Evaluation of Strong-Motion Parameters in Qom Province in Iran Using Generalized Inversion Method

Being located on the seismic belt, Iran has experienced frequent and sometimes intense seismic activities. Many researchers discussed about seismotectonic characteristics of Iran, and based on similarities in seismotectonic parameters, they divided Iran into different tectonic provinces. Most acceptable seismotectonic divisions for Iran is provided by Mirzaie which introduces five tectonic provinces; Alborz-Azerbaijan (North and Northwest of Iran), Kopet Dagh (Northeast of Iran), Zagros (Southwest of Iran), Makran (Southeast of Iran) and Central-East of Iran. In this study, Qom region located in Central-East province is discussed, which is a seismic active region and faced large earthquakes such as Avaj (22/6/2002) with magnitude 6.1 epicentered in 35.71˚N and 49.02˚E, Silakhor(30/3/2006) with magnitude 6.2 epicentered in 33.69˚N and 48.95˚E and Kahak (18/6/2007) with magnitude 5.7 epicentered in 34.54˚N and 50.94˚E, hence it is very important to have estimates of earthquake parameters such as anelastic and geometrical attenuation, average stress parameter, and site amplification for this region, which has high seismicity rates and especially has populated areas with high seismic risk. In this study, generalized inversion method is used which is common approach for deriving the modeling parameters from the empirically observed data. Generalized inversion method introduced by Andrews. Andrews recast the spectral ratios approach into an inverse problem for earthquakes and stations with multiple records. This method has been later improved by other researchers. Ground-motion amplitudes are affected by source, path and site effects. Source term includes the effect of magnitude, stress parameter and the type of faulting on the observed ground-motion amplitude. Path term includes both anelastic and geometrical attenuation. Site term includes the effect of the near surface features like geology and topography on the observe data that consists of site amplification and high frequency attenuation. High frequency attenuation is calculated by estimating kappa using Hough-Anderson method. Convolution of these effects is what stations are recording as earthquake records. Generalized inversion method introduces a way to divide recorded spectrum into mentioned effects. At first, by transforming time-domain data into frequency-domain and then with the aid of natural logarithm operator, a linear equation is achieved. Afterward, we can derive modeling parameters for each of the effective terms. In this study, for source term, the Brune source model (omega square) is used. To remove the trade-off between anelastic attenuation and geometrical spreading in path effect, geometrical spreading model from previous studies is used. In this formulation, all the site spectra may be multiplied by an arbitrary function of frequency and all the source spectra divided by the same function without changing the fit to the data that adds a degree of freedom to our system, which in this study is resolved by using predefined site term for reference site (Arak station) as it is located on hard rock (Vs30 >700 m/s) and can be evaluated using Boore-Joyner generic rock site model. Database is constructed from 39 number of strong-motion records, recorded in 14 stations from 9 earthquakes with magnitude greater than 4 within target region. At first, horizontal and vertical components are extracted (horizontal component is considered as geometric mean of two horizontal components) and corrected using wavelet de-noising method. Shear wave extracted from all records using Husid method for beginning of S-wave and Kinoshita method for end of S-wave and Fourier spectrum is sampled in frequencies between 0.4 and 15 Hz. A system of over-determined equations is resulted. Stress drop, corner frequency, moment magnitude and seismic radius is calculated for each earthquake using Grid-search method. The relation between S-wave energy and moment magnitude is expressed by , regional stress drop is 81 bar and frequency dependent quality factor is that aligns well with previous studies in this region. Calculated quality factor indicates that the region of study has high seismicity rates. Also site amplification is calculated for all the stations in the selected frequency range of 0.4-15 Hz. Results of this study can further be used for a better evaluation of seismic hazard in a region of interest.