Iranian Journal of Geophysics

Iranian Journal of Geophysics

Parametric estimation of S wave source spectra in a simple folded belt, the Zagros

Document Type : Research Article

Authors
Reza Davoudian 1
Ahmad Sadidkhouy 2
Hamid Zafarani 3
1 Ph.D. Student, Institute of Geophysics, University of Tehran, Tehran, Iran
2 Associated Professor, Institute of Geophysics, University of Tehran, Tehran, Iran
3 Professor, International Institute of Earthquake Engineering and Seismology, Tehran, Iran
10.30499/ijg.2024.463306.1608
Abstract
The Generalized Inversion Technique (GIT) was used to estimate region-specific seismic parameters in the Simply Folded Belt (SFB), located in the east of the Zagros. To this aim, we prepare a database including 114 three-component accelerometers and 420 three-component acceleration waveforms in hypo-central distance range between 6 and 300 km with moment magnitude between 5 and 6.3. The maximum depth of the events is 34 km. The used database was made by calculating the Fourier spectrum of the acceleration waveforms in both vertical and horizontal components. Based on the signal-to-noise ratio, achieved by the modified wavelet de-noising method (Ansari et. al. 2010), the spectra were calculated in the frequency range between 0.4 and 15 Hz. Horizontal spectrums, an average of Fourier spectrums of two orthogonal components in the horizontal plane of the propagated wave, were calculated by two different averaging methods called geometrical and vector averaging. This study shows that the difference in averaging methods has a negligible effect on the final result, including seismic moment rate spectrum and seismic parameters such as stress drop and kappa. Using the slope of the Fourier spectrum in high frequencies, the kappa parameter for both horizontal and vertical components was estimated to be equal to 0.039 and 0.028, respectively. In continuation, the GIT was employed to achieve a spectrum of seismic sources. Also, we achieved a parametric form of seismic source spectrum (fc  and γ, based on ω square source model of Brune 1970) and stress drop for each event. For this aim, we ran a non-linear inversion procedure, Simple Grid Search. For the sake of optimum use of processing resources, the initial range of model parameters was selected after several times of trials and errors.
    The computed stress drop values are between 2 and 331 bar, with 85.5 bar as an average value. These values were derived from horizontal components of Fourier spectra based on the geometrical averaging method. Although stress drop values based on geometrical and vector averaging are not the same, the average values of stress drop based on these two methods are approximately the same, approximately. 
As a result, this study showed that SFB events do not follow the scaling rule of Aki (1967) completely. We showed that the decreasing rate of the seismic moment versus corner frequency equals 1.77. Although using 3 as the rate, the average stress drop in the area was derived to be equal to 57 bar.
Our results indicate that, at low frequency, site amplification versus averaged velocity in the upper 30 meters (Vs30) decreases more slowly rather than high-frequency range.
Keywords
Source effect, Site effect, Seismic parameters, Generalized inversion, Zagros

Subjects

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Iranian Journal of Geophysics
Volume 19, Issue 4
September and October 2025
Pages 39-57