Insights into the geohazards triggered by the 2017 Ms 6.9 Nyingchi earthquake in the east Himalayan syntaxis, China

Abstract On 2017-11-18, an Ms 6.9 (Mw 6.4) magnitude earthquake struck the NBTN (Namcha Barwa tectonic node), located in the EHS (east Himalayan syntaxis) on the south Tibetan Plateau, China, with an epicenter at 29.87°N and 95.02°E and a focal depth of approximately 10 km. Multiple remote sensing interpretations and field investigations revealed that at least 1816 landslides and 3 dammed lakes were triggered and formed by the Nyingchi earthquake, and the landslide-affected area was approximately 527 km2. Most geohazards are concentrated in the southeast section of the epicenter, and compared to the outer section (503 landslides), the inner section (1313 landslides) exhibits a notable concentration. The total landslide area is 32.7 km2 and follows an exponential distribution, namely, Y = 0.023*e[-(Log(x)-3.06)^2/0.2738]. The estimated landslide volume is (14.096 + 3.023/-1.844)*106 m3. Rock avalanches are the most common failure pattern in the landslide-affected area, and the largest landslide area reaches 3.6*106 m2. >60% of the landslides occur at elevations ranging from 2600 to 3800 m and at distances to the river midline ranging from 400 to 2400 m, and this range coincides with the upper-top section of the slope. Landslides occurred most frequently in schist and gneiss (1313 landslides, 72.3%). Among the 3 dammed lakes, two dammed lakes were directly triggered by the Nyingchi event, and one dammed lake was formed by an active debris flow gully, namely, the Sedongpu gully. Moreover, numerous dangerous deposits remain in the main flow stream, and special attention should be given to these deposits. Finally, the AHP (analytic hierarchy process)-CF (certainty factor) method is adopted to establish a landslide susceptibility map for further risk assessment.