Performance of Built Environment in the 2015 Gorkha Earthquake and its Implications on Seismic Risk Mitigation Strategies

Abstract:

The M7.8 Gorkha earthquake of April 25, 2015 caused unprecedented loss of life and devastation in Nepal including Kathmandu valley and affected adjoining areas in the Indo-Gangetic (IG) basin. The largest of aftershocks, the M7.3 of May 12, 2015, originated from the same rupture area, caused further damages and casualties. A field investigation of the earthquake affected areas provided an opportunity to study the vulnerability of the built environment and re-assessment of the present day risk exposure of the region which was worst affected in the M8.1 1934 Bihar-Nepal earthquake.

The recent earthquake exposed the seismic vulnerability of the built-environment caused by lack of earthquake-resistant construction practices. Heavy damages were observed to old masonry buildings including several heritage structures. Even recent constructions of RC buildings in Kathmandu suffered moderate damage to complete collapse due to many unique and inherently poor construction features, such as open ground storey, inadequate size and poor reinforcement detailing of columns, poor geometric configuration of the buildings, insufficient spacing between adjacent buildings, projection of walls beyond the column lines, weak and slender masonry infill walls, lack of proper site investigation for constructions on sloping ground, etc.

The combination of high hazards due to large earthquakes of the Himalayan collision boundary and ever-growing vulnerable built and human environment creates a situation of unacceptably high levels of risk in the Nepal and densely populated IG basin. Since maximum shaking intensity in the heavily populated IG basin was between VI and VII, most structures did escape serious damage, but they continue to remain vulnerable for future design level earthquakes. Moreover, the large scale of liquefaction and soil instability observed in the larger shaking of M8.1 1934 earthquake, may have multiplying effect on structural damages.

Despite the available knowledge base of effective risk mitigation strategies, it is unfortunate that societies are not adequately prepared due to lack of implementation and, therefore, the seismic risk in the region capable of large to great earthquakes has risen to unacceptable levels which may lead to a large-scale disaster, if not mitigated.