The hazards associated with earthquakes are referred to as seismic hazards and these are one of the most devastating of all the natural hazards. The practice of earthquake geotechnical engineering involves in the identification and to model the rupture mechanism at the source of an earthquake, evaluate the propagation of waves through the earth to the top of bed rock, determine the effect of local soil profile and thus to develop a hazard map indicating the vulnerability of the area to the potential seismic hazard. The geotechnical engineer is responsible for providing the structural engineer with appropriate site-specific design ground motions for earthquake resistant design of structures. Many earthquakes in the past have left many lessons to be learned which are very essential to plan the infrastructure and even to mitigate such calamities in the future. Microzonation has generally been recognized as the most accepted tool in seismic hazard assessment and risk evaluation and it is defined as the zonation with respect to ground motion characteristics taking into account source and site conditions (TC4-ISSMGE, 1999). Making improvements on the conventional macrozonation maps and regional hazard maps, microzonation of a region generates detailed maps that predict the hazard at much smaller scales. Damage patterns of many recent earthquakes around the world, including the 1999 Chamoli and 2001 Bhuj earthquakes in India, have demonstrated that the soil conditions at a site can have a major effect on the level of ground shaking. For example, in the Chamoli earthquake the epicenter is located at more than 250 km away from Delhi caused moderate damage to some of the buildings built on filledup soil or on soft alluvium. The Bhuj earthquake caused severe damage not only in the epicentral region, but even in Ahmedabad, about 250 km away, which is attributed to increased ground shaking of the soft alluvium. Mapping the seismic hazard at local scales to incorporate the effects of local ground conditions is the essence of microzonation.
Earthquake damage is mainly controlled by three interacting factors:
source and path characteristics local geological and geotechnical conditions and type of the structures.
This requires analysis and presentation of a large amount of geological, seismological and geotechnical data. History of earthquakes, faults/sources in the region, attenuation relationships, site characteristics and ground amplification, liquefaction susceptibility are few of the important inputs required. Widespread structural failures during past earthquakes resulted from the effect of soil condition on the ground motion that translates to higher amplitude. It also modifies the spectral content and duration of ground motion. Site specific ground response analysis aims at determining this effect of local soil conditions on amplification of seismic waves and hence estimating the ground response spectra for future design purposes. The response of a soil deposit is dependent upon the frequency of the base motion and the geometry and material properties of the soil layer above the bedrock. Seismic microzonation is the process of assessment of the source & path characteristics and local geological & geotechnical characteristics to provide a basis for estimating and mapping potential damage to buildings or it is the quantification of hazard. Hence, another important aspect of microzonation is the development of hazard maps for the use of planners, developers, insurance companies and common public.
Need for the Study
Peninsular India (PI) once believed to be a stable continent, has experienced many earthquakes. In particular, Latur earthquake on 30th September 1993 (M 6.3), Jabalpur earthquake on 22nd May 1997(M 6.0) and Bhuj earthquake on 26th January 2001(M 7.9) have influenced the need for study of earthquakes and their effects on Indian cities in PI. As part of the national level microzonation programme, Department of Science and Technology, Govt. of India has initiated microzonation of Bangalore. The seismic hazard analysis and microzonation of Bangalore region is carried out as part of this project. Apart from this the specific need to study the site specific seismic hazard analysis and microzonation of Bangalore is as follows:
Bangalore city a fast growing urban center, with low to moderate earthquake history and highly altered soil structure (due to large reclamation of land in the dried up/silted tanks).
There were over 150 tanks, though most of them are dried up due to erosion and encroachments leaving only 64 at present in an area of 220 sq km.
Bangalores rapid growth has put pressure on its infrastructure and dilution in standards of building/ infrastructure construction. Present constructions include mud buildings to steel construction and RCC framed structures ranging from one storied to high rise buildings of more than 20 floors.
Department of Civil Engineering
Indian Institute of Science, Bangalore
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This page was last updated on August 25, 2011