RECONSTRUCTION OF EARTHQUAKE DAMAGED RURAL HOUSES (REDRH)
The Sikkim region falls between Nepal and Bhutan and comprises the lesser active part of the 2500 km stretch of the active Himalayan belt. The seismic hazard scenario of Sikkim Himalaya appears to be underestimated considering its zone IV status in the seismic zonation map of India, unlike zone V for most of the Himalayan front (IS1893, 2002). One cannot overlook the fact that this region is surrounded by Great earthquake occurrences during the past, namely the 1934 Bihar - Nepal earthquake (M 8.3) to the west, the 1897 Shillong earthquake (M 8.7) to the southeast and the 1950 Assam earthquake (M 8.7) to the east (Raju 2007). In the recent past, Sikkim experienced an M 6.1 earthquake on 19th Nov, 1980 and more recently on 14th Feb, 2006 by an M 5.3 earthquake. Seismic hazard in the Sikkim region is further accentuated due to site amplification which increases from north to south along the Tista and Gangtok lineaments (Nath 2000, 2004). This is attributed to the presence of sedimentary and low grade metamorphic rocks in the Lesser Himalayas. Sikkim was rocked by an M 6.8 earthquake of intensity VII at 18:10 IST on 18th Sept 2011. The earthquake was centered about 64 northwest of Gangtok, Sikkim at a shallow depth of 19.7 km. This 2011 Sikkim Earthquake caused strong shaking in many areas adjacent to its epicenter lasting 30 - 40 seconds. Although earthquakes in this region are usually interplate in nature, preliminary data suggests the Sikkim earthquake was triggered by shallow strike-slip faulting from an intraplate source within the over-riding Eurasian Plate. Initial analyses also indicate a complex origin, with the perceived tremor likely being a result of two separate events occurring close together in time at similar focal depths. The strongest shaking occurred in Sikkim, with tremors felt in Nepal, Bhutan, Bangladesh and China. In India, the tremors were felt in Assam, Meghalaya, Tripura, parts of West Bengal, Bihar, Jharkhand and as far away as Uttar Pradesh, Rajasthan, Chandigarh and Delhi. This earthquake was followed by three aftershocks occurring at magnitudes of 5.7, 5.1, and 4.6 magnitudes within 30 minutes of the initial earthquake.
Assessment of Damage: The state of Sikkim has a fragile ecology being the steepest and highest state in the country, and the third highest landscape globally. It is a mountainous state crisscrossed by narrow valleys and steep cliffs. The young fold mountains are characterized by a weak geology, comprising of sedimentary and low grade metamorphic rocks which are prone to landslides. The state also experiences heavy monsoons with the average annual rainfall being to the tune of 2800 mm. There was colossal collateral damage due to a combination of precipitous terrain, weak geology, fragile ecology and heavy rainfall which amplified the impact of the earthquake creating a multiplier effect resulting in hundreds of natural calamities in the form of landslides, boulder falls and flash floods thereby magnifying the damage to human life and property many times. Dzongu region in North District which was the closest to the epicenter was the worst hit. In coastal areas, Tsunami poses a formidable post-earthquake threat as was evident during 2011 Japan Earthquake and the 2009 Thailand Earthquake. While in fragile mountain areas like Sikkim, the major damage was caused by the shower of landslides and boulder falls. Post earthquake fresh landslides in North Sikkim are visible as brown scars in the satellite map of NASA. The National Remote Sensing Centre, ISRO, Dept of Space also identified 354 new landslides post earthquake using remote sensing images. The increasing frequency and intensity of earthquakes with growing hazard due to rapid urbanization has renewed the urgency in improving the preparedness and making the infrastructure earthquake resistant. The rural housing sector has long been a priority sector of the government and has received sizeable public investments. It is an important social infrastructure, providing safety, identity and dignity to the owner. Presently there are a total of 92,000 rural households with a population of 4.5 lakh in the state as per Census of India, 2011. The assessment report of fully and severely damaged 12,548 rural and 478 urban houses was received from LR&DMD vide letter no 22/LR&DMD dated 15/11/2011.
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