Estimation of geometrical spreading, quality factor and kappa in the zagros region

We have investigated the attenuation characteristics of the S-wave of 129 earthquakes recorded by 180 accelerometers in the Zagros region of Iran. A total number of 1641 Fourier amplitude spectra were used for a step by step estimation of the high-frequency attenuation, geometrical spreading parameters and the frequency dependence of S-wave quality factor. Two different criteria for isolating the S-wave window were examined in order to assess the effect of window selection on the estimated parameters. For the horizontal and vertical zero-distance, kappa values of 0.043 and 0.033 were obtained, respectively. The S-wave window selection method does not affect the estimated kappa values. We used Bayesian approach to estimate the tri-linear geometrical spreading parameters. The hinge positions of the tri-linear model (63 ± 5 and 124 ± 18 km) and the geometrical spreading coefficient (−0.8) for distances smaller than 63 km were found to be independent of frequency and window type. Both spreading coefficients for distances beyond the first hinge are highly frequency dependent, while at distances beyond 124 km, the coefficients at all frequencies are sensitive to the window type. The frequency-independent spreading coefficients were estimated by simultaneous inversion of all frequencies, and the absolute value of the coefficients at longer distances were found to be larger for longer S-wave windows. The average of the coefficients over both windows for the second and third hinges are −0.17 and −0.55, respectively. The S-wave quality factor is influenced by window type only at frequencies larger than 5 Hz. The frequency dependence of the quality factor at lower frequencies was determined as 141f0.84.