The Mw 6.9 Sikkim-Nepal Border Earthquake of September 18, 2011
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EERI Special Earthquake Report — February 2012 Learning from Earthquakes The Mw 6.9 Sikkim-Nepal Border Earthquake of September 18, 2011 Several teams from India investigat- Introduction shaking; a small number of these ed the effects of the Sikkim earth- collapsed or suffered irreparable quake between September 25th and An Mw 6.9 earthquake struck near the structural damage. Poor perfor- November 10th, 2011. This report is Nepal-Sikkim border on September 18, mance and widespread damage based on observations of the follow- 2011, at 18:10 local time. The earth- are of concern in important govern- ing contributors: C.V.R. Murty, S.T.G. quake triggered a large number of ment buildings, such as the sec- Raghukanth, Arun Menon, Rupen landslides and caused significant retariat, police headquarters and Goswami, A.R. Vijayanarayanan, damage to buildings and infrastruc- legislative assembly, perhaps some S.R. Gandhi and K.N. Satyanaray- ture. Sikkim was the most affected of the few engineered buildings in ana of the Indian Institute of Tech- state of India, followed by West Ben- Gangtok. The total loss of life in nology (IIT) Madras; Alpa Sheth of gal and Bihar. Neighbouring countries India is reported to be 78, 60 in VMS Consultants Pvt. Ltd., Mumbai; of Nepal, Bhutan, Tibet (China) and Sikkim, and the rest in West Bengal Arvind Jaiswal of EON Designers, Bangladesh sustained damage and and Bihar. The total loss has been Secunderabad; Hemant B. Kaushik losses to varying extent (Figure 1a). estimated at around US $500 mil- and Kaustubh Dasgupta of IIT Gu- The maximum shaking intensity is lion. wahati; Ajay Chaurasia of CBRI, estimated to be around VI+ on the Roorkee; Colonel Shashi Bhushan MSK scale. The earthquake was fol- Sikkim has a total population of of the Indian National Disaster Man- lowed by a series of aftershocks, two about 600,000 people (Figure 1b). agement Authority (NDMA); Deba- of which were M4.5 and M5.0 and hit Gangtok is the densest region in sis Roy of IIT Kharagpur; and R. within 75 minutes of the main shock. Sikkim, with a density of about Pradeep Kumar of the Indian Insti- 98,700 persons in an area of 77 sq tute of Information Technology, Hy- The landslides, rock falls, and mud- km (as per 2011 Census of India). derabad. Keya Mitra of Bengal En- slides were responsible for most loss This contrasts sharply with the gineering, Kolkata, and Durgesh C. of life and damage to infrastructure, density in North District of Sikkim, Rai, Goutam Mondal, Vaibhav Sing- as well as associated economic which has the smallest, at just about hal, Neha Parool, and Tripti Pradhan losses. There was also extensive loss 10 persons per sq. km. In the past of IIT Kanpur also reported on the of Buddhist monasteries and temples; decade, Sikkim has recorded an damaged sites. Eleven of the con- these heritage structures are built astounding 153% growth in its urban tributors were supported by the in random rubble masonry with mud population and a 5% decrease in NDMA. VMS Consultants and EON mortar. Most multi-story reinforced Designers sponsored the visits of concrete (RC) buildings were non- their members. The Building Materi- engineered and sustained consid- als and Technology Promotion Coun- erable damage due to earthquake cil, New Delhi, supported the visit of one contributor from IIT Mad- ras. IIT Guwahati supported the visit of its two faculty members. The IIT Kanpur team received support from the Poonam and Prabhu Goel Foundation at IIT Kan- pur. The reconnaissance teams together covered all four districts of Sikkim, with greater focus on the most affected region of North Sik- kim and the capital city of (a) Gangtok, and parts of North Bengal. Figure 1. ▲(a) key earthquake-affected areas; and ► (b) areas covered during the reconnais- This report is edited by sance, location of epicenter, and the rivers (b) C.V.R. Murty and Alpa Sheth. Teesta and Ringit. 1 EERI Special Earthquake Report — February 2012 (a) (b) Figure 2. Acceleration time histories for the main shock of 18th September 2011 Sikkim event recorded at (a) Gangtok, and (b) Siliguri (http://pesmos.in/2011/). its rural population. There may be a (Main Boundary Thrust [MBT] and The acceleration time history of further push for urban agglomera- Main Central Thrust [MCT]) crossing the main shock was recorded at tions in the future, which may make the state. A great earthquake of Mw Gangtok and Siliguri (Figure 2); the it easier for governance and disaster 8.1 struck in January of 1934 along PGA was 0.15g at Gangtok. The management in the challenging the Bihar-Nepal border along the inter- stochastic seismological projection terrain. plate boundary; other significant earth- is presented of the estimated mean quakes in this area in the last 50 years peak ground acceleration (PGA) Some parts of North Sikkim (Lac- include the September 2009 Bhutan contour in Figure 3a. The axi- hung and Lachen) and the hills of earthquake of Mw 6.1, the February symmetric contours are attributed Darjeeling in West Bengal are im- 2006 Sikkim earthquake of Mw 5.3, to the shallow focal depth of the portant international tourism desti- the August 1988 Bihar-Nepal earth- earthquake. The projected PGA in nations and have seen a growth of quake of Mw 6.5, and the November the meisoseismal region is as high multi-story RC buildings. Addition- 1980 Sikkim earthquake of Mw 6.0. as 0.35g; at Lachen, Ravangla, ally, North Sikkim shares a border Teesta, Chungthang and Mangan, with China and, for strategic rea- The Gangtok and Teesta lineaments, it is in the range 0.25g-0.29g, and sons, it is expedient for the Indian transverse to the Himalayas, are at Gangtok in the range of 0.15- government to maintain excellent responsible for many earthquakes in 0.20g. Sikkim lies in Seismic Zone roads and connectivity, even if it the region. Other prominent tectonic IV as per the Indian Standard may focus on a few key tourism and features in and around Sikkim include (IS:1893-Part 1, 2007) Criteria for strategic villages. These villages the Arun lineament and Dhubri fault in Earthquake Resistant Design of need to be better equipped for dis- the southeast, and the Kanchenjunga Structures, in which the design aster management and as tactical and Purnea-Everest lineament in the peak ground acceleration is 0.24g outposts. north. The focal mechanism solution for this zone. Pseudo-spectral for this event (as reported by USGS) accelerations of the ground motion Seismological Aspects indicates strike slip faulting and the recorded at Gangtok are similar to nodal plane coincides with the Teesta that of the code spectrum for the The state of Sikkim is spread on the and Kanchenjunga lineaments. The zone; code design spectral accel- Himalayan mountain range, also strike, dip, and focal depth of the fault erations at lower frequencies are known as the “Alpine-Himalayan plane have been reported as 313°, higher than those generated in this belt,” with the two main thrust faults 73°, and 35 km, respectively. event (Figure 3b). 2 EERI Special Earthquake Report — February 2012 In general, the maximum inten- sity of ground shaking in the entire affected area was VI+ on the MSK scale, but some poc- kets had higher shaking owing to site-specific amplification of ground motion on a hillock,e.g., at the ITBP quar- (a) (b) ters at Pegong in Chungthang (Fig- Figure 3. (a) Surface level peak ground acceleration (g) generated from stochastic seismological ure 4). Falling model. Black dots indicate location of landslides; (b) Comparison of recorded spectral accelera- rocks caused sig- tions during the earthquake and Indian Seismic Code design response spectrum at Gangtok. nificant damage to buildings and locations north of Dikchu; this is pos- reactivation in some cases was hydropower tunnel mouths in the sibly due to proximity to the epicenter not co-seismic, but developed over town of Chungthang. and steeper relief in the mountainous several days after the earthquake terrain. In the Chaday–Sankalang– because of heavy rainfall in area. Poor building performance was Dzongbu area, often these failures On-the-ground inspection revealed prevalent in towns sited on soft soil were found to have affected the signs of continual distress that layers of relatively flat river terraces highly weathered mica schists and indicated the post-earthquake land- and gradually sloping mountain phylites. The left bank of Lachung slides were inducements of already meadows, probably due to sus- chu in Chungthang was particularly ongoing slow mass movements. tained amplification of ground affected by several slope failures. Enquiry with the locals in Tukdah- shaking. Mudslides originating from Rangolikhola also corroborated mountain springs led to damage to Some incidents of reactivation of the landslide to be episodic, with buildings in Lachung (Figure 5). older landslides were reported, for a large event taking place several Even now, large boulders of fragile example, the Teendharia (Figure 8) decades back. sedimentary rocks are precariously and Balason landslides of the Dar- balanced along hill slopes at many jeeling district and the Health Sec- Site-specific scientific approaches locations. retariat landslide in Gangtok. The are required for studying relatively Geotechnical Impacts While landslides are frequent in this region during rainy seasons, an Figure 4. estimated 354 new landslides were Classic dam- caused by the event and 48 old age in lower ones reactivated (Figure 6). These stories of RC slides damaged roads and bridges frame build- and disrupted relief operations to ing with large towns and villages that were com- block masonry pletely cut off, some for over three infill panels at weeks (Figure 7).