STUDIES OF SEISMICITY OF INDIA AND AN ANALYSIS OF SOME STRONG EARTHQUAKE SEQUENCES

Abstract

The work presented in this thesis consits of two parts: the first part includes studies of the seismicity of India both as a single unit as well as region-wise, whilst the second relates to the study of energy released by primary and in some cases secondary aftershocks of some strong earthquakes. The earthquake data for these studies have been largely abstracted from seismicity of the Earth by Gutenberg and Richter and from the records published by the United States Coast and Geodetic Survey. However, in some cases where these were not available, records of the Indian observatories have been used. For the purposes of the seismicity studies, India has been divided into 6 regions. These are the Andaman-Nicobar, Assam, Bihar-Nepal, Kashmir, T<utch and South India. Of the above regions the data pertaining to Assam and the Himalayan region in general were most extensive which enabled the preparation of tectonic flux maps and the delineation of trends of prominent seismic activity for these regions. The map for Assam (Choahan, Gaur and Mithai, 1966) shows significant correlation with the known thrust zones.' Por other regions it was only possible to obtain the frequency magnitude relations 2 and the strain accumulation curves. Further, on the basis of these curves strain accumulation and relaxation curves were drawn for various region starting from an arbitrary store of strain around the year 1920. These curves show that every region has a minimum characteristic strain level which is perhaps never crossed and which could therefore be regarded as a reference from which to esti mate the maximum probable size of a shock that could be expected in that region in future. These estimates com pare well with some recent shocks. Finally the strain rebound characteristics of India (Chouhan,l906) as a single unit has also been made. The study of aftershock sequences relate to the well recorded sequences of some recent earthquakes: (Assam (1950), Alaska (March, 1964 \ Pakistan (February, 1966), NE China (March, 1966), Soloman Island (Tune 1966N and Northern California -(September, 1966 \ The manner of strain release in the aftershock sequences of the 1st five earthquakes are clearly of the compressional type whereas those pertaining to the latter three nhow both the compressional and shear creep recovery phases. Further these data have been used to estimate the extent of the strained zone, the magnitude of stresses prior to p*»a fracture, the probable fault slip and the creep relaxation times for the corresponding focal regions.