In the aftermath of the Gorkha Earthquake that hit Nepal on 25 April, ICIMOD joined hands with regional and international experts and institutions to map the positions of landslides and debris flows and where they had blocked river valleys. The information collected by the team was provided directly to the Government of Nepal to assist in relief efforts and was instrumental in the formation of a Geohazards Task Force by the government.

Later, ICIMOD in collaboration with other experts undertook several studies including field surveys, airborne observations, and remote sensing mapping to assess the occurrence and impact of the geohazards induced by the earthquake and its aftershocks. Many professionals around the world sacrificed their days and nights to gather and share information. This publication presents the results of this work together with findings from several other related studies.

One important lesson we have learnt in the wake of the Nepal Earthquake is the importance of regional and global cooperation in the face of disaster of such a scale. Individuals will rise to the cause, and their results will be stronger with an organized platform. Countries in the Hindu Kush Himalayas, like Nepal, are highly susceptible to geo-hazards posing grave risk to settlements and infrastructures. This is where knowledge and specialized institutions can play critical role by providing geo-information to the governments and other actors with satellite-based data and analyses to inform rescue and relief.

The Hindu Kush Himalayan (HKH) region is geologically fragile with unstable slope-land systems, and geohazards such as landslides and debris flows are common. The people of the region are very vulnerable to such natural hazards, a vulnerability compounded by the social conditions. The region also falls in a high seismic zone; earthquakes are a frequent phenomenon and cause significant loss of lives and property.

The Gorkha Earthquake devastated large parts of the country. The main shock of 25 April and several other aftershocks including that of 12 May caused the death of about 9,000 people, injured 22,000, and was responsible for loss and damage equivalent to USD 7 billion. This study indicates that the main geohazards induced by the Gorkha Earthquake were landslides, river channel constriction and damming, and avalanches with debris flow and airburst. The number of landslides was large (more than 4,000), but much less than that induced elsewhere by other earthquakes of similar magnitude.

Results from the study have shown that damage from earthquake-induced geohazards can be considerable, and there is the need to treat geohazards separately since their nature and effects and mitigation and adaptation options are different. Similarly, the secondary effect of the damage from geohazards is likely to be comparatively much higher than that of the direct earthquake impact in terms of loss of livelihoods, blocking of movement of people, goods, and services, and loss of revenue from trade and energy supply.

New data has revealed that the Gorkha Earthquake did not release all of the stress that had built up underground along the Himalayan arc. Due to the residual stress, likelihood of another large earthquake occurring in the future remains. Seismologists say the damage from a possible large earthquake could be much worse.

We hope that the findings and the recommendations provided by the authors in this publication will help policy and decision makers in Nepal and other regional member countries in their efforts to prepare for geohazards and improve geohazard management.

I gratefully acknowledge the timely support provided by the Governments of India (Indian Space Research Organization), Pakistan (Space and Upper Atmosphere Research Commission), China (Chinese Academy of Sciences), and Nepal, as well as other bodies like the National Aerospace and Space Administration, the University of Arizona, United States Agency for International Development, Environmental System Research Institute, Japan Aerospace Exploration Agency, Digital Globe, US Geological Survey, signatories to the International Charter on Space and Major Disasters, and the many professionals who gave willingly their precious time in a time of need.

David James Molden

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