تغییرات عمق موهو در برخی از زون‌‌های زمین‌‌ساختی ایران با استفاده از روش تابع انتقال گیرنده P

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

1 مؤسسه ژئوفیزیک دانشگاه تهران، ایران

2 مؤسسه تحقیقاتی علوم زمین پتسدام، آلمان

3 دانشگاه آزاد اسلامی‌‌واحد قم، ایران

چکیده

روش تابع انتقال گیرنده قابلیت زیادی در شناسایی و تعیین عمق ناپیوستگی‌‌های درون زمین دارد. به همین منظور بررسی تغییرات عمق موهو از ایستگاه دماوند در زون لرزه‌‌زمین‌‌ساختی البرز تا ایستگاه شوشتر در زون لرزه‌‌زمین‌‌ساختی زاگرس، با استفاده از روش پیش‌گفته صورت گرفت. با چرخش مؤلفه‌‌های زلزله‌‌های دور‌‌لرز حول زاویه آزیموت پشتی و زاویه برخورد و به‌دست آوردن توابع انتقال گیرنده P برای هر ایستگاه،  با توجه به گستردگی طول نیم‌رخ، منطقه به چهار زون زمین‌‌شناسی البرز، ارومیه دختر، سنندج– سیرجان و زاگرس تقسیم شد. توابع انتقال گیرنده به‌‌دست آمده در گروه البرز، افزایش عمق موهو که ناشی از کوتاه‌‌شدگی پوسته در اثر فرورانش صفحه عربستان به زیر صفحه اوراسیا است را نشان می‌‌دهند. همچنین، بیشترین عمق موهو در زیر ایستگاه DAMV، حدود 5/67 کیلومتر به‌دست آمد. ضخامت پوسته در کمان ماگمایی ارومیه- دختر تا 42 کیلومتر در زیر ایستگاه GHVR کاهش می‌‌یابد و در زون سنندج- سیرجان، در زیرایستگاه KHMZ تا 67 کیلومتر افزایش می‌‌یابد که گواه بر افزایش ناگهانی عمق موهو در زیر زون سنندج- سیرجان است. عمق موهو محاسبه شده در زیر تک ایستگاه موجود در زون کمربند چین‌‌خورده و راندگی زاگرس، SHGR، به 50 کیلومتر کاهش می‌‌یابد.
 
 

کلیدواژه‌ها


عنوان مقاله [English]

Variation of the Moho depth in some Iranian seismotectonic zones using P receiver functions

نویسندگان [English]

  • Najmieh Mohammadi 1
  • Ahmad Sadidkhouy 1
  • Forough Sodoudi 2
  • Fataneh Taghizadeh Farahmand 3
چکیده [English]

The convergence between the Arabian and Eurasian Plates has resulted in the extension of the Alborz mountains belt in the north and the Zagros mountains belt in the west-southwest of Iran, and in the different deformation zones with various distributions of seismicity and local topography which make geological structure interpretations difficult for the Iranian plateau. Detecting Moho depth and crustal thickness could be of great help in understanding the dynamics of the predominate tectonics which is the main objective of this study. The P- receiver function technique was selected for this work because it is a popular method for estimating crustal thickness and detecting Moho depth variations under a seismic station. We computed receiver functions for 9 permanent broadband seismic stations of the International Institute of Earthquake Engineering and Seismology (IIEES), which are installed between the Damavand station and the Shoshtar station in the limited region between 32.10°-35.63°N and 48.801°-51.97°E. All stations were equipped with Güralp CMG3T seismometers. The teleseismic events in epicentral distances between 30°-90° with magnitudes larger than 5.5 (mb) and a clear P onset with high signal-to-noise ratio recorded between 2006 and 2010 were selected. We applied observed backazimuth and incident angles derived from the eigenvalues of the covariance matrix for calculating P -receiver functions. Seismograms were then rotated into the ray coordinate system (L, Q, T) such that the components were oriented in the directions of the P-, SV- and SH-waves, respectively. By deconvolving the P-waveforms on the L-component from the corresponding Q- and T-components, the source and path effects were removed. We obtained approximately 110 P receiver functions for the study region. We increased the signal-to-noise ratio by stacking after the moveout correction for a reference slowness of 6.4 s/deg, which corresponds to an epicentral distance of 67°. PRFs for all stations were calculated and the distribution of the P to S piercing points at 40 Km plotted, which is the expected depth of Moho. To improve the spatial resolution, PRFs of all stations were stacked in bins of 0.04˚.Due to the different deformation zones that exist along the profile, our results reveal the significant variations of the Moho depths beneath the Iranian plateau. The depth of the crustal discontinuities as well as the Moho was estimated by calculating the time difference in the arrival of the converted Ps phase relative to the direct P wave. For depth estimation, we used the IASP91 reference model.
The estimated Moho depth beneath the Shoshtar station in the Zagros Fold and Thrust Belt (ZFTB) is estimated to be 50.5 km, which increases to a depth of about 67 km in the Sanandaj-Sirjan metamorphic zone (SSZ). Furthermore, the Moho depth decreases to approximately 42 km beneath the GHVR station located in the Uroumieh Dokhtar metamorphic zone (UDMA). A local crustal thickening of approxmiately 67 km is observed beneath the DAMV station located near the Damavand volcano in the Alborz zone. The Zhu & Kanamori method was also employed to determine the crustal thickness (H) and Vp/Vs ratio by using the arrival times of the crustal multiples.
 
 

کلیدواژه‌ها [English]

  • P-Receiver function
  • Moho Depth
  • Alborz zone
  • Sanandaj-Sirjan metamorphic zone