PREDICTION OF STRONG GROUND MOTION FOR PARTS OF NORTH INDIA

Abstract

The development of appropriate preventive measures to minimize the risk of damage from earthquakes is dependent on the proper evaluation of seismic hazard in the region. The source of damage lies most often with the strong shaking caused by the waves set up by the earthquakes. The records of strong ground motions from past earthquakes can serve to provide a wealth of information that will be indispensable in the design of earthquake resistant structures. In many regions like Himalayas, such records generated by past earthquakes are few or absent and one needs to use theoretical predictions for estimating the peak ground acceleration or complete time history of ground from expected earthquakes in order to assess the seismic hazard. However synthetic accelerograms are now increasingly being used in earthquake engineering. A knowledge of regional and local seismicity and seismotectonics, a suitable earth model and source characteristics of the design earthquake are required for this purpose. The work carried out in the present thesis has been divided into two parts. In the first part, new empirical attenuation relationships for estimating peak ground acceleration have been derived for Himalaya and northeast regions of India. In the second part, an improved method of generating synthetic accelerograms has been presented and discussed based on the convolution model of seismogram. The peak values of horizontal acceleration is one of the important parameter that is considered in the earthquake safe design of structures. In the present study, the strong motion data from eight earthquakes (three in Himalayan region and five in Shillong area) has been used. The Himalayan region and in Shillong area are characterised by two different tectonic environments, so two different attenuation relationships are developed. An integrated attenuation relationship has also been developed based on the relationships available in literature. An exercise has also been done to find out the attenuation relationship available in literature which predicts PGA values nearer to the observed ones. After comparing these three attenuation relationships it is observed that our developed relationships gives the good fit with the observed values. In the second part, a simple and fast method has been discussed and presented for generating synthetic accelerograms based on the convolution model of the seismogram. The spectrum of the ground motion expected at recording site are first computed from the knowledge of source parameters (source mechanism, size, geometry, time function, slip distribution on fault plane, radiation pattern etc.) and medium properties (elastic properties of earth materials in the immediate vicinity of the fault, geometrical spreading, frequency dependent attenuation, local site geology etc.). This spectrum is then inverse Fourier transformed to yield the desired synthetic accelerogram. The suitability of the method has been demonstrated by comparing the synthetic accelerograms with the observed accelrograms for two earthquake events of Uttarkashi, 1991 and Chamoli, 1999. The synthetic and observed accelerograms are compared at eight sites for Uttarkashi earthquake and at five sites in case of Chamoli earthquake. The observed and synthetic accelerograms are compared for selected portions of high amplitudes. The extent of matching has been quantitatively expressed by computing the r.m.s. errors. It has been found that matching between the synthetic and observed is quite comparable.

ACKNOWLEDGEMENTS I feel to express my profound indebtness, deep sense of gratitude and sincere thanks to Prof. V.N. Singh, Department of Earth Sciences and Dr. Tanuja Srivastava, Department of Mathematics, Indian Institute of Technology, Roorkee who not only imparted valuable instructions but also instilled in me a constant fountain of inspirational courage and evoked a keener sense of responsibility and devotion, during this endeavor. It is due to their inspiring novel ideas, expert and constant guidance, invaluable advice, constructive criticism without which it would not have been possible to complete this research work. I am highly obliged to both of them. I am extremely grateful to the Head and other faculty members of Department of Earth Sciences, IIT, Roorkee and to the Director and other officials of the Central Building Research Institute, Roorkee for providing the required facilities during the course of present work. Heart-felt thanks are due to my friends and colleagues Dr. Pradeep Kumar, Dr. Atul Kr. Agarwal, Scientists and Ms. Deepti Agarwal, B.S. Bisht, Tech. Staff of CBRI who have helped and co-operated me at various stages of the present work. Thanks are also due to Dr. K. K. Srivastava, Director, CESER, Roorkee for their constant encouragement and suggestions during the course of this work. Thanks are due to Ms. Himani, Vandana and Priyamvada, Research Scholars of IIT, Roorkee who helped me time to time. The good wishes, honest blessings, moral support and encouragement of my mother, sisters, brothers and other relatives in various ways are deeply appreciated and gladly acknowledged for completion of this task. I take pleasure in acknowledging my debt to all my colleagues and friends named or un-named , who provided me the environment in casting my work to present stage. No words, no language is ever adequate to express my heart-felt veneration for my dear husband Shri M.C. Mittal and sons Master Achal and Nishchal for their willing co-operation, whole hearted moral support, painstaking patience and continued understanding. Last but not the least, my heart goes to the memory of my father Late Shri J.P. Jain who had always desired and wished for my attaining a distinguished status. With profound love and regards, Idedicate this work to his cherished memories which are always with me. Deoember 2002 <Abha Mitta"