THE CRUSTAL VALUE OF ATTENUATION IN THE NORTHEAST OF IRAN

When a propagating wave is introduced into the Earth from a source, it is recorded with some changes after its travel through the Earth. The most evident of these changes is a loss or attenuation in energy and amplitude. In other words, the Earth introduces a reduction in the amplitude level and a change in the nature of the wavelet, which becomes broader and more asymmetric with increasing length of travel. The study of attenuation in high frequency seismic waves is useful for both the seismologist and the earthquake engineer as it is an essential parameter in predicting the earthquake ground motion in seismic hazard analysis. Why seismic waves attenuate or decrease in amplitude as they propagate? It is known that the reflection and transmission of seismic waves at discrete interfaces reduce their amplitude. There are four other processes that can reduce wave amplitude: geometric spreading, scattering, multipathing, and anelasticity. All four processes are important for seismic waves. The first three are described by elastic wave theory, and can increase or decrease an arrival's amplitude by shifting energy within the wave field. By contrast, anelasticity reduce wave amplitudes only because energy is lost from the elastic waves. The most commonly used measure of attenuation is the quality factor (Q) and its inverse (Q-1. In this study, for the measurement of ?? and Q?? -1, the extended coda normalization method is used (Yoshimoto et al., 1993). Coda normalization method is widely used for the estimation of attenuation per travel distance, site amplification factors, and source spectra on the basis of the spatially uniform distribution of coda energy at a long lapse time. The appearance of coda waves in seismograms is one of the most prominent observations supporting the existence of small-scale random heterogeneities in the earth (Aki, 1980).