تعیین پارامترهای ناهمسانگردی و راستای تنش محلی براساس تحلیل پس‌لرزه‌‌‌های زلزله 29 آذرماه 1389 محمدآباد ریگان (استان کرمان)

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

1 موسسه ژئوفیزیک، دانشگاه تهران، ایران

2 پژوهشگاه بین المللی زلزله شناسی و مهندسی زلزله، تهران، ایران

چکیده

بررسی پارامترهای ناهمسانگردی در پوسته‌ فوقانی یکی از روش‌های مناسب برای بررسی زمین‌ساخت فعال در منطقه است. بدین معنی که می‌توان با استفاده از روابط ناهمسانگردی، ارتباط میان رویداد زمین‌لرزه و فعالیت گسلی خاص و در حالت کلی‌تر، راستای تنش زمین‌ساختی حاکم بر منطقه را به‌دست آورد. چنانچه اندازه یک پارامتر در جهت‌های متفاوت اندازه‌گیری یکسان نباشد، در آن‌صورت محیط مورد بررسی نسبت به این پارامتر، ناهمسانگرد خوانده می‌شود. محققان ناهمسانگردی لرزه‌ای در اعماق کم پوسته (10-15 کیلومتر) را نتیجه‌ای از جهت‌گیری ترجیحی ریز‌ترک‌‌‌‌های قائم می‌دانند. در پژوهش حاضر با استفاده از فاز بُرشی Sg، براساس روش تینبای و همکاران ارائه شده در سال 2004، پارامتر‌های ناهمسانگردی در پوسته فوقانی منطقه محمدآباد ریگان تعیین شده است. زمین‌لرزه اصلی در ساعت 22:11:58 روز 29 آذرماه 1389 با بزرگی 5/6 در مقیاس امواج گشتاوری (MW) در 52 کیلومتری جنوب شرقی محمد‌آباد ریگان کرمان و در مرز استان‌های کرمان و سیستان و بلوچستان به وقوع پیوست. برطبق نتایج به‌دست آمده حاصل از بررسی 654 لرزه‌نگاشت مربوط به شش ایستگاه موقت نصب شده از سوی مرکز لرزه‌نگاری کشوری، آزیموت راستای غالب ناهمسانگردی در منطقه محمدآباد ریگان 9 ± 45 درجه و آزیموت راستای کمینه تنش اصلی در منطقه 9 ± 135درجه به‌دست آمده است. همچنین میانگین بزرگی ناهمسانگردی 004/0 ± 18/0 ثانیه محاسبه شده است که با میزان ناهمسانگردی در پوسته فوقانی مطابقت می‌کند. با مقایسه نتایج حاصل از این پژوهش با روند پس‌لرزه‌‌‌‌های تعیین محل شده و بررسی‌های جی پی اس می‌توان فعالیت‌های زمین‌ساختی اخیر در منطقه را به روند گسلی جدید در بخش جنوبی گسل کهورک نسبت داد.
 
 

کلیدواژه‌ها


عنوان مقاله [English]

Anisotropy parameter analysis in Mohammadabad, Rigan based on the aftershock analysis of the earthquake of December 20, 2010

نویسندگان [English]

  • Shirin Mirahmadi Shalamzari 1
  • Ahmad Sadidkhouy 1
  • Ali Rezaei Nayeh 1
  • Gholam Javan Doloei 2
چکیده [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.
 
 

کلیدواژه‌ها [English]

  • Anisotropy
  • Mohammadabad Rigan
  • shear wave Sg
  • Stress
  • upper crust
  • temporary seismic network

آقانباتی،1383، زمین‌‌شناسی ایران، انتشارات سازمان زمین‌‌شناسی واکتشافات معدنی کشور، تهران.

رضا،م.، 1391، تحلیل پس‌لرزه‌‌­های زمین‌لرزه 29/9/ 1389 محمدآباد ریگان، پایان نامه کارشناسی ارشد، پژوهشگاه بین‌‌المللی زلزله‌‌شناسی و مهندسی زلزله.

Agard, P., Omrani, J., Jolivet, L., and Mouthereau, F., 2005, Convergence history across Zagros (Iran): constraints from collisional and earlier deformation: Int. J. Earth Sci., 94, 401-419.

Babuska,V., and Cara, M., 1991, Seismic anisotropy in the Earth, Kluwer Academic Publishers, 217.

Berberian, M., 1976, Contribution to the seismotectonics of Iran (part II), p. 518, Geological Survey of Iran, Tehran.

Booth, D. C., Crampin, S., Lovell, J. H., and Chiu, J. M., 1991, Temporal changes in shear wave splitting during an earthquake swarm in Arkansas: J. Geophys. Res., 95(11), 151-11,164.

Bowman, J. R., and Ando, M., 1987, Shear-wave splitting in the upper-mantle wedge above the Tonga subduction zone: Geophys. J. R. Astr. Soc., 88, 25-41.

Crampin, S., 1984, An introduction to wave propagation in anisotropic media: Geophys. J. R. Astron. Soc., 76, 17-28.

Crampin, S., and Zatsepin, S. V., 1997, Modelling the compliance of crustal rock, II-response to temporal changes before earthquakes: Geophys. J. Int., 129, 495-506.

Everitt, B. S., Landau, S., and Leese, M., 2001, Cluster Analysis: Fourth Ed., Arnold, London.

Hatzfeld, D., Tatar, M., Priestley, K., and Ghafory-Ashtiany, M., 2003, Seismological constraints on the crustal structure beneath the Zagros Mountain belt (Iran): Geophys. J. Int., 155,403-410.

Nuttli, O., 1961, The effect of the Earth’s surface on the S wave particle motion: Bull. Seism. Soc. Am., 51, 237-246.

Savage, M. K., 1999, Seismic Anisotropy and Mantle Deformation; What have we learned from shear wave splitting?: Reviews of Geophysics, 37(1), 65-91.

Silver, P. G., and Chan, W. W. J, 1988, Implications for continental structure and evolution from seismic anisotropy: Nature, 335, 34-39.

Silver, P. G., and Chan, W. W. J., 1991, Shear–wave splitting and subcontinental mantle deformation: J. Geophys. Res., 96, 16429-16454.

Tapley, W., Tull, J., Miner, L., and Goldstein, P., 1990, SAC command reference manual, version 10.5.

Teanby, N. A., Kendall J. M., and Vander Baan, M., 2004, Automation of shear wave splitting measurements using cluster analysis: Bull. Seism. Soc. Am., 94, 453-463.

Tsukada, S., 1994, Analysis of shear wave splitting by using high density seismic array data- a quantitative study of anisotropy in the upper crust, Earthquake Research Institute: University of TOKYO.

Vernant, P., Nilforoushan, F., Chery, J., Bayer, R., Djamour, Y., Masson, F., Nankali, F., Ritz, J. F., Sedighi, M., and Tavakoli, F., 2004, Deciphering oblique shortening of central Alborz in Iran using geodetic data: Earth and Planetary Science Letters, 223, 177-185.

Vidale, J. E., 1986, Complex polarization analysis of particle motion: Bull. Seism. Soc. Am. 71, 1511-1530.

Walker, R., and Jackson, J., 2004, Active tectonics and late Cenozoic strain distribution in central and eastern Iran: Tectonics.23, pp. TC5010 1-24.

Zatsepin, S. V., Crampin, S., 1997, Modelling the compliance of crustal rock: The response of shear-wave splitting to differential stress: Geophys. J. Int., 129, 477–494.

Zhang, Z., and Schwartz, S. Y., 1994, Seismic anisotropy in the shallow crust of the Loma Prieta segment of the San Andreas fault system: J. Geophys. Res., 99, 9651–9661.