Risks associated with earthquakes occurring in and around urban centres pose a safety challenge to planners, administrators, engineers and architects. Seismic microzonation has been recognised as the most accepted tool in seismic hazard assessment and risk evaluation.
The concept of seismic microzonation is based on the observation that the intensity of ground shaking in certain frequency bands gets modified by the presence of a pile of unconsolidated sediments, or in a larger sense, local geological conditions alter the characteristics of ground motions. Microzonation thus is with respect to ground motion characteristics taking into account source and site conditions. It involves subdivision of a region into zones that have relatively similar exposure to various earthquake related effects.
Microzonation studies generate detailed maps that predict hazards at much larger scales. These maps can be used to help forecast effects on existing facilities such as schools and public utility buildings. It also allows ranking of priority areas or structures in terms of seismic vulnerability, providing a rational basis for the distribution of funds for upgrading, retrofitting and other remediation efforts. In addition, information on seismic microzonation could be incorporated into building codes to improve seismic design of new structures in terms of local site effects. Accordingly, development of seismic microzonation maps of major urban centres in the country has been recognised as one of the priority areas of the National Centre for Seismology (NCS) of the Earth System Science Organization/Ministry of Earth Sciences (ESSO-MoES). It may be noted in passing that earlier the seismic zonation map for India (Fig 1) was done by the Bureau of Indian Standards (BIS). The Earthquake Risk Evaluation Center under the India Meteorological Department (IMD) undertook microzonation of the national capital territory of Delhi and generated an integrated seismic hazard map of Delhi (Fig. 1). In 2014 the IMD Centre was merged with the NCS, Ministry of Earth Sciences.
Fig. 1: Seismic Zonation Map
There are three major steps involved in seismic microzonation: evaluation of the expected input motion, local site effects and ground response analysis, finally followed by the preparation of microzonation maps depicting hazard, vulnerability and risk. This requires collection and analysis of a large amount of seismological, geological and geo-technical data. Damage pattern of many recent earthquakes in India such as Jabalpur (1997), Chamoli (1999) and Bhuj (2001) earthquakes have demonstrated that the soil conditions at the sites had a major effect on the level of ground shaking. The Chamoli earthquake caused moderate damage to some buildings in Delhi built on soft alluvium although the epicenter was 250 km away. The 2001 Bhuj earthquake caused severe damage not only in the epicenter region, but even in Ahmedabad—250 km away from epicenter due to increased shaking of the soft alluvium.
As per the seismic zoning of India, several urban centres fall under ‘moderate’ to ‘severe’ seismic hazard zones. Following are some of the examples of the cities where seismic microzonation has been done.
Given the cultural and administrative importance of Delhi, seismic hazard microzonation of Delhi was undertaken by the Department of Science Technology (DST) in Delhi and was made public in 2005. For this, in keeping with the recommendations by the expert group, a multi disciplinary and multi institutional approach was adopted. Accordingly, four main components of implementation relating to identification of seismogenic sources; evaluation of ground motion characteristics; estimation of ground motion modifications; and estimation of secondary effects of ground motion such as liquefaction potential were identified. The products of this study were assessed to be useful for qualitative assessment of the seismic hazard of NCT Delhi (Fig. 2). Similarly, a Deterministic Seismic Hazard Analysis (DSHA) for the Bangalore, has been carried out by considering the historic earthquakes, assumed subsurface fault lengths and point source synthetic ground motion model. The sources have been identified using satellite remote sensing images and seismotectonic atlas map of India.
Microzonation studies are relevant for various sectors ranging from, disaster mitigation and management, risk assessment to existing life and property, public utilities and services on one hand to urban development, planning, design and construction, defense installations, as well as heavy industry on the other.
India Spend. 2015. 38 Indian cities in high-risk seismic zones.
National Center for Seismology. 2016. A report on seismic hazard microzonation of NCT Delhi on 1:10,000 scale.
Author is currently Chair, ESSO-MoES and former, Secretary, Ministry of Earth Sciences (MoES) . email@example.com