SEISMIC MICROZONATION OF MEGA CITIES

Abstract

The rapid growth of the world's population over the past few decades has led to a concentration of people, buildings and infrastructure in urban areas. The tendency of such development in sedimentary valleys has increased their vulnerability to earthquakes, due to the presence of soft sediments. The importance of estimating the seismic ground motion in such areas, especially the mega cities, for earthquake risk mitigation is well recognized. One of the important prerequisites for mitigation of the devastating effects of earthquakes is the proper assessment of seismic hazard associated with the region and seismic microzonation incorporating the local site effects. Seismic microzonation is a process that involves incorporation of geologic, seismologic and geotechnical concerns into economically, sociologically and politically justifiable and defensible land-use planning for earthquake effects so that architects and engineers can site and design structures that are less susceptible to damage during major earthquakes. Microzonation should provide general guidelines for the types of new structures that are most suited to an area and it should also provide information on the relative damage potential for the existing structures in a region. The process involves a multi-disciplinary approach with major contributions from geology, seismology, geotechnical and structural engineering. It comprises the assessment of the source and path characteristics of the earthquakes and the study of the effects of the local geological and geotechnical conditions on the damages caused by earthquakes at the site. In general terms, it is the process for estimating the response of soils under earthquake excitations and thus variation of earthquake characteristics on the ground surface. Seismic microzonation essentially involves assessment of seismic hazard at bedrock level on the basis of past earthquake history, geology and tectonics (Seismic Hazard Assessment), studying the characteristics of the overlying soil and topology of the area (Geotechnical information and Data), analyzing response of the overlying soil layers to the bedrock motion and finally microzoning of the area for significant spatial variation of ground motion and its effects. The main objective of the present work has been to develop a convenient and cost effective technique for seismic microzonation of a mega city. Seismic microzonation is a multidisciplinary task and many studies have been undertaken to accomplish the job. Studies related to seismotectonics, seismicity, soil properties and local site conditions were undertaken for estimation of seismic hazard and determination of local amplification of ground motion. The study area namely Srinagar city which is capital of Jammu and Kashmir, lies in one of the seismically very active zones, i.e. Himalayas and is placed in Seismic Zone V in the seismic zonation map of IS-1893. The present study reviewed the major limitations of seismic microzonation techniques, determined their causes, and offered optimum solutions. Traditional procedures for seismic microzonation suffer from several limitations especially in earthquake catalogues, methodologies for seismic hazard assessment, geotechnical data collection and finally the methodologies for carrying out seismic microzonation. Also, a methodology is proposed for data validation by comparing the PGA values obtained from seismic microzonation with those obtained from observed data on isoseismal maps for major past earthquake events in the region. Various methods for seismic microzonation have been reviewed and the limitations have been discussed. The methodologies adopted for seismic microzonation of Wellington iii (New Zealand), Mexico city (Mexico), Perth (Australia), Nasca (Peru), Bogota (Colombia), Greater Bangkok (Thailand), Armenia (Colombia), Basel (Switzerland), Delhi and Jabalpur (India), Yeneshir and Silivira (Turkey) have been reviewed. One of the requirements for seismic hazard assessment is a complete earthquake catalogue with a uniform magnitude scale where the limitations stem from incompleteness and heterogeneity of seismicity data which is compiled from various sources. The completeness has been checked using statistical methods and its effects on hazard parameters have been demonstrated. The data homogenization has been done using the recommended conversion formulae. A new catalogue which is complete and homogeneous has been generated and used for the seismic hazard estimation. The Probabilistic Seismic Hazard Assessment (PSHA) methodology adopted in the present work uses independent seismogenic sources (based on seismotectonic modeling of the area thus recognizing the physical sources present), doubly truncated Gutenberg Richter relationship and Poissonian distribution of earthquake occurrence. The estimated seismic hazard parameters match with the general earthquake occurrence in the area. The attenuation relationship has been selected based on the prevalent seismotectonics and its closeness to the recorded strong motion data in the Himalayan region. At bedrock level the horizontal component of ground acceleration is almost uniform in the entire Srinagar city with a variation of a little over 5%. Envelops of the response spectra obtained from 50 sites have been estimated for four return periods i.e., 225, 475, 1000 and 2500 years. Quantification of amplification of ground motion in the study area and determination of natural resonance frequency was the main objective of the site response study. The most important information for the ground response analysis is a subsurface model that represents the variation in thickness of the soil layers along with its geotechnical properties. The iv methods for geotechnical investigations have been reviewed in detail. Non destructive method using ambient noise has been implemented for the site investigations which can be conveniently employed in urban areas and is cost effective and faster than the traditional methods. A technique to collate borehole data with the results of ambient vibrations has been used to characterize the site for geotechnical parameters. The profile ofbedrock based on the analysis of microtremors reveals undulating nature with several humps and depressions varying from 18 to 57 m. The layered structure of the soil columns were determined by collating the data from bore holes to estimate the site response using ID analysis. The relatively denser selection of the test sites in the city allowed interpolation to get the picture ofthe strong ground motion in the city area for its further use in the seismic microzonation. The seismic microzonation of Srinagar city has been carried out using the spectral accelerations obtained at the surface. While at 30 out of the 49 sites studied in the city, deamplification of motion occurred during the propagation of waves from bedrock to surface with surface PGA values ranging between 0.166 g and 0.196g for a return period of 475 years, the strong motion got amplified at the remaining 19 sites and the maximum value of surface PGA was estimated to be 0.245g. The southern part of the city, where surface PGA values range between 0.166g and 0.195g, is relatively seismically safer for future development than the rest of the city. Three seismic microzones have been proposed for Srinagar city with PGA varying as 120, 150 and 180 gals, 170, 210, 250 gals, 250, 300 and 350 gals and 370, 430 and 500 gals for return periods of225, 475, 1000 and 2500 years, respectively. The patterns of seismic microzones reveal the dominance of the effect of local site conditions onthe strong ground motion estimated at the surface. The general structures in Srinagar city are two and three storeyed buildings. Srinagar city has also been divided into two seismic microzones based on spectral acceleration with 0.3 sec time period for 475 years return period which pertains to the recommendations for structures of relatively smaller heights and belongs to the more general types of the buildings found in the city. Similarly, the city has been divided into three seismic microzones based on spectral acceleration with 1.0 sec time period for 475 years return period which pertains to the recommendations for relatively taller structures.