), marking the boundary between the Lesser and Higher Himalayas . The geology of the study area comprises high-grade metamorphic rocks such as quartzite, migmatite, and gneiss of the Higher Himalayan Crystallines, and phyllite, schist, carbonate, and amphibolite of the Lesser Himalayan strata (Shrestha and Shrestha, 1986).

METHODS
Landslides induced by the 2015 Gorkha earthquake and its aftershocks are interpreted from Google Earth images. The landslides were detected from images taken after the main shock and major aftershock, and before the rainy season (i.e., May 3, May 14, and May 25, 2015) and were compared with images taken in December 2014. The morphometric criteria used for landslide identification included the arcuate shape of a landslide scar, exposure of debris or bedrock in the scar, deposition of coarse debris downslope, and stronger surface reflectivity relative to the surrounding vegetation (Tiwari et al., 2017). Ice-and snow-covered areas on satellite images taken in May 2015 are not included because the landscape frequently changes due to snowfall, meaning that landslide detection is difficult. The landslides were mapped as polygons and subdivided into two datasets consisting of new and enlarged landslides. New landslides are those only recognised in the post-earthquake image, and enlarged landslides are those identified in a preearthquake image and cover a greater area in the post-earthquake images, and are mapped together with the pre-existing landslides. We defined the source, transition, and deposition areas within Higher Himalayan Crystallines, outside our survey area. Those landslides occurred on very steep slopes and may have been controlled by joint systems within the crystalline rocks.
In comparison with landslides induced by the 2008 Wenchuan earthquake (Mw 7.9) (Chigira et al., 2010), the 2015 Gorkha earthquake induced more small-scale landslides ( Fig.  3) with no comparable gigantic landslides (Fig. 3). This difference may be attributed to the Gorkha earthquake occurring in the dry season, with no heavy precipitation recorded at the time (Tsou et al. 2017). Antecedent rainfall can contribute to landslide formation by raising the groundwater level and increasing the pore water pressure (Chigira, 2014;Higaki et al., 2015). Even though gigantic landslides were not induced by the 2015 Gorkha earthquake, Collins and Jibson (2015) conducted helicopter reconnaissance following the event and observed slope deformation such as ground cracks on ridge tops, which could develop into landslides and threaten downslope settlements. The cracks were also observed near the steep toe slopes of landslides, so their retrogressive development must be monitored to evaluate the possibility of future catastrophic failure. Fig. 4 shows the distribution of earthquake-induced landslides. Most landslides are concentrated on the steep slopes (>35°) of V-shaped inner gorges, bounded by convex slope breaks. This is evident in the Trishuli River catchment ( Fig. 5a  and b), where the steep inner gorges of gneiss and sandstone or quartzite are loosened by gravitational forces (Tsou et al., 2017). Discontinuities such as stratification, joints, and fractures each landslide polygon. To complement the image analysis, field investigations were conducted in the Gorkha, Nuwakot, Rasuwa, and Sindhupalchouk districts.

Landslide characteristics
A total of 13,097 new landslides and 750 enlarged landslides were identified (Fig. 1). Shallow-disrupted landslides are the predominant landslide type and are characterised by rock falls, debris slides, and rock (debris) avalanches. The highest landslide density was identified in the Gorkha, Nuwakot, and Sindhupalchouk districts. Landslide occurrence increased significantly to the north of PT2, where the terrain is steeper and receives a high mean annual precipitation, indicating that slopes destabilised by the Gorkha earthquake could be prone to further mass movements in the coming rainy seasons. The potential for debris flows to occur increases after an earthquake, as arose after the 1999 Chi-Chi earthquake in Taiwan (Lin et al., 2004) and the 2008 Wenchuan earthquake in China (Tang et al., 2016). Debris deposited on slopes or accumulated on valley floors and river beds may be mobilised as debris flows in the rainy season, and might continue to do so for years to come. The sediment load transferred downstream during a flood also increases along rivers with abundant earthquake-induced landslides in their catchment areas.
The individual landslides ranged from 10 to 3.2 × 10 5 m 2 in area (Fig. 2), covering a cumulative area of 5.4 × 10 7 m 2 or 0.7% of the study area. Our landslide inventories include landslides with areas < 10 3 m 2 , which account for 29% and 7% of the number of new and enlarged landslides, respectively (Fig.  2). These smaller landslides can destroy buildings and roads, and can cause more damage than larger landslides.
Numerous landslides can be seen on satellite images to the northeast of Kathmandu and to the west of Kodari in the  are present in the rocks, and are favourable for the generation of earthquake-induced landslides (e.g. Tsou et al., 2017). The Sun Koshi and Bhote Koshi catchments also have many landslides on inner gorge slopes (Fig. 4).
Many steep scarp slopes (infacing slopes >35°) along mountain ridges also failed in the Trishuli River, Bhote Koshi, Budhi Gandaki, and Daraundi Khola catchments. The scarp slopes are SW-facing, where the foliation dips into the slope and litho-structural control is mainly exerted by bedding and joints ( Fig. 5c and d). Landslides are particularly common on scarp slopes comprising carbonate rocks in the Bhote Koshi and Budhi Gandaki catchments. Dissolution of calcareous rocks is observed along fractures, creating voids that reduce the mechanical strength and resilience of the strata to shaking. This characteristic was also observed in the 2005 Northern Pakistan earthquake (Sato et al., 2007) and the 2008 Wenchuan earthquake (Chigira et al., 2010).
Landslides on terrace scarps were also observed ( Fig.  5e and f). Terrace deposits containing larger boulders failed and struck residential houses and destroyed roads, suggesting that terrace deposits are highly susceptible to earthquake shaking. This type of landslide was observed in Kodari and to the south of Arughat and Barpak. Lastly, large mass movements associated with deep-seated landslides were not induced by the earthquake, but their headscarps and steep toes were susceptible to shaking, and small debris slides formed because of the Gorka earthquake. Our results reveal that the majority of the earthquake-induced landslides are located on V-shaped inner gorge slopes, terrace scarps, and the steep scarp slopes of mountain ridges. Field observations confirm that shaking may be amplified in the convex parts of V-shaped inner gorges, terrace scarps, and crest slopes (Keefer, 1984;Wakai and Ugai, 2004).
Inner valleys are highly vulnerable to earthquake-induced landslides, and many hydropower stations and trunk roads were convex valley slope break; e.g., the Rasuwa Route (Pasang-Lhamu Road) passing through Dhunche, which is built in such a location, was scarcely damaged by the earthquake. In contrast, the Araniko Highway is located mainly along a river within inner valleys, and is at high risk of landslides during earthquakes, resulting in high levels of maintenance.
heavily damaged by the 2015 landslides (Chiaro et al., 2015). The construction of underground hydropower stations could reduce the costs of maintaining penstocks, power houses, and access roads, and would provide a stable supply of electricity, although construction costs would be much higher. Important trunk roads would be better located on gentler slopes above the

CONCLUSIONS
This study presented the distribution of landslides induced by the Gorkha earthquake in 2015, in a mountainous area of 7.8 × 10 3 km 2 between the epicentres of the main shock (Mw 7.8) and the largest aftershock (Mw 7.3). We mapped 13,847 earthquake-induced landslides, mainly shallow-disrupted landslides, which included 750 enlarged pre-existing landslides and 13,097 new landslides. The landslides often occurred on the steep slopes of V-shaped inner gorges, scarp slopes, and terrace scarps. The landslides on inner gorges and scarp slopes were associated mainly with fractured rock masses, whereas landslides on the terrace scarps occurred where large bounders failed from terrace deposits. Our findings suggest that these slopes are prone to earthquake-induced landslides and that geomorphological classification may mitigate future landslide disasters. Furthermore, attention needs to be paid to possible future disasters such as debris flows and retrogressive development of ground cracks. Gentler slopes above the convex valley slope break are suggested places for road construction.