EERI Newsletter, November 2011 Volume 45, Number 11
Learning from Earthquakes M 6.9 Sikkim Earthquake of September 19, 2011 This report was contributed by EERI 4.6 within 30 minutes of the members C.V.R. Murty, Indian In- main shock. The region falls stitute of Technology (IIT) Madras; into the second highest seis- Alpa Sheth, VMS Consultants, Mum- mic zone of the Indian Seis- Figure 1. bai; and Durgesh Rai, IIT Kanpur. mic Code IS:1893, Epicenter An earthquake of magnitude 6.9 Zone IV, with an ex- of Sikkim pected shaking inten- earthquake occurred at 6:11 pm local time on EPICENTER September 19, 2011, with an epicen- sity of VIII. and route ter at the India-Nepal border region As the region is mountain- of damage (Figure 1); the focal depth has been ous and rains preceded survey trip. estimated as 19.7 km. The region is the earthquake, the event known to be seismically active, lo- triggered massive land- cated in the Alpine-Himalayan seis- slides. Damage to build- panel walls mic belt characterized by two major ings and infrastructure due to weak ! "! caused by landslides masonry ciated with the collision of the Indian was more severe than and large and Eurasian plates. The event was damage due to direct unsupported ! ground shaking in some length- or height- Nepal-Sikkim Himalayas, which has regions (Figures 2 and 3). to-thickness ratio. been a region of intense seismic Landslides cut off the severely af- # $ % fected areas, especially at higher had a symmetric and uniform grid of shaking was experienced in India’s altitudes, hampered rescue and re- beams and columns. Some buildings northeastern state of Sikkim, fol- lief work, and required the help of that had open stories collapsed, lowed by Tibet, Nepal, the Indian Army helicopters. usually at mid-height (Figure 4), or had severe structural damage. Like- states of Bengal, Assam, Bihar, and The affected area has a low popula- wise, buildings with asymmetry in tion density of an average of 88 per- boring countries of Bhutan and sons/sq. km. The state capital of ing torsion, also were severely pun- Bangladesh. The maximum shaking Gangtok is the biggest city in the ished. Single RC buildings collapsed intensity attributed to this earth- $ far from the meizoseismal area; e.g., quake is VIII in the North Sikkim dis- empirically constructed reinforced at Jorethang in South Sikkim district trict in the regions of Chungthang concrete (RC) buildings of four to about 80 km from Gangtok (Figure 5). and Lachung. The total death toll is nine stories adjoining each other on Traditional Ekra housing made of about 112 persons, with 60 reported small plots, with buildings extending bamboo or wood framing with light- fatalities in the state of Sikkim alone. to the property lines. A majority of The earthquake was followed by these buildings exhibited extensive plaster behaved exceptionally well. aftershocks of magnitude 4.8 and damage to unreinforced masonry continued on next page
Figure 2. Landslide near the cable-stayed bridge at Nayabazar (South Sikkim District) restricted the use of the bridge for relief work after the earthquake (photo: Figure 3. Damage due to rockslide and mudslide at C.V.R. Murty). Lachung (photo: D. Rai).
6 EERI Newsletter, November 2011 Volume 45, Number 11
Figure 4. Pancaking of middle two stories in a Figure 5. Figure 6. All columns supporting roof of nine-story building at story house in Jorethang, South Sikkim Phudong Monastery, North Sikkim, failed. Gangtok, East Sikkim (photo: A. Sheth). (photo: A. Sheth). (photo: D. Rai). M 6.9 Sikkum Earthquake ings of the past few decades in- as the King’s monastery in Gangtok. clude those made in RC, with beams continued from previous page Sikkim is the second smallest state in India; about 31 hydropower Heavy damage has occurred to walls. In this RC construction, there projects have been sanctioned in temple structures (shrine halls) of is no frame action; the RC construc- the past three years, some of which various Buddhist monasteries (Fig- tion mimicked the historic wooden are close to completion. No serious ure 6). Historic monastery temples construction. Both sets of monas- damage to these projects, which &