در مطالعات مرتبط با زلزله‌شناسی مهندسی برای برآورد خطر لرزهای در مناطق مختلف و همچنین تعیین دقیق بزرگا، شبیه‌سازی جنبش نیرومند زمین و مطالعه انرژی مخرب حاصل از زمین‌لرزه در حوزه نزدیک تا متوسط، تخمین ضریب کیفیت امواج برشی و فشاری نقشی اساسی دارد. بدین منظور با توجه به قرار گرفتن چندین شهر بزرگ در پهنه شمال غربی فلات ایران، ضریب کیفیت امواج فشاری و برشی با استفاده از داده‌های هفده ایستگاه لرزه‌نگاری با بیش از 13000 زمین‌لرزه ثبت شده در شبکه مرکز لرزه‌نگاری کشوری (IRSC) و شبکه ملی لرزه‌نگاری نوارپهن ایران (INSN) برآورد شد. برای سه گروه داده به‌ترتیب با فاصله رومرکزی کمتر از 100 کیلومتر، از 100 تا 200 کیلومتر و صفر تا 200 کیلومتر بررسی تغییرات جذب امواج حجمی در نُه نوار بسامدی، با بسامدهای مرکزی در 3، 5، 7، 10، 14، 20، 28، 38 و 47 هرتز انجام گرفت و برای هرکدام از ایستگاه‌ها برای بسامدهای مختلف به‌طور جداگانه ضریب کیفیت تخمین زده‌شد. در این مطالعه از روش کُدای بهنجارشده برای برآورد ضریب جذب امواج فشاری و برشی حاصل از زمین‌لرزه به‌عنوان روشی قابل اعتماد با توجه به حذف شدن اثرات چشمه و ساختگاه در مدل‌سازی انجام گرفته، استفاده شد. برای پهنه شمال غربی ایران مقادیر وابستگی بسامدی ضریب کیفیت امواج فشاری و برشی در همه ایستگاه‌ها برآورد شد به‌طوری‌که مقدار متوسط آن برای محدوده شمال غرب فلات ایران از و پیروی می‌کند. با توجه به مقادیر پایین و لذا جذب بالای امواج برشی و فشاری در پهنه شمال غرب ایران دامنه امواج حین عبور از زمین به‌شدت تضعیف می‌شود که این اثر جذب امواج لرزهای، باعث کاهش خسارات ناشی از زمین لرزه‌ها در فواصل مناسب از گسل‌ها در زمان وقوع زمین‌لرزه خواهد شد.
Estimation of compressional and shear wave quality factor in North West of Iranian Plateau
نویسندگان [English]
Mojtaba Naghavi1؛ Habib Rahimi2؛ Ali Moradi1
1Institute of Geophysics, University of Tehran
2Institute of Geophysics, University of Tehran
چکیده [English]
The purpose of this study is to estimate compressional and shear wave quality factors of seismic waves by using local earthquakes occurred in the NW of Iranian Plateau. In seismological engineering studies, quality factor estimation of body and shear waves plays an important role in seismic risk assessment of different areas, determining the exact magnitude of the earthquake, strong ground motion simulation and study of destructive energy of earthquake from near to intermediate region. In this study, earthquakes recorded in the Iranian Seismological Center (IRSC) and Iranian National Broadband Seismic Network (INSN) for the longitudinal band from 43°E to 53°E and the latitude band from 36°N to 40°N were used. Among the 17 stations, 14 stations belong to the IRSC and the rest belong to the INSN. Due to the presence of some big cities in the northwestern part of Iranian Plateau, quality factors of body and shear waves were estimated by using the data of 17 seismological stations including 13000 recorded earthquakes of the IRSC and INSN.
This region of intense deformation is situated between two thrust belts of the Caucasus to the north and the Zagros Mountains to the south. The NW of Iranian Plateau is a part of Turkish–Iranian plateau and includes historical and destructive earthquakes and two volcanoes with lots of thermal springs around. The North Tabriz Fault (NTF) is one of the active faults in NW Iran that has a clear surface expression.
Seismic quiescence and large historical earthquakes in the region in more than the two last centuries have increased the seismic risk of this region. To estimate seismic hazard in an area, a two-step process is needed. First, we must understand the nature of the earthquake sources that generate potentially hazardous ground motion. This includes knowledge of the distribution of seismic source zones, predominant fault mechanisms and return times of large events. Second, we must understand the effects of the transmitting medium (the Earth) on the seismic waves. A synthesis of the source and path effects will allow us to calculate the ground motion at a given site. Seismic attenuation is also caused by intrinsic mechanisms that convert the wave energy to heat through friction, viscosity, and thermal relaxation processes. Scattering redistributes wave energy within the medium but does not remove energy from the overall wavefield. In contrast, intrinsic attenuation mechanisms convert the wave energy to heat through friction, viscosity, and thermal relaxation processes. Energy loss caused by inelastic behavior is called inherent or internal attenuation and is determined by the inverse of the Q parameter. Large values of quality factor mean that attenuation is low and when Q is equal to zero, attenuation is very high.
Aki (1980) used the normalized Coda for the first time in order to estimate absorption amplitude of the S waves. Since then, this method has frequently been used in seismological studies for estimation of the absorption parameters of seismic waves (see, for example, Yoshimoto, 1993; Hatzidimitriou, 1995).
For three categories of data with epicentral distances less than 100 km, from 100 to 200 km and 0 to 200 km, attenuation variation investigation of body waves was carried out in 9 frequency bands with central frequencies of 3, 5, 7, 10, 14, 20, 28, 38 and 47 Hz and the quality factor was estimated in different frequencies for each station, separately. For the northwesern part of Iran, the frequency dependence of the body and shear wave quality factors in all stations were estimated so that their average values are quantified as Qp=55f0.84 and Qs=38f 0.93 , respectively. Due to the low values of the Q parameter and thus high attenuation values of body and shear waves in North West of Iranian Plateau, the amplitude of the propagated waves are decreased severely in the interested area when these waves pass through it. The attenuation effect of seismic waves would reduce the damages caused by the earthquakes at appropriate distances from the faults at the time of probable earthquake occurence.
کلیدواژه‌ها [English]
Compressive and shear waves, NW of Iranian Plateau, Attenuation, Quality factor
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