كليدواژه :
مكران ساحلي , تابع گيرنده , ساختار عميق , مرزهاي ناپيوستگي
چكيده فارسي :
منطقه مكران بخشي از فرورانش فعال حاصل از همگرايي صفحات عربي و اوراسيا است. زاويه مخروطي كم و ضخامت زياد رسوبات در منشور افزايشي از ويژگي هاي متمايز كننده اين منطقه هستند. در اين مطالعه، با استفاده از داده هاي جمع آوري شده در منطقه مكران ساحلي توسط ايستگاه باند پهن چابهار توابع گيرنده P و S و با استفاده از چهار ايستگاه كوتاه دوره، تابع گيرنده P محاسبه شد. مهاجرت به عمق توابع گيرنده P نشان مي‌دهد كه يك مرز (احتمالاً مرز موهو) در عمق 27 كيلومتري در زير ايستگاه چابهار قرار دارد كه شيبي اندك، در حدود 2/5 درجه، رو به شمال دارد. بررسي تغييرات دامنه فاز‌هاي تبديلي بر روي مؤلفه‌هاي شعاعي و مماسي تابع گيرنده P در ايستگاه چابهار وجود يك لايه ناهمسانگرد تا عمق حدود 9 كيلومتر كه احتمالاً مرتبط با رسوبات فلسي شكل در گوه برافزايشي است را تأييد مي‌كند. مقدار عمق‌هاي تخميني 9 و 27 كيلومتر براي رسوبات و مرز موهو از مدل‌سازي هم‌زمان تابع گيرنده و منحني پاشش محاسبه شده است. محاسبه و برانبارش توابع گيرنده S براي ايستگاه CHBR يك پالس منفي با تأخير زماني 10 تا 13 ثانيه (متناسب با عمق 80 تا 100 كيلومتر) را در زير اين ايستگاه نشان مي‌دهد كه مي‌تواند مرتبط با مرز ليتوسفر اقيانوسي با استنوسفر باشد.
چكيده لاتين :
We analyzed the teleseismic data gathered by a broad-band (CHBR) and four short-period (CDK, CNT, KHB, KSM) seismometers, located in western coastal Makran, north of Chabahar, Iran. The data were gathered by the roughly north-south direction quasi-linear profile and used to calculate P (for all stations) and S (only for CHBR) receiver functions utilizing iterative deconvolution technique of Ligorria and Ammon (1999). Because of backazimuth gaps in south and western directions, we used PKiKP and Pdiff phases to calculate receiver functions in a similar processing approach. Calculated P receiver functions are migrated to depth to clarify the geometry of velocity boundaries at the base of sediments and Moho. The result shows that there is a dipping interface lying at a depth of 27 km (beneath CHBR) to 31 km (beneath CDK), which imply a 2.5o dipping Moho boundary beneath the study region. To avoid the trade-off between velocity model and reported depth, we jointly modeled the stacked receiver function, and group velocity dispersion curve for CHBR and the output model was considered for any time to depth migration of receiver functions. We analyzed the effects of P and S anisotropy on teleseismic converted waves to map the presence, the strike, and the depth of anisotropic structures. High-resolution PRFs are considered for such analysis. The following criteria are considered to select the high-quality receiver function (Schulte-Pelkum and Mahan, 2014): the signal-to-noise ratio of the three components of the seismograms is at least 1.5; the convolution of the PRF with the vertical component of the seismogram reproduces at least 60% of the horizontal component (defined as variance reduction by Ligorria and Ammon, 1999); the PRF shows a positive polarity direct P arrival; the receiver function amplitude does not exceed 1; any arrivals’ pulse length does not exceed 3.5 s. The latter two criteria are employed because very high amplitudes and long oscillatory pulses are typical characteristics of an unstable deconvolution (Schulte-Pelkum and Mahan, 2014). The calculated PRFs were then binned in 5° azimuthal groups with 5° overlap. In CHBR station, we recognized signs of the top (at 1 km depth) and bottom (at 9 km depth) of an anisotropic layer with almost north-south anisotropic symmetry axis. In addition, we recognized a flat interface beneath CHBR station at 27 km depth that is not in consistency with the result of migration to a depth of RFs showing a 2.5o dip Moho at the same place. For this reason, we utilize forward modelling to calculate synthetic PRFs to explain periodic amplitude variation of P to S converted phases with back-azimuths in each station that could be a signature for anisotropic velocity features. The forward modeling indicates that the horizontal interface makes a similar pattern on simulated PRfs as a low angle dipping interface with dip less than 10o. Migration of S receiver functions reveals a deep velocity discontinuity at depth around 80 to 100 km that might be considered as a shallow lithosphere-asthenosphere boundary beneath the study region.
