محاسبه کاهندگی طیف دامنه جنبش زمین در ناحیه شمال‌غرب ایران

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

دانشگاه تحصیلات تکمیلی علوم پایه زنجان

چکیده

در این مقاله با تلفیق داده‌های ثبت‌شده توسط 8 ایستگاه دائم شبکه‌ تبریز متعلق به شبکه‌ لرزه‌نگاری کشوری موسسه ژئوفیزیک دانشگاه تهران در بازه زمانی 1996 تا 2009 و داده‌های ثبت‌شده توسط 16 ایستگاه شبکه موقت لرزه‌نگاری دانشگاه تحصیلات تکمیلی علوم پایه زنجان در بازه‌ زمانی سال‌های 2008 تا 2011 در شمال‌غرب ایران، 3514 نگاشت از 943 زمین‌لرزه با بزرگای بین 6/1 تا 2/5 تحلیل و شکل منحنی کاهندگی با استفاده از الگوریتمRobust Lowess به‌صورت گرافیکی ارائه شد. شکل منحنی کاهندگی نشان می‌دهد که تغییر ضریب گسترش هندسی در فاصله‌ کانونی 70 کیلومتر اتفاق می‌افتد و سبب می‌شود تا یک مدل کاهندگی دو‌خطی برای برازش در بسامد‌های کم (کمتر از 5 هرتز) استفاده شود. ضریب گسترش هندسی تا فاصله 70 کیلومتر برای بسامد‌های مختلف به‌طور متوسط 28/0±1/1- به‌دست آمد. ضریب گسترش هندسی در فواصل بیش از 70 کیلومتر در بسامد‌های کمتر از 5 هرتز به‌طور متوسط 27/0±44/0- به‌دست آمد که در مقایسه با مقادیر گزارش‌شده برای سایر نواحی دنیا مقدار کوچکی است و نشان از وجود مرز ناپیوستگی موهو با تباین سرعتی ضعیف‌تر نسبت به آن مناطق دارد. ضریب گسترش هندسی در بسامدهای 5 هرتز و بالاتر، در فواصل کمتر از 70 کیلومتر و بیش از آن به هم شبیه است و مدل کاهندگی را از حالت دوخطی به حالت یک‌تکه تبدیل می‌کند. با استفاده از ضریب کاهندگی جذب ذاتی، تابع کیفیت موج برشی به‌صورت معرفی می‌شود.
در این مقاله با تلفیق داده‌های ثبتشده توسط 8 ایستگاه دائم شبکه‌ تبریز متعلق به شبکه‌ لرزه‌نگاری کشوری موسسه ژئوفیزیک دانشگاه تهران در بازه زمانی 1996 تا 2009 و داده‌های ثبت‌شده توسط 16 ایستگاه شبکه موقت لرزه‌نگاری دانشگاه تحصیلات تکمیلی علوم پایه زنجان در بازه‌ زمانی سال‌های 2008 تا 2011 در شمال‌غرب ایران، 3514 نگاشت از 943 زمین‌لرزه با بزرگای بین 6/1 تا 2/5 تحلیل و شکل منحنی کاهندگی با استفاده از الگوریتمRobust Lowess  به‌صورت گرافیکی ارائه شد. شکل منحنی کاهندگی نشان می‌دهد که تغییر ضریب گسترش هندسی در فاصله‌ کانونی 70 کیلومتر اتفاق می‌افتد و سبب می‌شود تا یک مدل کاهندگی دو‌خطی برای برازش در بسامد‌های کم (کمتر از 5 هرتز) استفاده شود. ضریب گسترش هندسی تا فاصله 70 کیلومتر برای بسامد‌های مختلف به‌طور متوسط 28/0±1/1- به‌دست آمد. ضریب گسترش هندسی در فواصل بیش از 70 کیلومتر در بسامد‌های کمتر از 5 هرتز به‌طور متوسط 27/0±44/0- به‌دست آمد که در مقایسه با مقادیر گزارش‌شده برای سایر نواحی دنیا مقدار کوچکی است و نشان از وجود مرز ناپیوستگی موهو با تباین سرعتی ضعیف‌تر نسبت به آن مناطق دارد. ضریب گسترش هندسی در بسامدهای 5 هرتز و بالاتر، در فواصل کمتر از 70 کیلومتر و بیش از آن به هم شبیه است و مدل کاهندگی را از حالت دوخطی به حالت یک‌تکه تبدیل می‌کند. با استفاده از ضریب کاهندگی جذب ذاتی، تابع کیفیت موج برشی به‌صورت  معرفی می‌شود.
 
واژه‌های کلیدی: رابطه کاهندگی، تابع کیفیت، شمال‌غرب ایران

کلیدواژه‌ها


عنوان مقاله [English]

Attenuation of ground-motion spectral amplitudes in the NW Iran

نویسندگان [English]

  • Khalil Motaghi
  • Zahra Zarunizadeh
  • Abdolreza Ghods
Institute for Advanced Studies in Basic Sciences, Zanjan
چکیده [English]

Estimation of seismic wave attenuation due to anelasticity and geometrical spreading has attracted major interests among earthquake engineering community in recent decades. The choice of ground-motion model has a significant impact on hazard estimates in an active seismic zone such as the NW Iran. Estimation of ground motion for a typical frequency range of 0.5–10 Hz is required for the proper design of earthquake resistant structures and facilities and is considered as input for engineering stochastic ground motion relationships. For seismological purposes, appropriate attenuation models make it possible to calculate more accurately the source parameters such as magnitude and seismic moment. The NW Iran has experienced very few large events during the operation of the accelerometer network of the Building and Housing Research Center (BHRC). The BHRC network has been operating since 1973 but has recorded ground acceleration for few events in the study area, because of the low seismicity rate. The availability of the abundant weak-motion waveform data from the short-period local seismograph network of the Institute of Geophysics of the University of Tehran (IGUT) provides an opportunity to derive a new and more reliable ground-motion relationship for small events to complement those of strong-motion results. In this study, we analysed 3514 records of 943 small and moderate events that were recorded by 8 permanent stations of Tabriz network (belonging to the IGUT) and 16 temporary stations of the Institute for Advanced Studies in Basic Sciences (IASBS) to prepare a dataset including week ground-motion spectral amplitudes for different magnitudes and hypocentral distances. We graphically found the distance at which the nature of geometrical spreading attenuation changes significantly using a locally weighted scatter-plot smoothing called robust LOWESS. A bilinear function with a hinge at distance of about 70 km describes the geometric spreading attenuation with distance. Geometrical spreading and intrinsic attenuation coefficients were calculated using nonlinear regression in different frequencies and an average value of  was found as geometrical spreading coefficient for distance range of 10–70 km. This value is consistent with geometrical spreading in a layered Earth. The average geometrical spreading coefficient of  was found for the frequency range 0.79–5 Hz and the distance range of 70–200 km. This value is smaller than the values reported for other regions in the world (e.g. +0.09 for Central Alborz: Motaghi and Ghods, 2012; +0.2 for North Iran: Motazedian, 2006; +0.2 for SE Canada and the NE United States: Atkinson, 2004; +0.1 for SE Australia: Allen et al., 2007) and indicates that the velocity contrast in the Moho discontinuity is smaller than that in the other regions. The low-velocity uppermost mantle in NW Iran was manifested by different types of tomographic results obtained for the region. The geometrical spreading coefficient   does not change before and after 70 km distance for frequencies ≥ 5 Hz. Thus, the attenuation relationship in this frequency range changed from bilinear to linear function. Using anelastic attenuation coefficients calculated at different frequencies, the shear-wave quality factor,  , obtained equal to   for frequencies greater than 1.5 Hz. In fact, the   values show a U-shaped behavior in all of the frequency ranges and the function that describes it is defined as .
Estimation of seismic wave attenuation due to anelasticity and geometrical spreading has attracted major interests among earthquake engineering community in recent decades. The choice of ground-motion model has a significant impact on hazard estimates in an active seismic zone such as the NW Iran. Estimation of ground motion for a typical frequency range of 0.5–10 Hz is required for the proper design of earthquake resistant structures and facilities and is considered as input for engineering stochastic ground motion relationships. For seismological purposes, appropriate attenuation models make it possible to calculate more accurately the source parameters such as magnitude and seismic moment. The NW Iran has experienced very few large events during the operation of the accelerometer network of the Building and Housing Research Center (BHRC). The BHRC network has been operating since 1973 but has recorded ground acceleration for few events in the study area, because of the low seismicity rate. The availability of the abundant weak-motion waveform data from the short-period local seismograph network of the Institute of Geophysics of the University of Tehran (IGUT) provides an opportunity to derive a new and more reliable ground-motion relationship for small events to complement those of strong-motion results. In this study, we analysed 3514 records of 943 small and moderate events that were recorded by 8 permanent stations of Tabriz network (belonging to the IGUT) and 16 temporary stations of the Institute for Advanced Studies in Basic Sciences (IASBS) to prepare a dataset including week ground-motion spectral amplitudes for different magnitudes and hypocentral distances. We graphically found the distance at which the nature of geometrical spreading attenuation changes significantly using a locally weighted scatter-plot smoothing called robust LOWESS. A bilinear function with a hinge at distance of about 70 km describes the geometric spreading attenuation with distance. Geometrical spreading and intrinsic attenuation coefficients were calculated using nonlinear regression in different frequencies and an average value of  was found as geometrical spreading coefficient for distance range of 10–70 km. This value is consistent with geometrical spreading in a layered Earth. The average geometrical spreading coefficient of  was found for the frequency range 0.79–5 Hz and the distance range of 70–200 km. This value is smaller than the values reported for other regions in the world (e.g. +0.09 for Central Alborz: Motaghi and Ghods, 2012; +0.2 for North Iran: Motazedian, 2006; +0.2 for SE Canada and the NE United States: Atkinson, 2004; +0.1 for SE Australia: Allen et al., 2007) and indicates that the velocity contrast in the Moho discontinuity is smaller than that in the other regions. The low-velocity uppermost mantle in NW Iran was manifested by different types of tomographic results obtained for the region. The geometrical spreading coefficient   does not change before and after 70 km distance for frequencies ≥ 5 Hz. Thus, the attenuation relationship in this frequency range changed from bilinear to linear function. Using anelastic attenuation coefficients calculated at different frequencies, the shear-wave quality factor,  , obtained equal to   for frequencies greater than 1.5 Hz. In fact, the   values show a U-shaped behavior in all of the frequency ranges and the function that describes it is defined as  .
 

کلیدواژه‌ها [English]

  • seismic attenuation relation
  • Quality factor
  • NW Iran
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