@article { author = {M.R, Najaftomraei and H, Rahimi and M, Rezapour}, title = {Estimation of spectral attenuation relation in the Zagros Region}, journal = {Journal of the Earth and Space Physics}, volume = {41}, number = {1}, pages = {13-24}, year = {2015}, publisher = {Institute of Geophysics, University of Tehran}, issn = {2538-371X}, eissn = {2538-3906}, doi = {10.22059/jesphys.2015.53419}, abstract = {Amplitudes of seismic waves decrease with distance according to anelastic properties of the earth and geometrical spreading. The attenuation of ground-motion amplitudes in the frequency domain is an important problem in engineering seismology. It is of particular practical interest in the regions such as Zagros (Iran). The Zagros fold- thrust belt, as a part of Alpine- Himalayan orogenic belt is one of the most active continental collision zones on the earth, which extends from the Tarus mountains in southeastern Turkey to the Minab fault in the east of the Strait of Hormoz in southern Iran. Structurally, its formation is related to the continuing convergent movement between the Arabian plate to the southwest and the Central Iranian Micro continent to the northeast, resulting from the north- northeastward drift of Afro-Arabia against Eurasia and so this region is seismically active. Hence, attenuation studies as well as other seismic studies seem to be necessary. The ground-motion relations are key inputs to seismic hazard analysis for engineered structures. In such cases, an empirical attenuation model determined from events provides critical input to the models of ground-motion generation. The attenuation model is used to play back attenuation effects to determine the apparent source spectrum for each earthquake in the database. The lack of an appropriate ground-motion prediction model may result in undesirable outcomes, such as unrealistically high or low loading standards in the design and construction of critical infrastructure such as large dams, power stations, and hospitals. For seismological purposes, appropriate attenuation models make it possible to calculate more accurately the source parameters such as magnitude and seismic moment. In this study about 10000 records due to 998 events recorded at the International Institute of Earthquake engineering and Seismology (IIEES) stations during 2006-2013 across the Zagros region, were selected in order to estimate the average attenuation relation parameters. All data were divided into two parts: a) acceleration data (that is derived from velocity data) with moment magnitude between 4-7and b) velocity data with moment magnitude between 2-4. We combined the two north–south and east–west seismograms into a single seismogram for a given azimuth. For each rotated combined horizontal record, a shear-wave window was selected and a 5% taper was applied at each end of the window. After correcting for instrument response, the Fourier spectrum of the shear-wave window and a noise window with the same length as the shear-wave window were calculated and binned in increments of 0.2 log frequency units for a central frequency range of 1-10 Hz. We fit the observed Fourier velocity amplitudes at each frequency to a Hinged-Trilinear function. The distances at which the nature of geometrical spreading attenuation changes significantly was graphically found for both data in 110 km and 200 km using a locally weighted scatterplot smoothing (LOWESS, local regression smoothing method) called robust LOWESS. A trilinear function with hinges at distances of about 110 and 200 km can describe the geometric spreading attenuation with distance. Using unconstrained Nonlinear Optimization algorithm, we found that for acceleration data with  and for velocity data with  minimize the average absolute value of the Fourier spectrum amplitude residuals. Using an anelastic attenuation coefficient at different frequencies, the direct average of calculated quality factor for both dataset, Q in the Zagros region is obtained as }, keywords = {Spectral attenuation relation,Geometrical spreading coefficient,Anelastic coefficient,Hinged trilinear,quality factor}, title_fa = {برآورد رابطه کاهندگی طیفی در پهنه زاگرس}, abstract_fa = {دامنه امواج لرزه‌‌‌ای به دو علت‌‌ غیرکشسان بودن زمین و گسترش هندسی با افزایش فاصله کاهش می‌‌یابد.‌ در چند دهه گذشته تعیین روابط کاهندگی برای یک ناحیه، به‌‌منظور اجرای بررسی‌های تحلیل خطر زلزله، همواره مورد توجه بوده است. در این تحقیق به‌منظور برآورد رابطه کاهندگی در پهنه زاگرس ،‌ حدود 998 زلزله رویداده با حدود 10000 نگاشت در این ناحیه با بزرگی بین 2 تا 7 طی سال‌های 2006 تا 2013 که با دستگاه‌‌های شبکه لرزه‌نگاری رقمی پژوهشگاه بین‌المللی زلزله‌شناسی ثبت شده بود، بررسی شد. برای برآورد روابط کاهندگی سرعت و شتاب، علاوه بر دادهای سرعت، داده‌‌های شتاب نیز از مشتق داده‌‌های سرعت برآورد شد. بعد از پردازش‌های اولیه شامل حذف تصحیح خط مبنا، میانگین و تصحیح دستگاهی، پردازش سیگنال در حوضه بسامد برای پنجره زمانی موج برشی صورت گرفت. در مرحله بعد، نقاط تغییر ضریب گسترش هندسی که ناشی از بازتاب‌‌‌‌های حاصل از وجود ناپیوستگی‌‌های سخت کره است از الگوریتمی موسوم به الگوریتم  به‌دست آمد که با رعایت فرض، نقاط تغییر این ضریب‌ها برای هر دو دسته داده پیش‌گفته، ، در نظر گرفته شد. در نهایت با به کارگیری الگوریتم بهینه‌سازی غیر خطی بدون قید مقادیر پارامترهای مجهول برای این ناحیه محاسبه شد، به‌طوری‌که ضریب‌های گسترش هندسی برای داده‌‌های سرعت در  و برای داده‌‌های شتاب در  لحاظ شد. در ‌نهایت از روی ضریب c(f)، فاکتور Q(c) برای هر دو دسته، در بسامد‌‌های مورد ‌‌نظر به‌دست آمده و میانگین تابع کیفیت موج عرضی و مقدار خطای آن برای این دو دسته به‌‌صورت  به‌دست آمد.}, keywords_fa = {Spectral attenuation relation,Geometrical spreading coefficient,Anelastic coefficient,Hinged trilinear,quality factor}, url = {https://jesphys.ut.ac.ir/article_53419.html}, eprint = {https://jesphys.ut.ac.ir/article_53419_6683c5fdc4dbc6c92ee132c5788d553f.pdf} }