Sikkim Earthquake Of 14 February 2006

Sikkim Earthquake Of 14 February 2006

• Moderate earthquake (reported as Mw 5.3 by USGS and as ML 5.7 by IMD)  occurred in the state of Sikkim (India) on February 14 2006 at 06:25:23 a.m. local time.
•  At 27.35°N 88.35°E, near Ralang (South Sikkim), with a focal depth of 30 km (www.usgs.gov)
•  At 27.7°N 88.8°E, near Lachung (North Sikkim), with a focal depth of 33 km.
• The shaking was also felt in the North-Eastern states of India and in the neighboring countries.
• Sikkim is located in the seismic zone IV of Indian seismic zoning map with the expected maximum intensity of shaking as VIII on the MSK scale.
• The entire Himalayan belt is one of the most seismically active regions in the world.
•  In last 110 years, four great earthquakes (magnitude equal to or greater than 8.0) have occurred along this subduction zone causing large scale damage in the surrounding areas (Bilham 2004).

Construction Practice

• Common practice in Sikkim to build residential buildings using wood/bamboo.
• Until tourism industry got a boost.
• Most major old buildings in Sikkim are made of stone masonry with mud mortar.
• Except for a very few RC buildings involving major projects
•  Analysis and design are generally not carried out.
• the structural drawings are prepared simply based on previous experiences of engineers on the basis of a few thumb-rules
• Roof generally consists of GI sheets supported on wood/bamboo trusses which laterally connect the parallel walls

Masonry Constructions

Undressed stones used without proper bonding between adjacent courses of masonry, and also at the corners.The mud mortar used as bonding material in these buildings further aggravates their lateral strength capacity.These buildings were only “repaired” and not adequate retrofitted against future events.

Reinforced Concrete Constructions

• Floating columns in upper storeys.
• Intermediate soft storey in multistoried buildings.
• Poor reinforcement detailing.
• Tashiling Secretariat, State Legislators’ Hostel, Geological Survey of India (GSI) building at Deorali, suffered varying degree of damages

  

• The nine-storey masonry infill RC frame hostel building at
• Sikkim Manipal Institute of Medical Sciences (SMIMS)
•  Tadong, Gangtok suffered damages in walls and columns.
• Poor material quality,
• Poor connection between perpendicular masonry walls
• Resulted in out-of-plane failure of one infill wall, and severe damages to other infill walls in the building.

Lifeline Structures

On National Highway 31A, which connects Sikkim to the rest of country. In the Tarkhola Bridge between Melli and Rangpo, spalling of concrete was observed at the top of central pier near bearings.

An RC pedestrian foot bridge constructed in 1989 near STNM Hospital at Gangtok sustained minor damage in the form of spalling of cover concrete in one RC beam corrosion of reinforcement bars.