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The M6.9 Sikkim Earthquake of 18 September 2011 caused widespread devastation in the state and adjoining areas from disrupting the road network to damaging structures of commercial, public, and religious values. This event presented another opportunity to further the understanding of earthquake risk of the affected region as well as of the North-Eastern Himalayan region, which have similar patterns of seismicity, built environment and construction practices. The general pattern of damage to structures, landslides, rockfalls, etc. was consistent with the shaking associated with this event. However, collapses of many buildings and damages to structures were disproportionate to the observed intensity of shaking, primarily due to poor compliance with seismic codes, inferior quality of raw materials and shoddy workmanship. Consequently, the seismic risk in the region is growing at an alarming pace with increasing inventory of vulnerable construction. The current event provides a preview of what is likely to happen in the event of a larger earthquake which the Himalayan region has witnessed in the past. This article discusses the construction practice in Sikkim Himalayas and their seismic performance in the recent earthquake, highlighting the vulnerability of inventories and suggests steps to mitigate the seismic risk for future events.

Potential failure mechanisms representing various limit equilibrium states of geosynthetic-reinforced soil walls/slopes are employed to study the behavior of reinforced soil walls/slopes. Parametric analyses were conducted to investigate the effects of varying the values of pore water pressure, soil strength, geosynthetic strength, soil-geosynthetic interaction coefficients, vertical spacing of geosynthetic, surcharge load, facing slope, and backslope. In the analyses, geosynthetic-reinforced soil walls/slopes are assumed to have competent foundations. Seepage forces and other dynamic forces such as earthquake forces have not been taken into account. Similarly, the stabilizing effects of facing rigidity and strength are neglected. The results are presented in a nondimensional graphical form. Methods of optimizing the design of geosynthetic-reinforced soil structures are suggested.

Buddhist monasteries in the Sikkim region have conserved and portrayed the art of Tibetan and Chinese architectural style through centuries. These historic structures have sustained varied degrees of damage due to earlier earthquakes. Their performance in the recent Sikkim earthquake of M 6.9 on 18 September 2011 shows their high seismic vulnerability. A quick seismic assessment using certain simplified indices suggests higher vulnerability of damage for these heritage structures. A post-earthquake ambient vibration test established these monastery temples as shortperiod structures with fundamental period of 0.23 to 0.37 s. A finite element analysis of one of these temples has been done to study its dynamic behaviour. The response spectrum and static nonlinear pushover analysis highlighted vulnerable portions of stone masonry walls and provided useful insights for proper retrofitting to mitigate damage in future earthquakes.

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