Investigation of deep and lateral changes of coda wave quality factor using short-time Fourier transform in northwest of Iran

The amplitude of seismic waves is generally reduced while passing through the earth under the influence of the two main factors of geometrical spreading and apparent attenuation of seismic waves. Scattering attenuation, as an elastic phenomenon, redistributes seismic wave energy due to the collision of seismic waves (P, S and surface waves) with randomly distributed heterogeneities. The attenuation of coda wave, backscattered waves form heterogeneities, is one of the most important parameters in the estimation of seismic wave attenuation. In this study, the attenuation of seismic coda waves as scattered body waves has been estimated using single back-scattering model.

The most common method to estimate attenuation is single back-scattering model. Here, we use short-time Fourier transform (STFT) instead of bandpass filter. For each individual frequency, the envelopes of STFT coefficients of extracted waveforms for lapse times of 30, 40, 50, 60, 70, 80 and 90 seconds are measured as the BPF method. The obtained envelopes have been used in order to estimate attenuation at each individual frequency. Since the length of the window is constant in the STFT method, it is possible to determine fairly accurate frequency in a short window. However, to this purpose, each waveform is divided into 1 s windows (50 samples) overlapped by 90% (45 samples), and the STFT has been calculated for each window.

In this study, attenuation parameter has been estimated in northwestern Iranian plateau using the STFT method. The results show good correlation between seismicity and tectonic of the study area for lapse times greater than twice the S- wave travel time, especially for lapse time of 40 s. At lapse time of 40 s, the frequency-dependent relationship has been estimated as Q = (103±1)f(0.88±0.04) using the STFT method. It is concluded that the STFT model can be used as an appropriate time-frequency tool to study the energy attenuation of high-frequency coda waves due to the high correlation coefficients and low standard deviations of the relationship. Furthermore, the relatively low values of quality factor in frequency attenuation relation show that there is high heterogeneity among shallow layers of Iran northwestern region that is possibly due to the relatively high seismicity of the region.