Seismic Behavior of Historic Masonry Monasteries in Sikkim in the 2011 M 6.9 Sikkim Earthquake: Case Study of Hee Gyathang Monastery, North Sikkim, India

The Himalayan region is, one of the most seismically vulnerable and tectonically active zones in the world. Sikkim is a state in North-East India, situated on the Himalayan arc. Sikkim is dotted with numerous Buddhist monasteries dating back to the 17th century. These historical monastery structures are a part of the rich cultural heritage of the state, being structures of social, cultural and religious significance to the community. The old monasteries are typically load bearing structures where stone masonry and timber are used extensively as columns, beams, floors, roofs and also in staircases. The monasteries follow some spatial characteristics such as regular geometrical shapes in plan with an assembly hall and front verandah, and are usually one or two storeys high with diminishing upper storeys topped by light weight sloped roofs. The M6.9 earthquake on September 18, 2011, in Sikkim has clearly demonstrated the vulnerability of the historical monastery structures. The evaluation of the earthquake vulnerability of monasteries is a necessary first step for their protection in future earthquakes. This paper analyzes the performance of one of the non-engineered monastery structure made with load bearing stone masonry the century old hilltop Hee Gyathang Monastery located in the North District about 30 kms from Mangan. Structural analysis by demand capacity method of this monastery proves that the monastery is unsafe after the 2011 M 6.9 Sikkim Earthquake.


Introduction
Sikkim is a state in North-East India, situated on the Himalayan arc that is made up of a number of parallel fault systems, namely, Himalayan Frontal Fault (HFF), Main Boundary Thrust (MBT), Main Central Thrust (MCT), Trans Himalayan Fault and the Indus-Tsangpo Suture [1]. In the Sikkim Himalayas, the MBT and MCT are not parallel which has been reported as a major controlling factor for earthquakes that has made the entire state highly earthquake prone [2]. Sikkim has about 250 Buddhist monasteries, the oldest dating back to the seventeenth century. The monasteries are made with load bearing stone masonry along with timber frame. An earthquake of magnitude 6.9 occurred on 18 th September at 18:11 hrs IST in Sikkim-Nepal Border region. The preliminary hypo-central parameters of this earthquake, as estimated by the Seismic Monitoring Network of India Meteorological Department (IMD) are given in Table 1. This region the monasteries are situated in hilly terrain not far from the China-Tibet border. The monasteries located in this area experienced extensive damages during this earthquake. Hee Gyathang Monastery was built in 1914 and is representative of the architectural and structural typology mentioned earlier. The monastery caters to the devotional needs of the Nyingmapa Sect of Buddhism. This remotely located monastery is rich in architectural value and is of special significance to devotees, adding to its socio cultural value for the community.

Location/Accessibility of Hee Gyathang
Monastery The Hee Gyathang Monastery ( Figure 1)   on the road from MangantoGor via Sankalan. The monastery is located 2.5 kms from the village and can be accessed only on foot through an arduous one hour hike through a steep pathway through dense forests, without any human habitation along the way ( Figure 2). The monasteryis built on a small patch of flat ground about 1600 sqm area on a hill top at an altitude of 1669 meter ( Figure 3). The geographical coordinates are simultaneously 27.4750 N and 088.5010 E.

Planning and Site Consideration
The old two storied load bearing monastery structure sits on a 500 mm high plinth amidst soft landscape.The ground floor has an assembly hall (8.7 m × 10.35 m) with an altar of 775 mm width at its rear end. The ground and first floors have the same plan footprint (Figures 4-6). Presently, the statues have been shifted to the new monastery while the old one lies abandoned. Both the floors have similar fenestration with two windows located on the northern and southern walls. The front of the monastery has a wooden verandah of 900 mm width (Figure 7). The fenestration pattern on the rear side is not the same on both the floors. While the first floor has two window openings, the ground floor has none ( Figure 8). It is noticed that the old windows have been replaced by new ones (Figure 9).

Planning and Site Consideration
The old two storied load bearing monastery structure sits on a 500 mm high plinth amidst soft landscape. The ground floor has an assembly hall (8.7 m × 10.35 m) with an altar of 775 mm width at its rear end. The ground and first floors have the same plan footprint (Figures 4-6). Presently, the statues have been shifted to the new monastery while the old one lies abandoned. Both the floors have similar fenestration with two windows located on thenorthern and southern walls. The front of     the monastery has a wooden verandah of 900 mm width (Figure 7). The fenestration pattern on the rear side is not the same on both the floors. While the first floor has two window openings, the ground floor has none ( Figure 8). It is noticed that the old windows have been replaced by new ones (Figure 9).

Structural System
The old monastery is made with Random Rubble stone masonry structure ( Figure 10) with a thickness of 600 mm. Four timber posts are located at the centre of the assembly hall, only two horizontal beams are observed above the wooden posts spanning only in the "X" direction ( Figure 11).The wooden posts are decorated at the top ( Figure 12). Two decorated wooden posts are located at the entrance of the front verandah ( Figure 13). No bands are present in the monastery. Floors are made with timber planks. Wooden rafters hold the timber plank in the first floor ( Figure 14). Roof is covered with G.I sheets (Figure 15).

Seismic Load Analysis
The Demand Capacity Ratio (DCR) method is an useful tool to determine the capacity of a building to resist seismic action, through       determination of the seismic demand and the calculation of the ratio of demand vis a vis capacity [3]. DCR values less than or equal to one indicate that a structure is safe. DCR values in excess of one indicate that a structure is unsafe since the structural capacities are less than the seismic demand imposed on the building. The combined stress due to direct axial compression and stress is given by expression P/A ± M/Z where, P=Axial compressive force on the determined wall A=area of the wall under consideration.
M=Moment at the base of wall.

I=Moment of Inertia.
A negative value for the above expression indicates that the structure will be unsafe, that is, the masonry wall is subjected to tensile stress, which it is inherently incapable of resisting.

Findings and Concluding Remarks
After the 2011 earthquake, it was observed that the old structure had suffered edirreparable damages. Visual inspection revealed the deteriorated condition of the monastery due to these damages ( Figures  17-19). The random rubble masonry walls suffered bulging due to out of plane bending (Figure 20), diagonal and plaster cracks developed on the inside walls plaster (Figure 21) The timber posts also suffered bending (Figure 22). The damages sustained may be attributed to the inability of the walls to withstand the tensile stresses generated during earthquake induced shaking. Clearly, these cracks have weakened the structure and remedial measures need to be taken to prevent their further propagation. However, it is noteworthy that the structure has not collapsed despite suffering such widespread damage. The DCR values indicate that the structure should have sustained Grade    NOTE: This design horizontal seismic coefficient A h is determined by the following expression provided that for any structure with T ≤ 0.1 Sec the value of A h will not be taken less than Z/2 whatever the value of I/R. Sa/g=Spectral Acceleration Co -efficient depends on i) Fundamental Natural Period 'T' and ii) the type of soil at the site. Soil type of Sikkim is Hard Rock.
The Sa/g value can be derived from the graph (Figure 16) -Response Spectra for Rocky Soil Sites For 5% Damping.          So this analysis has pointed out that the historical monasteries have become vulnerable and are in jeopardy. So far very little work has been done to formally assess the vulnerability of the historical structures in the earthquake prone state Sikkim. It is not too late to evaluate the earthquake vulnerability to all old historic monasteries and suggest a way forward for their protection and preservation. 8. http://www.sikkimeccl.gov.in/History/Monasteries/North/HeeGyathang.aspx 9. http://www.imd.gov.in/section/nhac/dynamic/eq.pdf