ATTENUATION OF SEISMIC WAVES IN THE KUMAON HIMALAYA

Abstract

The study of attenuating nature of seismic waves in a region is important for the estimation of earthquake source parameters, for the simulation of earthquake strong ground motions and, hence, useful in evaluating seismic hazards of a region. The overall attenuation of seismic waves is dependent on several factors such as geometrical spreading, scattering due to in homogeneities, inelasticity and multipathing. A part of the energy in the waves is dissipated while propagating through non-elastic medium. This type of attenuation of seismic wave is described by the dimensional quantity known as quality factor 'Q', which describes the decay of wave amplitude during propagation in the medium. The Q can be estimated using the different types of seismic waves- P-waves (Q) S waves (Q) and coda-waves (Q). The objective of the present study is to understand the attenuation characteristics of Dharchula site of Kumaon Himalaya using the different parts of the accelerograms. For this purpose, the extended coda normalization method has been used to develop the frequency dependent relationships for Q and Q11. The single back-scattering model of Aki and Chouet (1975) has been used to estimate relationship for coda-Q. The waveforms of 23 earthquakes recorded at the sampling rate of 100Hz at Dharchula station has been used in present analysis. The accelerograms have been filtered using Butterworth bandpass filter with six frequency bands i.e. 1-2 Hz, 2-4 Hz, 4-8 Hz, 8-16 Hz, 16-24 Hz and 24-48 Hz. The frequency dependent relationships for Q obtained in the present study have been compared with those of obtained on different parts of Himalaya and other regions of the world. Also, a comparison between the values of Q, Qp, and Q have been made. A comparison between Qi and Q. has also been drawn to see which out of the two is predominant.