Earthquake-induced landslide distribution and hazard mapping Megh Raj Dhital Central Department of Geology, Tribhuvan University
Contents Geological setting of Gorkha earthquake Peak ground acceleration and landslides Frequency of High-intensity rainfall Types of failure Discontinuities and failure pattern Newmark displacement and failure relationship Conclusions
Geological map of the 25 April 2015 earthquake-affected area
Peak ground acceleration due to the 25 April Gorkha earthquake (Source: USGS)
Rockslide hazard map of earthquake-affected area and contours of rainfall frequency (exceeding 100 mm in 24 hrs for 30 years)
Types of landslides triggered by the 25 April Gorkha earthquake
Rockslide partially damming the Bhote Koshi at Rasuwagadhi
Shallow rockslides and rock falls in the Mailung Khola and Trishuli River
Shallow failures near Chaku, right bank of Bhote Koshi, Sindhupalchok Wedge failure Plane failure Talus Talus
Shallow failures controlled by discontinuities in Bhote Koshi, Sindhupalchok Wedge slide Wedge slide Plane slide Wedge slide Rock fall Rock fall Wedge slide Talus Talus Talus Upper Bhote Koshi Powerhouse
Failures controlled by discontinuities in Khokundol Khola, Sindhupalchok Wedge failure Plane failure Talus
Position of instabilities on the landscape, Bhote Koshi, Sindhupalchok Tension cracks Debris fall Debris slide Wedge slide Rock fall
Shallow slides near Sundrawati, Upper Tama Koshi Road
Soil slide at Km 10+500 of the Upper Tama Koshi Road Before earthquake After earthquake
Rock falls and talus cones near Gongar confluence, Tama Koshi River
Three sets of mutually perpendicular discontinuites on Arniko Highway Sindhupalchok 969 Measurements between Bahrabise and Liping
Slope orientation and distribution of landslides Most of failures fall on the dip-normal slope
Geological map of the Bahrabise-Liping area, Sindhupalchok District Metamorphic rocks are resistant and gently dipping due north
Static Safety factor of Bhote Koshi area, Sindhupalchok District where C = effective cohesion in Pa (N/m 2 ) γ = unit weight of soil (N/m 3 ) m = Z w /Z (dimension less quantity) Z = depth of failure surface below the slope (m) Z w = height of water table above failure surface (m) γ w = unit weight of water (N/m 3 ) β = slope angle (in degrees) φʹeffective angle of friction
Critical acceleration map of the Bhote Koshi area, Sindhupalchok District Newmark (1965)
Newmark displacement map of the Bhote Koshi area, Sindhupalchok District (Method I) Jibson et al. (1998, 2000) Hsieh and Lee (2011) Wilson and Keefer (1985)
2 Newmark displacement map of the Bhote Koshi area, Sindhupalchok District (Method II) Hsieh and Lee (2011) Dobry et al. (1978) Wilson, in Jibson (1993)
Internal relief map of the Bhote Koshi area, Sindhupalchok District Slopes with higher (more than 50 m) internal relief suffered the most
Conclusions Mainly resistant metamorphic rocks prevail in the area affected by the Gorkha earthquake. Peak ground acceleration and landslides do not correlate well. Frequency of High-intensity rainfall is highest near Pokhara and further west. Mainly shallow rockslides were triggered by the quake, whereas old and deep slides were insensitive to the earthquake. Newmark displacement and seismically induced failures are not in good argreement. Hence, some other factors, such as slope height, amplification of vibration at the crest, and resonance of shear waves.
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