Spatial variations of coda-wave attenuation in the Khurgu region, south-east of the Zagros from analysis of local earthquakes
Introduction
The attenuation features of high-frequency seismic wave of earth's lithosphere are essential parameters used to determine earthquakes and seismic source properties that have generated strong ground motion characteristics; therefore, assessing the seismic hazards of an area is imperative. Moreover, propagation distance is used to attenuate the seismic wave energy, principally because of geometric spreading and two mechanisms corresponding to loss, in particular, scattering attenuation and intrinsic absorption. The intrinsic absorption is considered as an irreversible energy loss since the medium is inelastic; however, the dispersive attenuation is taken into account as the redistribution of the seismic energy which takes place when the interaction between seismic waves and the medium heterogeneity occurs. The attenuation of seismic waves in the lithosphere, in particular, the spatial variation, regional values, Qc−1, and also the decay rate of coda amplitudes, was advantageous in quantifying the seismicity and tectonics of the region (Jin and Aki, 1988).
There are various methods for estimating the seismic wave attenuation quality factor in a region. The attenuation of a high frequency seismogram employing a deterministic method is difficult to measure, because too many parameters are needed to sufficiently describe a high frequency seismogram. To conquer this problem, a statistical approach is used to study seismic waves with high frequency. The complete and statistic treatment of all parts of a seismogram is impossible, since a specific nature of the path among source and station are used to determine their characters (Aki, 1969). Another model was developed for the rate of coda decay by Aki (1969); it is indicated that seismic-coda wave of a local earthquake is considered as back-scattered body wave from a large number of distributed heterogeneities in no particular order in the curst of earth, and also a statistical method can be used to treat it. Since the longer wave originates from further distance, and because of some type of averaging upon numerous samples of heterogeneities, the later portions of a seismogram can be considered, suggesting a statistical treatment in which average features of the heterogeneous media are characterized by a few parameters (Aki and Chouet, 1975). Moreover, analysis of the local earthquake coda is one of the highly encouraging ways to investigate seismic scattering and attenuation in the lithosphere and earth's crust (Aki, 1969). Coda is the end of a local earthquake seismogram subsequent to the body as well as surface waves including an extremely complex and slowly decaying signal.
Coda attenuation values were obtained throughout the world, Qc They were compared with. Since lithosphere is characterized by heterogeneity, studies were carried out between 1 and 30 Hz in the frequency range. Qc values vary in each area by above a factor of 10. Different areas exhibit various tectonic activities. Such a condition can be observed in changes in Qc−1 measurements from region to region. Low-frequency dependency values in the seismically active regions in various sections of the universe have been achieved. These low-values have been a cause for procedures like fault that can lead to robust heterogeneity.
According to our research, to determine vertical as well as lateral change in attenuation of coda wave in Khurgu, southeastern of the Zagros (Fig. 1), recorded events by 25 temporal stations in this region is analyzed at various coda window and frequency bounds. In two steps, Coda Q can be estimated. Initially, the coda Q is evaluated at 25 stations to discover the spatial variation of Qc in the area using the epicentral station paths. A common Qc relationship is presented over these areas by finding similar Qc values for different stations. In the second step, coda Q is determined for six lapse times to study the attenuation at various depths. The variations which are depth-dependent, are therefore measured and compared to another investigation carried out in this research field. The findings are compared to the measured attenuation of the coda wave in some representative areas of the Iranian Plateau namely, Silakhor region; western region of Zagros; Jiroft, Bam and Zarand; East-central Iran; the Alborz region, and also the central region of the Iranian plateau. The results of our study are also compared with the values related to coda Q, reported in the same type of seismic and tectonic regions like the Himalayan collision zone in the world. In the end, some possible consequences are discussed.
Section snippets
General tectonic setting
According to the Vernant et al. (2004), Iran stands between two Arabia as well as Eurasia lithospheric plates; its convergence rate is about 23 millimeters annually at 26.5° N, 56.5° E. Moreover, accomodation of consequent shortening has been done via thrust faulting and folding in the Kopeh Dagh and Alborz mountains in northern part, and also mountain belt of Zagros or Makran Subduction zone in the southern part, as well as by slipping over number of significant NS trending strike slip faults
Result and discussion
We calculated the Qc (quality factor of coda-waves) for Khurgu area; SE of Zagros by utilizing single-back scattering approach (Aki and Chouet, 1975). The local earth-quakes registered by 25 temporal stations during a period of 3.5 months data were utilized in the present research. Moreover, we observed the high-frequency dependency of estimated Qc values. Finally, an analysis for lateral and vertical coda Q variations is performed our research.
Conclusion
Detailed crustal Q0 distribution has been considered to be one of the crucial input parameters for anticipating the ground motion and seismic hazards assessment. Moreover, vertical and lateral changes in coda Q were studied in the present research. In addition, Khurgu region coda Q study showed significant lateral and vertical variation in the north and south of High Zagros Fault (HZF), which is well correlated by the reported velocity structure. The relation to calculate the mean frequency for
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
IIEES(International Institute of Earthquake Engineering & SeismologyIIEES(International Institute of Earthquake Engineering & Seismology) has supported the present research. Therefore, the IIEES team would be appreciated for supportive, administrative, as well as field-work help under challenging conditions. We would like to thank Dr. Habib Rahimi for his useful help, and also thanks to an anonymous reviewer for their valuable comments. We also kindly appreciate the people of the Hormozgan
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