Geotechnical Field Reconnaissance: Gorkha (Nepal) Earthquake of April 25, 2015 and Related Shaking Sequence

The April 25, 2015 Gorkha (Nepal) Earthquake and its related aftershocks had a devastating impact on Nepal. The earthquake sequence resulted in nearly 9,000 deaths, tens of thousands of injuries, and has left hundreds of thousands of inhabitants homeless. With economic losses estimated at several billion US dollars, the financial impact to Nepal is severe and the rebuilding phase will likely span many years. The Geotechnical Extreme Events Reconnaissance (GEER) Association assembled a reconnaissance team under the leadership of D. Scott Kieffer, Binod Tiwari and Youssef M.A. Hashash to evaluate geotechnical impacts of the April 25, 2015 Gorkha Earthquake and its related aftershocks. The focus of the reconnaissance was on time-sensitive (perishable) data, and the GEER team included a large group of experts in the areas of Geology, Engineering Geology, Seismology, Tectonics, Geotechnical Engineering, Geotechnical Earthquake Engineering, and Civil and Environmental Engineering. The GEER team worked in close collaboration with local and international organizations to document earthquake damage and identify targets for detailed follow up investigations. The overall distribution of damage relative to the April 25, 2015 epicenter indicates significant ground motion directivity, with pronounced damage to the east and comparatively little damage to the west. In the Kathmandu Basin, characteristics of recorded strong ground motion data suggest that a combination of directivity and deep basin effects resulted in significant amplification at a period of approximately five seconds. Along the margins of Kathmandu Basin structural damage and ground failures are more pronounced than in the basin interior, indicating possible basin edge motion amplification. Although modern buildings constructed within the basin generally performed well, local occurrences of heavy damage and collapse of reinforced concrete structures were observed. Ground failures in the basin included cyclic failure of silty clay, lateral spreading and liquefaction. Significant landsliding was triggered over a broad area, with concentrated activity east of the April 25, 2015 epicenter and between Kathmandu and the Nepal-China border. The distribution of concentrated landsliding partially reflects directivity in the ground motion. Several landslides have dammed rivers and many of these features have already been breached. Hydropower is a primary source of electric power in Nepal, and several facilities were damaged due to earthquake-induced landsliding. Powerhouses and penstocks experienced significant damage, and an intake structure currently under construction experienced significant dynamic settlement during the earthquake. Damage to roadways, bridges and retaining structures was also primarily related to landsliding. The greater concentration of infrastructure damage along steep hillsides, ridges and mountain peaks offers a proxy for the occurrence of topographic amplification. The lack of available strong motion records has severely limited the GEER team’s ability to understand how strong motions were distributed and how they correlate to distributions of landsliding, ground failure and infrastructure damage. It is imperative that the engineering and scientific community continues to install strong motion stations so that such data is available for future earthquake events. Such information will benefit the people of Nepal through improved approaches to earthquake resilient design