### Assessment of existing and potential landslide hazards resulting from the April 25, 2015 Gorkha, Nepal earthquake sequence

By: Brian D. CollinsRandall W. Jibson**

                            https://doi.org/10.3133/ofr20151142

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-                                                                                                                             Document:                                                                                                                                                                                                                                                  **Report Version 1.1**                                                                                                     (20 MB pdf)                                                                                                                                     
- Dataset:  **Video data files**
- Version History:  **Version History**                            (2 kB txt)                        
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#### Introduction

First posted July 28, 2015

Revised August 24, 2015

**For additional information, contact:**
Geology, Minerals, Energy, and Geophysics Science CenterU.S. Geological Survey345 Middlefield Road, MS 901Menlo Park, CA 94025-3591http://geomaps.wr.usgs.gov/gmeg/

On April 25, 2015, a large (*M*7.8) earthquake shook much of central Nepal and was followed by a series of*M*>6 aftershocks, including a *M*7.3 event on May 12, 2015. This earthquake and aftershocks, referred to as the “Gorkha earthquake sequence,” caused thousands of fatalities, damaged and destroyed entire villages, and displaced millions of residents. The earthquakes also triggered thousands of landslides in the exceedingly steep topography of Nepal; these landslides were responsible for hundreds of fatalities, and blocked vital roads and trails to affected villages (fig. 1). Landslides caused by the Gorkha earthquake sequence continue to pose both immediate and long-term hazards to villages and infrastructure within the affected region. Some landslides blocked rivers and thus created another potential concern for villages located downstream.

With the support of the United States Agency for International Development (USAID), Office of Foreign Disaster Assistance (OFDA), and in collaboration with earthquake-hazard organizations from both the United States (for example, U.S. National Science Foundation Geoengineering Extreme Event Reconnaissance [GEER] Team) and Nepal (International Centre for Integrated Mountain Development [ICIMOD]), the U.S. Geological Survey (USGS) responded to this crisis by providing landslide-hazard expertise to Nepalese agencies and affected villages. In addition to collaborating with an international group of remote-sensing scientists to document the spatial distribution of landsliding in the first few weeks following the earthquake, the USGS conducted in-country landslide hazard assessments for 10 days beginning May 24, 2015. Much of the information obtained by the USGS during their time in Nepal was conveyed directly to affected villages and government agencies as opportunities arose. Upon return to the United States, data organization, interpretation, and synthesis began immediately to provide a rapid assessment of landslide hazards for use by Nepalese agencies during the 2015 summer monsoon (typically occurring from June through September).

This report provides a detailed account of assessments performed in May and June 2015 and focuses on valley-blocking landslides because they have the potential to pose considerable hazard to many villages in Nepal. First, we provide a seismological background of Nepal and then detail the methods used for both external and in-country data collection and interpretation. Our results consist of an overview of landsliding extent, a characterization of all valley-blocking landslides identified during our work, and a description of video resources that provide high resolution coverage of approximately 1,000 kilometers (km) of river valleys and surrounding terrain affected by the Gorkha earthquake sequence. This is followed by a description of site-specific landslide-hazard assessments conducted while in Nepal and includes detailed descriptions of five noteworthy case studies. Finally, we assess the expectation for additional landslide hazards during the 2015 summer monsoon season.

#### Suggested Citation

Collins, B.D., and Jibson, R.W., 2015, Assessment of existing and potential landslide hazards resultingfrom the April 25, 2015 Gorkha, Nepal earthquake sequence (ver. 1.1, August 2015): U.S. Geological Survey Open-File Report 2015–1142, 50 p., http://dx.doi.org/10.3133/ofr20151142.

ISSN: 2331-1258 (online)

#### Study Area

#### Table of Contents

- Introduction
- Seismological Background
- Methods
- Results
- Hazard Assessment of Selected Areas
- Potential Landslide Hazards Associated with the 2015 Summer Monsoon
- Conclusions
- References Cited
- Appendix 1. Video and Geographic Reference Files from Helicopter Reconnaissance

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