برآورد ضریب کیفیت امواج SH در جزیره قشم با استفاده از پس‌لرزه‌های زلزله 6 آذر 1384

نوع مقاله : مقاله پژوهشی‌

نویسندگان

گروه فیزیک دانشکده علوم، دانشگاه هرمزگان، هرمزگان، ایران

چکیده

در این مطالعه تضعیف امواج SH در جزیره قشم مورد بررسی قرار گرفته است. امواج برشی دارای دو مؤلفه عمودی و افقی است. بخش افقی آن که موج SH نامیده می‌شود، مخرب‌ترین بخش امواج زلزله است. تعداد 661 پس‌لرزه ناشی از زلزله 4 آذر 1384 قشم که به‌دقت تعیین محل شده بودند انتخاب و 18343 نگاشت مربوط به این رویدادها مورد پردازش قرار گرفت. با چرخش مؤلفه‌ها، بخش افقی امواج برشی را جدا کرده و ضریب کیفیت امواج برشی افقی با استفاده از روش بهنجارش پساموج در پنج پهنای باند فرکانسی 2-1، 4-2، 8-4، 16-8، 32-16 هرتز با فرکانس مرکزی 5/1، 3، 6، 12، 24 هرتز در پنجره زمانی 30 ثانیه که با فیلتر میان‌گذر باترورث مرتبه 2 شده‌اند، برآورده شده است. بر اساس محاسبات انجام شده تضعیف امواج برشی افقی به‌صورت زیر می‌باشد: QSH = (11 ± 1.2) f(1.2 ± 0.105) که به معنای تضعیف زیاد امواج برشی است. همچنین تضعیف امواج برشی در دو امتداد شمال غرب- جنوب شرق و شمال شرق- جنوب غرب، به‌طور جداگانه به ترتیب به‌صورت QSH = (9 ± 1.5) f(1.49 ± 0.185) و QSH = (10 ± 1.2) f(1.23 ± 0.1) به دست آمد.
در بررسی آزیموتی، تضعیف امواج برشی افقی برای فرکانس‌های کم، در دو راستای شمال‌‌ شرق- جنوب ‌غرب و شمال ‌غرب- جنوب ‌شرق تقریباً برابر شده است که به نظر می‌رسد در این منطقه تضعیف مؤلفه افقی امواج برشی تحت تأثیر امتداد ساختارهای زمین‌ساختی قرار نگرفته و بیشتر تابع جنس مواد تشکیل‌دهنده‌ی پوسته در این منطقه می‌باشد. در فرکانس‌های بیشتر از 6 هرتز، تفاوت قابل ملاحظه‌ای بین تضعیف در دو امتداد وجود دارد که می‌تواند ناشی از ابعاد ناهمگنی‌های موجود در منطقه باشد. مقدار پارامتر n که بیانگر میزان پراکنش امواج لرزه‌ای است در امتداد شمال غرب- جنوب شرق به‌صورت قابل ملاحظه‌ای بیشتر بوده که دلالت بر پراکنش بیشتر امواج برشی در این امتداد دارد.

کلیدواژه‌ها


عنوان مقاله [English]

Estimation of quality factor for SH waves in Qeshm Island, using aftershocks of 27 November 2005

نویسندگان [English]

  • Nazanin Arbab
  • Abbas Gholamzadeh
Physics Department, Faculty of Science, Hormozgan University, Hormozgan, Iran
چکیده [English]

The attenuation quality parameter (Q) is a phenomenological quantity depending on the observations and on the underlying theoretical models. Attenuation of seismic waves is expressed with inverse quality factor (Q-1) and helps understand the physical laws governing the propagation of seismic waves in the lithosphere. Attenuation is often found to be anisotropic (directionally dependent) due to a variety of factors such as the intrinsic anisotropy of the material, the presence of aligned fluid-fractures (Batzle et al., 2005), or interbedding of thin layers with different properties (Zhu et al., 2007). The magnitude of attenuation anisotropy can be much higher than that of velocity anisotropy, and the symmetry of the attenuation coefficient can be different than that of the velocity function (Liu et al., 2007). The observed seismic-wave amplitudes usually decay exponentially with increasing travel distance after the correction for geometrical spreading, and decay rates are proportional to Q−1 that characterizes the spatial attenuation for SH-wave. Qeshm Island, the largest island of the Persian Gulf, is important because of various aspects such as population, economics and some oil and gas reservoirs.
Since the most destructive part of the elastic waves, is the horizontal component of the shear waves, estimation of attenuation of the horizontal component will provide us with very useful information. Horizontal components of shear waves are also affected by the structure of the earth.
 In this study, 661 well-located aftershocks are selected and 18342 seismograms are used to the calculation. by rotation of the components, the horizontal part of the shear waves are separated and horizontal shear wave quality factor was determined by using the Coda normalization method in five frequency bands 1-2, 2-4, 4-8, 8-16, 16-32 (Hz) with a central frequency of 1.5, 3, 6, 12, 24 Hz, in the lapse time of 30 seconds. Based on the calculations, the frequency dependence relation for shear waves: QSH = (11 ± 1.2)f(1.2±0.105).
The relationship between the frequency dependence for horizontal shear waves shows the attenuation in the Qeshm Island is very high and consequently the region is seismically active. Besides, a small amount of the quality factor for horizontal shear waves associated with the low velocity of shear wave propagation in the crust that may relate to the presence of gas and oil fluids and some salt dome. In the azimuthal study, the attenuation of horizontal shear waves are calculated in two directions: northeast-southwest and northwest-southeast. For low frequencies, the attenuation in the northeast-southwest direction is close to northwest-southeast direction, which seems the horizontal component of the shear waves are not affected by tectonic structures, so it seems mostly to be dependent on the material of the earth, whereas for high frequencies greater than 6 Hz, there are significant differences between two azimuthal attenuation, that can be due to some small-scale heterogeneity of the region.

کلیدواژه‌ها [English]

  • Attenuation
  • Quality factor
  • SH
  • horizontal shear wave
  • Qeshm
  • Zagros
شاه پسندزاده، م.، و حسامی، خ.، 1385، بررسی گسیختگی‌های سطحی ناشی از زمین‌لرزه ششم آذرماه 1384 جزیره قشم: پژوهشنامه زلزله‌شناسی و مهندسی زلزله، 2، 34-41.
رحیمی، ن.، و غلام‌زاده، ع.، 1396، برآورد رابطه وابستگی فرکانسی ضریب کیفیت امواج فشارشی در جزیره قشم: مجله ژئوفیزیک ایران، 11(2)، 118-110
Aki, K., 1969, Analysis of the seismic coda of local earthquakes as scattered waves: Journal of Geophysical Research, 74(2), 615-631.
Aki, K., 1980, Attenuation of shear-waves in the lithospher for frequencies from 0.05 to 25 Hz: Physics of the Earth and Planetary Interiors, 21, 50-60, DOl 10.1016/0031-9201(80)90019-9.
Aki, K., and Chouet, B., 1975, Origin of coda waves: source, attenuation, and scattering effects: Journal of Geophysical Research, 80(23), 3322-3342.
Berberian, M., 1995, Master “blind” thrust faults hidden under the Zagros folds, active basement tectonics and surface morphotectonics: Tectonophysics, 241(3).
Bindi, D., Parolai, S., Grosser, H., Milkereit C., and Karakisa, S., 2006, Crustal attenuation characteristics in northwestern Turkey in the range from 1 to 10 Hz: Bulletin of the Seismological Society of America, 96(1), 200-214.
Farrokhi, M., and Hamzehloo, H., 2017, Body wave attenuation characteristics in the crust of Alborz region and North Central Iran: Journal of Seismology, 21.4, 631-646.
Haghipour, A., 2006, Geology of Qeshm, Qeshm Free Zone Organization Publication: P. 5.
Havskov, J., Ottemöller, L., 2010b, Routine data processing in earthquake seismology: Springer Science + Business Media BV, DOI 10: 978-90.
Horasan, G., and Boztepe-Gu¨ney, A., 2004, S-wave attenuation in the Sea of Marmara, Turkey: Physics of the Earth and Planetary Interiors, 142, 215–224.
Hoshiba, M., 1995, Estimation of nonisotropic scattering in western Japan using coda wave envelopes: Application of a multiple nonisotropic scattering model: Journal of Geophysical Research: Solid Earth, 100(B1), 645-657.
Hudson, J. A., 1981, Wave speeds and attenuation of elastic waves in material containing cracks: Geophysical Journal International, 64(1), 133-150.
Jackson, J., and McKenzie, D., 1984, Active tectonics of the Alpine—Himalayan Belt between western Turkey and Pakistan: Geophysical Journal International, 77(1), 185-264.
Kinoshita, S., 1994, Frequency-dependent attenuation of shear waves in the crust of the southern Kanto area, Japan: Bulletin of the Seismological Society of America, 84(5), 1387-1396.
Mahood, M., 2014, Attenuation of high-frequency seismic waves in Eastern Iran: Pure and Applied Geophysics, 171(9), 2225-2240.
Ma’hood, M., Hamzehloo, H., and Doloei, G. J., 2009, Attenuation of high frequency P and S waves in the crust of the East-Central Iran: Geophysical Journal International, 179(3), 1669-1678.
Rahimi, H., and Gholamzadeh, A., 2017, Coda Q in Qeshm Island, south of Iran, using aftershocks of the Qeshm earthquake of November 27, 2005: Arabian Journal of Geosciences, 10.11, 239.
Rahimi, H., Hamzehloo, H., and Kamalian, N., 2010, Estimation of coda and shear wave attenuation in the Volcanic area in SE Sabalan Mountain, NW Iran: Acta Geophysica, 58.2, 244-268.
Sato, H., 1977, Energy propagation including scattering effects single isotropic scattering approximation: Journal of Physics of the Earth, 25(1), 27-41.
Sato, H., Fehler, M. C., and Maeda, T., 2012, Seismic wave propagation and scattering in the heterogeneous earth , Berlin: Springer, 496.
Vernant, P., Nilforoushan, F., Chery, J., Bayer, R., Djamour, Y., Masson, F., Nankali, H., 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.
Yaminifard, F., Tatar, M., Hessami, K., Gholamzadeh, A., and Bergman, E. A., 2012, Aftershock analysis of the 2005 November 27 (Mw 5.8) Qeshm Island earthquake (Zagros-Iran): Triggering of strike-slip faults at the basement: Journal of Geodynamics, 61, 138-147.
Yoshimoto, K., Sato, H., Ohtake, M., 1993, Frequency-dependent attenuation of P and S waves in the Kanto area, Japan, based on the coda-normalization method: Geophysical Journal International, 114, 165–174.
Yoshimoto, K., Sato, H., Iio, Y., Ito, H., Ohminato, T. and Oh-Take, M., 1998, Frequency-dependent attenuation of high-frequency P and S waves in the upper crust in western Nagano, Japan: Pure and Applied Geophysics, 153, 489–502.