عنوان مقاله [English]
Attenuation of seismic waves is considered a very important characteristic of the wave propagation path because this physical parameter affects how the seismic waves propagate, and consequently knowing the amount of it is essential in accurate calculation of seismic source parameters, modeling and reducing seismic risk in seismic areas. Attenuation of seismic waves in an environment is the result of two physical processes, elastic and inelastic. In the elastic process, energy remains constant in the propagation medium; however, the amplitude of the waves may increase or decrease due to the geometrical spreading, multipath, and scattering. These factors depend on the type of wave, frequency, degree of heterogeneity and characteristics of the propagation medium. In the inelastic process, part of the energy of the seismic waves is converted to heat and the amplitude of the seismic waves decreases due to the loss of a part of the energy. In the inelastic process, factors such as inelastic properties of the environment and physical properties of the environment (wave velocity, density and temperature) play an important role. Not only the scattering and the intrinsic absorption are important factors in reducing the amplitude of direct waves, but they also affect the appearance of a seismogram.
Decreasing the amplitude of seismic waves by increasing the propagation distance from the source and the frequency changes made in the time history of the earthquake is called attenuation. In general, the factors that weaken the waves and the energy emitted from the seismic source are reflection and passage of waves through the boundaries of the layers, multi-path, geometrical spreading, wave diffraction, attenuation and inherent absorption of waves due to heterogeneity in the propagation path. The attenuation of seismic waves is described by the dimensionless quantity of the quality factor Q, which indicates a decrease in the amplitude of the wave along the propagation path. This parameter is a function of frequency, seismic wave type, time window intended for seismic analysis, geological characteristics below the seismic recording site and tectonic activity of the region. Physically, Q is the ratio of wave energy to energy wasted in each cycle of oscillation. By examining and calculating the range of Q changes from the data processing of seismic waves emitted in the Earth's crust, the characteristics of its various parts can be realized. Parts of the Earth's crust that have very low attenuation have very high Q values, and parts with severe attenuation have very low Q values. Tectonically active regions have a relatively high heat flux, so they are more strongly adsorbed than other regions, so they have a lower Q value.
The attenuation of coda waves, Qc, has been estimated in Fariman region, NE-Iran, using a single back-scattering model of S-coda envelopes. For this purpose, we used the time histories of 124 aftershocks of Do-Ghaleh Fariman earthquake (Mw6, 2017), recorded by local seismic network belong to International Institute of Earthquake Engineering and Seismology (IIEES). In this research, the frequency-dependent Qc values are estimated at central frequencies of 2, 4, 8, 16 and 24 Hz using different lapse time windows from10 to 60 s and the frequency-dependent relationships obtained for 30 s is Qc=(73±11) f (0.89±0.06). It is observed that the exponent n decreases and Qo increases as lapse time increases. Any increase of Q with depth or with distance from the seismic source to receiver would cause the increasing of coda-Q with lapse time. The average Qc values estimated and their frequency dependent relationships correlate well with a highly heterogeneous and highly tectonically active region. Our results regarding QC factor is the first study in this area and would be significant for reassessment seismic hazard and risk management in southern part of Khorasan Razavi.