نوع مقاله : مقاله تحقیقی (پژوهشی)
1 مؤسسه ژئوفیزیک دانشگاه تهران، ایران
2 پژوهشگاه بینالمللی زلزلهشناسی و مهندسی زلزله، تهران، ایران
3 دانشگاه تحصیلات تکمیلی علوم پایه زنجان، ایران
عنوان مقاله [English]
The attenuation of seismic waves is one of the basic physical parameters used in seismological studies and earthquake engineering, and is closely related to the seismicity and regional tectonic activity of a particular area. Seismic wave attenuation is caused by two major factors: scattering at heterogeneities in the earth and intrinsic absorption by anelasticity of the earth. The inverse of quality factor represents the attenuation. There are different methods for estimating the quality factor of Shear and Coda waves. In this study, the quality factors of Shear waves (QS) and Coda waves (QC) have been estimated in the Hormuzgan regionin the south of Iran. This region is located in the southeastern Zagros seismotectonic region. Several faults exist in this region, including the Main Zagros Reverse Fault (MZRF), High Zagros Fault (HZF), Zagros Foredeep Fault (ZFF), Mountain Front Fault (MFF) and Minab Fault. Recordings from the Bandar-Abbas (BNDS) station (located in the Hormuzgan region, 27.40º N_56.17º E) of the Iranian National Seismic Network (INSN) of local earthquakes with signal-to-noise ratios greater than 3were used for this study. These events were recorded from June 2004 through August 2009 and registered magnitudes of between 2.5 and 5.1 (ML), epicentral distances of less than 100 km and average focal depths of about 15 km. In this study, the Coda Normalization Method (Aki, 1980) and the Single Back-Scattering Method (Aki & Chouet, 1975) are used for the estimation of QS and QC at the seven central frequencies of 1.5, 3.0, 4.5, 6.0, 9.0, 12.0 and 18.0 Hz. The Shear waves on 183 North-South (N-S) components and 142 East-West (E-W) components, and the Coda waves on 200 Vertical (U-D) components, have been analyzed to estimate QS and QC, respectively. Time windows of the Shear waves were determined by the Kinoshita algorithm and the velocity of the Shear waves was estimated at 3.5 km/s. The estimated frequency-dependent relationships of QS on N-S and E-W components are and , respectively. The QC values were computed at five lapse time windows (20, 30, 40, 50 and 60 sec), starting at double the time of the primary Shear wave from the time of origin. The frequency-dependent relationships of QC vary from at 20 sec to at 60 sec lapse time window. The results show an increase in QC value with increasing lapse time windows. The estimated QC at a greater lapse time window indicates attenuation at a greater depth. In the Hormuzgan region, the values of Q at 1.0 Hz are less than 200 for the frequency-dependent relationships of QS and QC. Therefore, the Hormuzgan region is a highly tectonically and seismically active region; also, the medium is highly heterogeneous. The results reflect sedimentary deposits and salt domes in the Hormuzgan region. The QS frequency-dependent relationship in the Hormuzgan region is similar to that of the Ardabil and Avaj regions in northwestern Iran, and of the Strait of Messina in the south of Italy. Moreover, the QC frequency-dependent relationship in the Hormuzgan region is similar to that of the Alborz and Zagros regions in Iran and in the northwest of the Himalayan region. These regions are all tectonically and seismically active.