LANDSLIDES TRIGGERED BY THE 2015 GORKHA , NEPAL EARTHQUAKE

The 25 April 2015 Gorkha Mw 7.8 earthquake in central Nepal caused a large number of casualties and serious property losses, and also induced numerous landslides. Based on visual interpretation of high-resolution optical satellite images preand post-earthquake and field reconnaissance, we delineated 47,200 coseismic landslides with a total distribution extent more than 35,000 km, which occupy a total area about 110 km. On the basis of a scale relationship between landslide area (A) and volume (V), V = 1.3147 × A, the total volume of the coseismic landslides is estimated to be about 9.64 × 10 m. Calculation yields that the landslide number density, area density, and volume density are 1.32km, 0.31%, and 0.027m, respectively. The spatial distribution of these landslides is consistent with that of the mainshock and aftershocks and the inferred causative fault, indicating the effect of the earthquake energy release on the pattern on coseismic landslides. This study provides a new, more detailed and objective inventory of the landslides triggered by the Gorkha earthquake, which would be significant for further study of genesis of coseismic landslides, hazard assessment and the long-term impact of the slope failure on the geological environment in the earthquake-scarred region. * Corresponding author


INTRODUCTION
The 25 April 2015 Mw7.8 Gorkha (Nepal) earthquake caused at least 8800 deaths and tens of thousands of injuries (Hashash et al. 2015).This shock also triggered a large number of landslides and other secondary geological effects.The studies on inventory, mechanism, spatial distribution, and assessment of coseismic landslides are of great importance to mitigation of the potential subsequent hazards in the affected area (Harp et al. 2011;Xu et al. 2014a;Xu et al. 2014b;Xu et al. 2014c;Xu 2015;Xu et al. 2018).This work presents a new inventory of the landslides induced by the 2015 Gorkha event based on visual interpretation of high-resolution satellite images before and after the earthquake as well as verification at selected sites in the field, which is much more detailed and complete than previous work (Kargel et al. 2016;Gnyawali and Adhikari 2017;Martha et al. 2017;Roback et al. 2017).Our results show that the earthquake triggered at least 47,200 landslides, with a total occupation area of about 110km 2 .These landslides are distributed in an ellipse with an area of about 35,700km 2 .This inventor would be useful for further study of genesis of seismic landslides, impact of slope failure on the geological environment and hazard assessment in the affected region.

GEOLOGIC SETTING
The 2015 Gorkha Mw 7.8 earthquake occurred in the central Nepal Himalaya, where the Indian plate is underthrust beneath Asia (Powell and Conaghan 1973;Tapponnier et al. 1986;Royden et al. 2008).The present Himalayan orogen is dominated by three approximately parallel main thrusts with low dip angles (~10°), i.e. the Main Frontal Thrust (MFT), the Main Boundary Thrust (MBT), and the Main Central Thrust (MCT), from south to north in the order of increasing ages of thrust initiations (Upreti 1999) (Figure 1).This fault system is one of the most active tectonics in the world.The 2015 Gorkha earthquake ruptured from east to west, and most aftershocks occurred in a rectangle area about 140 km in NWW-SEE direction and 50 km in NNE-SSW direction east to the mainshock epicentre.The seismogenic fault of this earthquake is considered to be the Main Frontal Thrust (MFT), but the rupture did not reach the ground surface, confined to depths 5-15km below (Hubbard et al. 2016).Within 45 days of the main shock, a total of 553 aftershocks were recorded with Mw4.0+ (Adhikari et al. 2015).The largest aftershock occurred on May 12, with magnitude Mw7.3, which is located on the eastern boundary of the aftershock distribution area.It means the region between the Mw7.8 mainshock and the largest aftershock of Mw7.3 represents rupture extent of the seismogenic fault.

DATA AND METHODS
In this study, by combination of visual interpretation of highresolution satellite images and field investigations (Xu et al. 2015), a detailed landslide inventory for the Gorkha earthquake was prepared.The pre-earthquake satellite images, mainly from the Google Earth platform, and other high-resolution and good-

Langtang landslide:
It is the largest landslide triggered by the Gorkha earthquake, which buried the Langtang village and killed about 400 victims (Figure 7).Its volume is estimated to be more than 10,000,000m 3 .The landslide is composed of debris materials, ice and snow.The comparison of preearthquake satellite image (Figure 6a) from Google Earth platform and post-earthquake GF1 (China) satellite image (Figure 7b) reveals the extent of the landslide damage.The Langtang village was completely destroyed by the large, wet, debris-and ice-laden avalanches.

Landslide inventory
Statistics show that at least 47,200 landslides were triggered by the Gorkha earthquake, which are distributed in an oval area about 35,700 km 2 with its long axis in NWW-SEE direction, which is consistent with the seismogenic fault and distribution of aftershocks.The total occupation area of these landslides is about 110km 2 (Figure 8).According to the landslide power law relationship of "area (A) -volume (V)", V = 1.3147 × A 1.2085 (Xu et al. 2016b), the total volume of these landslides is estimated to be about 964 million cubic meters.In the landslide distribution area, the landslide point density is 47200/35,664km 2 = 1.32km -2 , the landslide area percentage is 110.46km 2 /35,664km 2 × 100% = 0.31%, and the landslide volume density is 964 × 10 6 m 3 /35,664km 2 =0.027m.The largest landslide has an area more than 600,000 m 2 , and the smallest one only 1~2 m 2 .Of the 47,200 coseismic landslides, 22 landslides have area exceeding 100,000m 2 , 2,042 between 10,000m 2 and 100,000m 2 , 18,359 between 1,000m 2 and 10,000m 2 , 24,109 between 100m 2 and 1,000m 2 , and the remaining 2,668 less than 100m 2 .
Figure 8. Spatial distribution of landslides triggered by the Gorkha earthquake

Landslide density
This work took 1 km as the search radius and selected the Kernel function type to construct the coseismic landslide point density distribution map with resolution of the grid set to 100 m (Figure 9).The maximum landslide density is as high as 132.2km -2 .The area was divided into nine types based on classification of landslide density, which are 0, 0-1 km -2 , 1-2 km -2 , 2-5km -2 , 5-10 km -2 , 10-20 km -2 , 20-50 km -2 , 50-100 km -2 , and 100-132.2km -2 (Figure 6).The high landslide density area is located between the Mw7.8 mainshock and the Mw7.3 aftershock.The length and width of the area are about 170 km and 40 km, respectively.These data can help us to understand quantitatively the development and spatial distribution of Gorkha earthquake-triggered landslides.

CONCLUSIONS
This study established a detailed, objective, and completed inventory of landslides triggered by the April 25, 2015 Gorkha, Nepal Mw7.8 earthquake using remote sensing and field observations.The results show that the earthquake triggered at least 47,200 landslides which occupy a total area about 110 km 2 .The total volume of the landslides is estimated to be 964 million cubic meters.These landslides are distributed in an oval area of 35,700 km 2 with its long axis in the NWW-SEE direction.
Statistics show the landslide density, area percentage and volume density are 1.32 km -2 , 0.31% and 0.027m, respectively.The landslide density distribution map was also prepared, showing the maximum density of coseismic landslides is as high as 132.2 km -2 .This study provides a detailed landslide inventory for the 2015 Gorkha, Nepal earthquake, which would be useful for further analyzing the genesis of seismic landslides, long-term impact of slope failure on the environment and hazard assessment of in the affected area.

Figure 1 .
Figure 1.Map showing regional tectonic setting of the 2015 Gorkha earthquake.MFT, Main Frontal Thrust; MBT, Main Boundary Thrust; MCT, Main Central Thrust.
quality satellite images of multiple sources fully cover the study area.The post-earthquake satellite images have two main sources, one is from China Centre for Resources Satellite Data and Application, including GF1, GF2, ZY02C, ZY3, and CB04 (Figure2); the other is true colour or panchromatic images from Google Earth platform, such as CNES/Astrium and DigitalGlobe (Figure3).The coverage of the two sources of satellite images almost covers the entire landslide distribution area; only about 1% (~350km 2 ) of the region near the boundary of the landslide distribution area is beyond the coverage.The Landsat8 image of 15m resolution was supplemented to this 1% of the landslide distribution area.In addition, detailed field investigations were carried on along the Pasang Lhamu Highway that connects Kathmandu, Nepal and Gyirong County and the Araniko Highway that links Kathmandu, Nepal and Nielamu County, China for tens of days by cars or by walk(Xu et al. 2016a).

Figure 9 .
Figure 9. Map showing density of landslides triggered by the 2015 Gorkha earthquake