Quake-induced geohazards


KATHMANDU: The April 25 earthquake and its powerful May 12 aftershock not only damaged the man-made structures and buildings but also induced geohazards including landslides and avalanches in the affected region. The Gorkha earthquake sequence affected sources of water and blocked streams and rivers at many places. Several valley-blocking landslides were reported from Myagdi to Dolakha.

The main shock triggered a massive debris avalanche in the Langtang Valley of Rasuwa district, wiping out entire Langtang village and killing at least 300 people. Likewise, it caused avalanche in Mt Pumori and hit the Mount Everest base camp, where hundreds of climbers and their guides had gathered.

According to a USGS report, experts saw thousands of landslides during reconnaissance. In general, landslide concentrations were highest near the epicentre in Gorkha, but significant landslide concentrations extended about twice as far to the east than the west, the report said. “This likely was the result of the eastward-directed fault rupture of the main shock as well as the occurrence of the M 7.3 aftershock east of the main shock,” it added.

The International Center for Integrated Mountain Development in collaboration with international agencies and scientists mapped and assessed the earthquake-induced geohazards.

“With the help of international volunteer group of scientists and analysts from USGS, NASA and Europe, satellite images were examined and the hazards mapped,” said Dr Arun Bhakta Shrestha, programme Manager for ICIMOD’s Programmes on River Basins and Cryosphere and Atmosphere. “We mapped 4,312 landslides across the affected region.”

Further data analysis showed 5,159 significant landslides had occurred in 14 districts affected by the earthquake sequence.

The hazards assessment revealed that 464 landslides had directly affected physical infrastructure.

Examination of satellite images showed glacial lakes did not face threat of sudden outburst though some of them were affected by tremors.

According to the National Planning Commission, the earthquakes damaged 21 hydropower projects. This inflicted a loss of at least USD 180 million. Likewise, landslides directly damaged 42 km roads while affected 236 km only.

Though landslides occur regularly in Nepal due to its steep terrain and heavy precipitation during monsoon, vulnerability may increase during subsequent seasons as a result of the widespread landslides and ground fracturing that occurred from the 2015 earthquakes, according to the USGS.

“As the Gorkha earthquake and its aftershocks have destabilised the topography, landslides have been further induced and they will continue to occur in future also,” Dr Shrestha said, pointing out the need for risk assessment and mapping of landslides to determine whether certain areas are fit for human settlement.

Terming the Gorkha earthquake a wake-up call, he further opined that the government should facilitate building up a risk monitoring system as Nepal continuously faces threats of geohazards including earthquakes and landslides.

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  • Fault segments of the 1833 earthquake and the 2015 Gorkha Earthquake overlapped
  • The 1505 earthquake that hit western Nepal devastated Delhi and Lhasa also.
  • After 1505, no big earthquake has occurred in western Nepal
  • No big earthquake, except the Magnitude 8.7 Assam earthquake in 1950, has occurred in the region east of Koshi since 1255
  • Kathmandu Valley witnessed about 1 m uplift while the Himalayas subsided by about 0.6 m
  • Coordinates of Kathmandu Valley and many other places have significantly changed after the April 25 earthquake while Birgunj has come closer to Kathmandu as India plate slipped below the Tibetan plate
  • Mount Everest’s height was not affected much as it lies beyond the Gorkha-Dolakha fault segment