EFFECT OF MEDIUM CHARACTERISTICS ON SH WAVE PROPAGATION

Abstract

The destruction at a location is due to the ground motion generated by an earthquake. However, the intensity of destruction may vary from location to location. This variation of intensity could be due to the amplification or deamplification of seismic waves propagated through the medium between the earthquake source and site location. Therefore, the propagating medium is a significant factor in the shaping of ground motion. The seismic waves released from the source are generally influenced by propagation path and local site effects. The work done in this thesis deals with the propagation path effect and local site effect. The shear waves (S-waves) which are produced during an earthquake are more disastrous because it create horizontal ground shaking which leads to the destruction of buildings and infrastructures. The understanding of propagation velocity and quality factor of SH helps in predicting the severity and distribution of seismic shaking, assessing seismic hazards, and constructing earthquake-resistant buildings that can withstand horizontal forces. The main objectives of this thesis are (i) to study the medium characteristics of Garhwal Himalaya and Kumaon Himalaya in terms of shear wave velocity (VS) and attenuation factor and (ii) to study the effect of shear wave velocity and topography on the SH wave propagation during an earthquake. In this work, the three-dimensional (3-D) S-wave attenuation tomography study has been done for the Garhwal Himalaya, Uttarakhand to understand the attenuation characteristics of S waves in the region. The strong ground motion (SGM) data collected from 19 earthquakes have been used for this purpose. The technique proposed by Hashida and Shimazaki (1984) which is later modified by Joshi (2006a, 2007) has been used for this purpose. The study shows that the anomalous zone of S-wave quality factor has been observed in the vicinity of the Alaknanda fault. The obtained results are compared with those obtained for the Kumaon Himalaya which has the same tectonic arrangements as that of the Garhwal Himalaya using similar technique by Kumar et al. (2015b). It has been found that the attenuation rate of seismic waves is higher in the Kumaon Himalaya in comparison to that in the Garhwal Himalaya. The regional frequency-dependent quality factor of order 𝑄𝛽 = 107𝑓0.82 has been developed for the Garhwal Himalaya using the results obtained from 3-D attenuation tomography.