روش‌ها و چالش‌های قرائت فازهای بازتابی از موهو

نوع مقاله : مقاله پژوهشی‌

نویسندگان

1 دانشجوی دکتری، موسسه ژئوفیزیک، دانشگاه تهران، ایران

2 استادیار، گروه زلزله‌شناسی، موسسه ژئوفیزیک، دانشگاه تهران، ایران

3 دانشیار، گروه زلزله‌شناسی، موسسه ژئوفیزیک، دانشگاه تهران، ایران

چکیده

فازهای PmP و SmS فازهای ثانویه‌ای هستند که از مرز موهو بازتاب می­شوند. مسیر عبور فازهای بازتابی از موهو، از کل ساختار پوسته می­گذرد؛ بنابراین زمان سیر آنها حاوی اطلاعات مفیدی از پوسته است. در این تحقیق از زمین‌لرزه­های محلی واقع در شمال غرب ایران در سال­های 2017-1996 با بزرگای بیش از 3 و عمق کانونی کمتر از 35 کیلومتر استفاده شده است. لرزه‌نگاشت­های کوتاه­دوره و باندپهن هر زمین‌لرزه به‌صورت مجزا بررسی و پس از حذف روند و میانگین و استفاده از فیلترهای مناسب، فازهای بازتابی قرائت شده‌اند. در این بررسی، به دلیل یکسان بودن پاسخ دستگاه برای لرزه­نگاشت­های مختلف در محدوده فرکانسی مورد نظر (1 تا10 هرتز)، از حذف پاسخ دستگاه صرف‌نظر شده است. علاوه بر فازهای بازتابی، فازهای مستقیم P و S نیز قرائت شده است. در این تحقیق برای شناسایی فازهای بازتابی موهو، از برون­راند فازهای بازتابی، شباهت حرکت ذرات فاز بازتابی PmP و فاز مستقیم P و همچنین فاز بازتابی SmS و فاز مستقیم S استفاده شده است. با صرف‌نظر کردن از داده­هایی که اختلاف زمان سیر فازهای مستقیم و بازتابی مشاهده‌ای و محاسبه­شده آنها بیش از 75/0 ثانیه است، دقت قرائت فازهای بازتابی افزایش یافته است. این روش که به زمان سیر تفاضلی موسوم است، باعث می­شود مقدار خطای قرائت فازها در محدوده خطای تعیین محل زمین‌لرزه­ها قرار گیرد. پس از بررسی­های مختلف، فیلتر میان­گذر باترورث 4-5/0 هرتز برای قرائت فاز PmP و فیلتر میان­گذر باترورث 4-2/0 هرتز برای قرائت فاز بازتابی SmS مناسب تشخیص داده شد. در این مطالعه فازهای P و PmP در مؤلفه عمودی و فاز SmS در مؤلفه­های عرضی و شعاعی بررسی شدند، اما برای اجتناب از هرگونه تأخیر زمانی که به دلیل ناهمسانگردی ایجاد می­شود، فازهای S و SmS روی مؤلفه­های افقی مشابه قرائت شدند.
 

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

How to read the reflection phases of Moho

نویسندگان [English]

  • Fatemeh Alidoost 1
  • Esmaeil Bayram Nezhad 2
  • Zaher Hossein Shomali 3
1 Ph.D. Student, Institute of Geophysics, University of Tehran, Tehran, Iran
2 Assistant Professor, Seismology Department, Institute of Geophysics, University of Tehran, Tehran, Iran
3 Associate Professor, Seismology Department, Institute of Geophysics, University of Tehran, Tehran, Iran
چکیده [English]

PmP and SmS are secondary phases that are reflected from Moho discontinuity. Moho reflected phases contain information from the entire crust, thus their travel time and waveforms contain significant information about crustal structure. We used seismogram data associated to earthquakes with magnitude larger than 3.0 and focal depth shallower than 35 km occurred during 1996-2017 in NW Iran. The seismographs of each earthquake are investigated separately. After removing mean and trend, we examined appropriate band pass filters. In this study, the response of sensor in our interest frequency (1-10HZ) is similar; thus, we did not remove instrument response.
   Particle motion of PmP reflected phase is similar to P direct phase as compressional waves and particle motion of SmS and S phases show that these phases get to stations as shear waves. If the travel time difference of picked Moho reflected and direct phases is less than 0.75 sec, picked reflected phase is reliable. We found that Butterworth band pass filter with corner frequencies of 0.5 and 4 HZ is appropriate for providing clear PmP reflected phases and the corner frequencies of 0.2 and 4 HZ are appropriate for SmS reflected phases. In general, the amplitude of the reflection phase could be shorter than the direct one due to large attenuation along the long ray path and missing some energy at the reflection boundary, but in some samples, amplitude of Moho reflected phase is larger than the direct phase, due to increment of Q values in lower crust with respect to upper crust and source of the earthquake. In this research, we often investigated the cases that amplitude of the reflection phases is larger than direct ones. We picked P and PmP from vertical component and considered the transverse and radial components for picking SmS reflected phase. S phase is read from the component where the SmS phase is more clear and noticeable. However, we can read S phase from each of the two horizontal components but in general, to avoid each time delays likely generated by anisotropy, it was read from the identical components used for picking SmS phase. Consistent with our research, the slope of the curve of difference between reflected and direct phases increases with focal depth and epicenteral distance, but for focal depth greater than Conrad discontinuity and epicenteral distance more than 80 km, the rate of increment of slope increases. Regarding this, a total of 216 PmP, 310 first P, 254 S and 161 SmS phases were picked from seismograms in the local earthquakes of NW Iran.

کلیدواژه‌ها [English]

  • Secondary phase
  • Moho discontinuity
  • particle motion
  • vertical and transverse component
  • local earthquakes of NW Iran
  • differential travel time
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