نوع مقاله : مقاله تحقیقی (پژوهشی)
1 موسسه ژئوفیزیک، دانشگاه تهران، ایران
2 پژوهشگاه بین المللی زلزله شناسی و مهندسی زلزله، تهران، ایران
عنوان مقاله [English]
Studying anisotropy properties is a proper procedure to survey the rate of tectonic phenomena in the upper crust and mantle. Therefore, analyzing specific phases of the shear wave, which is intensely sensitive to both the earthquake mechanism and any anisotropy along the path in lithosphere and upper astonosphere, is worthwhile. Consequently, the aim was to determine the relation between the strain and anisotropy, calculation of anisotropic parameters around the world, and determining how much formed by past and present lithospheric deformation and how much by crustal and asthenospheric sources.
Most of the researchers consider the anisotropy in the upper crust (10-15 km) as a result of micro crack orientations parallel to minimum main stress.
Seismic anisotropy is the variation of seismic wave speed with direction. Whenever a shear wave reaches the anisotropic media, it splits into two directions called fast and slow directions. Parameters which describe the seismic anisotropy are the directions of the polarization of the fast shear wave (φ) and the splitting time between the fast and slow component of shear waves (δt). Two techniques are more commonly used to calculate the anisotropy parameters. The first one is based on the calculation of the cross correlation coefficient of the two horizontal S wave components (Bowman and Ando, 1987). The second one is based on the diagonalization of the covariance matrix of the S waves (Silver and Chan, 1991). Silver and Chan (1991) demonstrated that these two techniques were theoretically equivalent. Zhang and Schwartz (1994) found that the same results were inferred from the application of the two techniques to crustal earthquakes.
The methodology in this research is based on the diagonalization of the covariance matrix of the S waves (Silver and Chan, 1991), which has been automated by Teanby et al. (2004.(
First, a shear-wave analysis window is defined. If anisotropy is present, the particle motion within this window will be elliptical. Second, a grid search over φ and δt is performed. The result that has the lowest second eigenvalue of the corrected particle-motion covariance matrix indicates linear particle motion after correction and is the solution that best corrects for the splitting.
In sum, the method consists of three steps: First φ and δt are calculated for a range of start and end times, and a 2D diagram of φ versus δt is plotted. Second, a stable region with tight clusters or desired compaction is specialized by a cluster analysis. Finally, the optimized clusters are applied and the window with the least error in the evaluation of φ and δt is determined (Teanby et al., 2004).
In order to select accurate wave forms, an important factor must be taken into consideration: The arrival angle of the S waves must be less than the critical angle. Because whenever S wave hits the surface at an angle (i) greater than the critical angle (ic= sin-1(Vp / Vs)), it can be recorded at the surface as elliptically polarized (Nuttli,1961; Booth and Crampin, 1985). Therefore, the arrival angel of S waves must be less than 35º (i < ic).
We applied a casual band pass filter in the frequency range 1-15 Hz to the waveforms to avoid the effects of long period surface waves.
This research is based on analyzing shear wave splitting by use of Sg phase to calculate the anisotropy at the upper crust in Mohammadabad Rigan (Southeast of Iran). Data were provided by the temporary seismic network, assembled by IranianSeismologicalCenter.
By analyzing 654 waveforms, we observed an anisotropy at each station and evaluated the direction of the fast component of shear wave in N45 ± 9 °E. The calculated splitting time was 0.18 ± 0.004 (s) on average, which was in accordance with the splitting time in the upper crust. The direction was not consistent with the act of known faults in this region. Therefore, it showed a new fault trend considering the locations of micro earthquakes and the focal mechanism of the main event.