محاسبه منحنی کاهندگی مقیاس بزرگای محلی برای ایران

نوع مقاله : Articles

نویسندگان

1 دانشگاه تحصیلات تکمیلی علوم پایه زنجان

2 دانشکده مهندسی، گروه نقشه برداری، دانشگاه زنجان

چکیده

در این مطالعه با استفاده از 48016 بیشینه دامنه­ی رکوردهای مصنوعی لرزه­نگاشت وود- اندرسون استخراج شده از شکل موج‌های مربوط به 2650 زلزله­ی ثبت شده توسط لرزه­نگاشت­های مرکز لرزه­نگاری کشوری وابسته به مؤسسه ژئوفیزیک دانشگاه تهران، پژوهشگاه بین­المللی زلزله­شناسی مهندسی زلزله و شبکه­­های موقت دانشگاه تحصیلات تکمیلی علوم پایه زنجان، رابطه کاهندگی برای بزرگای محلی برای کل ایران به‌صورت زیر محاسبه شده است: که در این رابطه­  فاصله کانونی بر حسب کیلومتر و  بیشینه دامنه­ جابه‌جایی موج برشی برحسب میلی­متر است. داده­­های استفاده شده برای محاسبه­ رابطه بالا مربوط به زلزله­های با فواصل کانونی 10 الی 800 کیلومتری است و در نتیجه رابطه محاسبه شده برای فواصل کانونی مساوی و کمتر از 800 کیلومتر معتبر است. در رابطه­ کاهندگی محاسبه شده، پارامتر پخش هندسی بیشتر از مقدار ارائه شده توسط رابطه هاتون و بور [1] است که پخش فوق کروی جبهه موج در فواصل نزدیک را نشان می­دهد. در صورت عدم استفاده از داده­های با فاصله کانونی کمتر از 60 کیلومتر، پارامتر پخش هندسی به مقادیر پخش کروی نزدیک می­شود. مقادیر تصحیح ایستگاهی در محدوده­ی 44/0- و 32/0 است. به‌طورکلی، برای بیشتر ایستگاه­های واقع در زاگرس، البرز و شمال غرب ایران مقادیر تصحیح ایستگاهی منفی محاسبه شده است که نشان­دهنده­ی تقویت دامنه امواج برشی و برای بیشتر ایستگاه­های ایران مرکزی و شمال شرق ایران تصحیح ایستگاهی مثبت به دست آمده است. رابطه­ی کاهندگی به دست آمده برای ایران به‌طور قابل توجهی در فواصل نزدیک کانونی بزرگای متفاوتی از روابط مورد استفاده در مراکز زلزله­نگاری ایران می­دهد. با توجه به گسترش شبکه­های لرزه­نگاری ایران و افزایش ثبت زلزله­ها در فواصل نزدیک کانونی، پیشنهاد می­شود از رابطه­ی محاسبه شده در این مطالعه برای تخمین بزرگای محلی در ایران استفاده شود.

کلیدواژه‌ها


عنوان مقاله [English]

Local Magnitude Calibration in Iran

نویسندگان [English]

  • Mehdi Maheri-Peyrov 1
  • Ahmad Mirhashemi 1
  • Abdolreza Ghods 1
  • Madjid Abassi 2
1 Department of Earth Sciences, Institute for Advanced Basic Sciences, Zanjan, Iran
2 , Department of Surveying Engineering, Zanjan University, Zanjan, Iran
چکیده [English]

Using 48016 synthetic maximum Wood-Anderson amplitudes read from waveforms of 2650 events recorded by stations of Iranian Seismological Center (IRSC, irsc.ut.ac.ir), Iranian National Seismograph Network (INSN, www.iiees.ac.ir) and temporary seismic networks belong to Institute for Advanced Studies in Basic Sciences(IASBS, iasbs.ac.ir), the empirical attenuation curve (0logA) for local magnitude of Iran has been calculated as follows:0log(1.556 0.06) log(0.001637 0.0009) (100) 3100RAR where Ris hypocentral distance in km and 0Ais maximum displacement amplitude of shear wave in millimeter. The empirical attenuation relationship is valid for hypocentral distances equal or smaller than 800 km. MLamplitude is the maximum amplitude observed on a Wood-Anderson (W-A) seismogram. We manually picked the maximum amplitudes on the shear window of synthetic W-A seismograms having S/N of larger than 5. We calculated synthetic W-A seismograms by removing the instrument response of each record and convolving the resulting signal with the response of the standard W-A torsion seismograph. We assumed a static magnification of 2080 for the W-A instrument. The selected MLamplitudes are belonging to events at hypocentral distance of 10 to 800 km. Except for the Makran and South Caspian Basin regions, the ray coverage of the MLamplitude covers properly the whole Iranian Plateau.To reduce the scatter of magnitude residues and ensure a reliable calculation of the attenuation curve, the selected events belong to 45 precisely relocated seismic clusterswith location uncertainties of 5 km or less. The cluster approach produces redundancy in amplitudes arriving from a cluster to a given station. The redundancy will facilitate easy recognition and removal of possible outliers and thus provide a reliable estimate for the magnitude station correction. The magnitude station corrections attempts to absorb the regional attenuation difference relative to that dictated by average attenuation relationship derived in this work. The calculated attenuation curve shows a larger geometrical spreading for hypocentral distances closer than 100 km, representing a super-spherical geometrical spreading, and a smaller value for intrinsic attenuation for distances farther than 200 km once compared with the currently used MLrelationship of Hutton and Boore (1987). Excluding amplitudes with hypocentral distances smaller than 60 km results in a geometrical spreading coefficient close to spherical spreading, emphasizing the importance of near distances data on accurate estimation of the geometrical spreading value. The difference in the attenuation parameters between our results and those of Hutton and Boore (1987) relationship clearly indicates the crustal disparity of Iranian Plateau and southern California. This necessitates using the new attenuation relationship for Iran.We calculated the local magnitude empirical attenuation relationship by inverting the amplitude data set for the geometricalspreadin]gand intrinsic attenuation. We didnot considermagnitude station correctionsin ourinversion to avoid any tradeoff between the station corrections and attenuation parameters. We have shown that the magnitude residuals calculated by our local magnitude empirical relationship do not vary systemically versus hypocentral distance or magnitude. Due to the cluster-wise approach in selection of our events and partially because of the precise location of the selected events, the standard deviation of magnitude residues is about 0.19, significantly smaller than those reported by others.We calculated the station corrections by averaging the magnitude residual in each station. The station corrections vary between -0.44 to 0.32. Generally, stations located in Zagros, Alborz and north west of Iran have negative station correction representing amplitude amplification in these regions relative to central Iran and north east of Iran. The new attenuation relationship provides better estimates for the attenuation parameters and especially provides precise magnitudes at close hypocentral distances. By time, the expansion of Iranian seismic networks reduces the average distance spacing of Iranian seismic stations and thus usage of better local magnitude formula such as ours becomes more important.

کلیدواژه‌ها [English]

  • Local Magnitude
  • Attenuation Curve
  • Station Correction
  • Iranian Plateau
  • Zagros
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