STUDIES ON SEISMIC SURFACE WAVE DISPERSION. CRUSTAL STRUCTURE OF HIMALAYA REGION AND GLOBAL SEISMICITY

Abstract

"In the present thesis the importance of the observational surface wave group velocity dispersion studies of the fundamental mode of Rayleigh and Love waves has been emphasised. Specifically* a) group velocity dispersion of Love waves has been investigated for Eurasia, which has been divided into thirteen regions according to their disper sion characteristics. b) crustal structure of the Himalaya and Tibet Pla teau region; where very low Rayleigh and Love wave group velocities have been observed} is delineated. c) based upon the Rayleigh wave division pattern, the lateral surface wave group velocity gradients have been studied and a new significant correlationship has been sought between these velocity gradients and global seismiclty and d) the recent seismic activity of the Koyna region has been comprehended in view of the much steeper velo city gradients near Bombay and some outstanding fea tures investigated for the seismic activity of Koyna region have been discussed. • In the beginning^the historical development of the surface wave dispersion studies is presented briefly. The observational difficulties of Love wave dispersion have been pointed out. Dispersion of the fundamental mode of Love waves along various paths in Eurasia has been studied and using the 'crossing path technique', Eurasia has been divided into 13 regions of similar group velocity disper sion character. This division pattern has been compared with Rayleigh wave division pattern. Rayleigh wave to Love wave group velocity ratios for the same period and region are calculated. Himalaya and the Tibet Plateau region is found to be of special interest in view of the very low Rayleigh and Love wave group velocities delineated for this region and the observed dispersion has been compared with theoretical dispersion curves computed for a three layer earth model. The results are supported by profile seismological observa tions and deep seismic soundings carried out in some of the common portions by Russian scientists. Similarity in Rayleigh and Love wave division patterns for Eurasia suggests that the Rayleigh wave division pattern; which Is based on 30 sec period waves; in general depicts the seismic wave velocity distribution in the upper 40-50 km section of the earth. This divisioning of Eurasia, Africa, Pacific Ocean, Atlantic Ocean and the Indian Ocean has been mainly used for calculating the lateral velocity gradients. Seismicity; for the first time; has been correlated with the lateral elastic wave velocity gradients. The investigation reveals that the regions of high seismicity most unambigu ously correspond to regions of steep gradients. The puzzling seismic activity in the Western side of the stable Indian Shield also seems understandable in view of the much steeper gradients near Bombay. The Koyna earth quake of December 10, 1967 has been studied in detail."