BHUBANESWAR: HIGHER-INTENSITY QUAKE NOT UNLIKELY
Bhubaneswar has been facing mild earthquakes from time to time. People feel the shock and run out of the buildings. Then they forget about it till another shock comes. While constructing or booking houses people hardly think about the structural safety of the building against earthquake. People should pay attention to earthquake zone maps of the area and make sure that the building is safe enough for the particular zone. The recent earthquake was indeed a lesson, and a slight increase in its magnitude would have wrecked devastation in the city.
Earthquakes are common phenomena, which occur most often irrespective of time and place. Earthquake is most feared among all natural disasters as it strikes suddenly without any prior indication and creates devastation to life and property. Earthquake is not predictable even with today’s advanced technologies.
Odisha lies in a stable continental region, which is not seismically active. However, minor to moderate earthquakes have occurred now and then at the State’s different localities which are not damaging. Rarely a building has collapsed or any human casualty reported ever. The maximum magnitudes of these earthquakes have been reported around 4.5 to 5.3 in the Richter’s scale. These earthquakes have occurred either due to local tectonic causes or due to external tectonic forces, e.g., high intensity earthquakes in the Himalayas, Sumatra or Gujarat regions, etc., which have an impact in Odisha. Despite the moderate quakes so far in Odisha, it cannot be confidently said that a higher-intensity earthquake is not unlikely in future.
The possible magnitude of an earthquake that is likely to occur in an area may be predicted. Earthquake zoning map of a country or region gives an idea of possible earthquake to occur in an area. Zoning maps are made basing on earthquake history and the geological condition of the region. Zones indicate possible magnitude, intensity of ground vibration for an earthquake in the region. This will help in the design of structures. However, a zone is not permanent always. It could change if a major quake hits the area in future.
Indian subcontinent was classified into five seismic zones as per IS 1893-1984. After the Latur earthquake of 1993, this map was further revised. In India, the seismic zoning map prepared by the Bureau of Indian Standards (BIS) based on the data provided by the Geological Survey of India (GSI), India Meteorological Department (IMD) and several other organisations forms the basis for macro level planning. The increased seismic activity during the last two decades required the reclassification into four seismic zones as per IS 1893 (Part I): 2002. Zone II being the region of lowest activity and Zone V is seismically most active, where earthquakes of magnitude 8.0 or more could occur.
The Himalayan region where the Eurasian tectonic plate is constantly being pushed by the Indian plate, coastal States like Odisha may suffer from large-scale earthquakes. Besides, in Odisha the Mahanadi basin can be vulnerable and can cause earthquakes. As per Seismotectonic Atlas of India, several deep-seated faults are situated beneath the Mahanadi delta. A fault line is a fracture along which the crust has moved. Since faults do not usually consist of a single, clean fracture, geologists use the term fault zone when referring to the zone of complex deformation associated with the fault plane.
Odisha can be divided into two seismic zones: Zone II (Low Damage Risk Zone) and Zone III (Moderate Damage Risk Zone). Regions lying in the valleys of rivers Mahanadi and Brahmani lie in Zone III, which stretch from Jharsuguda along the border with Chhattisgarh in a south-easterly direction towards the urban centres of Bhubaneswar and Cuttack on the Mahanadi delta.
As per the recent seismic Zoning Map of India (IS: 1893, 2002), Bhubaneswar city is located in Zone-III. Damage starts with Zone-III with intensity VII in the MSK scale. The city may be prone to quakes that can reach 6.5 on the Richter scale. However, none of the past earthquakes represented intensity of the order of VII. All quakes in Odisha recorded a maximum intensity of VI. However as a factor of safety, intensity VII is desired to be taken for design of structures in Odisha, located in Zone-III.
Nobody can assure earthquake safety unless everybody gets prepared. A massive awareness programme for making prepared individuals from all communities and stakeholders is required. The Bureau of Indian Standards (BIS) has laid down codes to start about the earthquake resistance of buildings. If these codes are followed, it gives sufficient resistance to building against earthquake. There should be a proper mechanism to ensure implementation of the codes for structural safety of the buildings during earthquake.
Existing buildings that are highly unsafe must be declared unfit for occupation, demolished or retrofitted appropriately. Safe building construction practices can certainly reduce the extent of damages. It is mandatory for every citizen to have elementary knowledge about earthquake, forces acting on structures, remedial measures to be taken for structural safety. Seismic-resistant structures are designed in such a way that they might face partial damage but will not totally collapse.
The primary objective of earthquake-resistant design is to prevent building collapse, thus minimising the risk of death or injury to people in or around those buildings. Reinforced concrete buildings have become common. A typical RCC building is made of horizontal members (beams and slabs) and vertical members (columns and walls) and supported by foundations that rest on the ground. The system consisting of RC columns and connecting beams is called a RC frame. When earthquake occurs, it transmits seismic waves which in turn cause ground motion of the earth’s surface. As structures rests on the earth surface, this ground motion is also passed onto them.
The base of the structure moves with the ground, but the roof tends to retain its position. But the roof is also forced to move as the walls and columns of the structure are connected. So, the structures generally tend to collapse. This can be prevented if the structure is ductile. Ductility is defined as an ability of a structure to face huge plastic deformation without loss in ultimate strength. The ductility of a structure enables to predict the amount of seismic energy that may be dissipated through plastic deformations, which is a very important factor for structural design under seismic loads.
More the ductility, strength and deformation capacity of a structure better is its seismic resistance whereas lesser the amount of deflection, lesser will be the vulnerability of the structure. Besides, greater the mass of building, more lateral force is exerted on buildings, and this alone is the major component behind building damages. Buildings with rigid layout (box like structure) with strong joints between different components are generally earthquake-proof because rigid buildings react as a single unit to earthquake forces.
The BIS has formulated guidelines which cover selection of materials and techniques to be used for repair and seismic strengthening of damaged buildings during earthquakes and retrofitting for upgrading of seismic
resistance of existing buildings. The BIS codes are not mandatory and are only guidelines. The construction as such is governed by the municipal and development authority’s byelaws which is within the jurisdiction of State Governments.
Structural measures would reduce the impact of disasters and non-structural measures would enhance the management skills and improve capacities of the community, local self-governments, urban bodies and the State authorities to prepare, prevent and respond effectively to disasters. The city authorities may use earthquake risk maps to control land use to minimise earthquake hazards. Besides, local authorities can implement guidelines for earthquake-resistant structures and set limits on the heights of the buildings. The buildings should be specially-designed as per the code to withstand strong tremors.