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تغییرات عمق موهو و نسبت VP/VS در گستره اصفهان با استفاده از تحلیل تابع انتقال گیرنده | ||
| فیزیک زمین و فضا | ||
| مقاله 2، دوره 38، شماره 3 - شماره پیاپی 1210297، آذر 1391، صفحه 15-24 اصل مقاله (393.13 K) | ||
| شناسه دیجیتال (DOI): 10.22059/jesphys.2012.29111 | ||
| نویسندگان | ||
| احمد سدیدخوی1؛ زهرا علیخانی2؛ فروغ صدودی3 | ||
| 1استادیار،گروه فیزیک زمین، مؤسسه ژئوفیزیک دانشگاه تهران، ایران | ||
| 2دانش‌آموخته کارشناسی ارشد ژئوفیزیک (زلزله شناسی)، دانشگاه آزاد اسلامی، واحد علوم تحقیقات، تهران، ایران | ||
| 3پژوهشگر، مؤسسه تحقیقاتی علوم زمین پتسدام، آلمان | ||
| چکیده | ||
| تابع‌های انتقال گیرنده، سری‌های زمانی هستند که از واهمامیخت مؤلفه قائم و مؤلفه شعاعی (مؤلفه افقی چرخش‌‌یافته در راستای چشمه زلزله) لرزهنگاشت دورلرز به‌دست میآیند و بیانگر پاسخ نسبی ساختار زمین درمحل گیرنده لرزهای هستند. به‌‌منظور به‌دست آوردن تابع‌های انتقال گیرنده ابتدا سه مؤلفه Z، N و E لرزه‌‌نگاشت‌‌های دورلرزه (30 تا 95 درجه) و بزرگای 5/5 و بالاتر در بازه زمانی 2000 تا 2007 را انتخاب و بعد از حذف منحنی پاسخ لرزه‌‌نگار، تحت زاویه آزیموت پشتی و زاویه ورودی موج دوران می‌‌‌دهیم تا مؤلفه‌‌های L، Q و T به‌دست آید. سپس با واهمامیخت در حوزه بسامد مؤلفه L از دو مؤلفه Q و T، اثر چشمه و مسیر از لرزه نگاشت‌ها برداشته می‌‌‌شود و با تبدیل معکوس فوریه تابع‌های واهمامیخت شده، آنچه روی مؤلفه Q باقی می‌‌‌ماند همان تابع انتقال گیرنده موج P است که شامل انرژی تبدیل شده P به S (PS) و حاوی اطلاعاتی در مورد ساختار زیر ایستگاه است. زمان رسید فاز تبدیلی Ps اولیه در تابع انتقال گیرنده نشان‌‌دهنده عمق ناپیوستگی موهو و دامنه فاز تبدیلی، اختلاف آکوستیک امپدانس محیط را نشان می‌دهد. در این مقاله با استفاده از روش تابع انتقال گیرنده موج P و استفاده از 200 لرزهنگاشت کوتاه‌دوره دورلرز ثبت شده در 5 ایستگاه شبکه لرزه‌نگاری اصفهان، وابسته به مؤسسه ژئوفیزیک دانشگاه تهران، عمق موهو و نسبت VP/VS در گستره اصفهان تعیین شد. تحقیقات گرانی‌سنجی نشان می‌دهد که ضخامت پوسته در نوار سنندج- سیرجان که بخش جنوب غربی ایران مرکزی را تشکیل می‌دهد حدود 50 تا 55 کیلومتر است و همچنین براساس این تحقیقات عمق موهو در گستره اصفهان بین 5/38 تا 43 کیلومتر به‌دست آمد. در این تحقیق نقشه تغییرات عمق موهو در گستره اصفهان به‌دست آمده است. با استفاده از فازهای تکراری پوسته (PpPs , PpSs + PsPs) و رسم نمودار ضخامت پوسته برحسب VP/VS برای 5 ایستگاه لرزه‌نگاری در گستره اصفهان نسبت VP/VS بین 71/1 تا 79/1 به‌دست آمد؛ بنابر این در این گستره میانگین عمق موهو 40 کیلومتر و میانگین نسبت VP/VS ، 74/1 به‌دست آمد. | ||
| کلیدواژه‌ها | ||
| امواج دورلرز؛ تابع انتقال گیرنده؛ فاز تبدیل شده Ps؛ نسبت VP/VS | ||
| عنوان مقاله [English] | ||
| The variation of Moho depth and VP/VS ratio beneath Isfahan region, using analysis of teleseismic receiver function | ||
| نویسندگان [English] | ||
| Ahmad Sadidkhouy1؛ Zahra Alikhani2؛ Forough Sodoudi3 | ||
| چکیده [English] | ||
| Iran is located in a roughly triangular deforming region, consisting of relatively undeformed shield areas to the southwest (Arabia) and northeast and the more recently deformed, though currently inactive, southwest Afghanistan block in the east. The current geological and tectonic setting of Iran is due to the ongoing convergence between the Arabian and Eurasian Plates, which resulted in the formation of the Iranian plateau, mountain building, extensive deformation and seismicity. The deformation involves intracontinental shortening except where the Oman Sea subducts towards the north beneath the southern east of Iran. The edges of the deformation zone are well defined by the distribution of seismicity and the local topography. It is concentrated in the mountain belts along the SW borders (Zagros), the southern shore of the Caspian Sea (Alborz) and along the NE (Kopeh Dagh) and eastern borders. These belts enclose a series of relatively aseismic and flat blocks. The Isfahan Seismic Network belongs to IGUT (Institute of Geophysics, University of Tehran), consists of 5 stations, which are located in Isfehan province. The short-period seismographs (SS-1) are connected to the central recording station via telemetry. The recording is performed on an event-triggered basis. Teleseismic data between 2000 and 2007 have been used in this study. More than 200 teleseismic events with magnitudes greater than 5.5 at epicentral distances between 30? and 95? have been used for P receiver function analysis. Then we have been processed all of data by using the P receiver function and Zhu and Kanamori (2000) methods to calculate the Moho depth and VP/VS ratio beneath Isfahan region. The teleseismic P receiver function method has become a popular technique to constrain crustal and upper mantle velocity discontinuities under a seismic station (e.g. Langston, 1977; Owens et al., 1984; Kind and Vinnik, 1988; Ammon, 1991; Kosarev et al., 1999; Yuan et al., 2000). Telesismic body waveforms recorded at a three-component seismic station contain a wealth of information on the earthquake source, the earth structure in the vicinity of both source and the receiver, and mantle propagation effects. The resulting receiver function is obtained by removing the effects of source and mantle path. The basic aspect of this method is that a few percent of the incident P wave energy from teleseismic events at significant and relatively sharp velocity discontinuities in the crust and upper mantle will be converted to S wave (Ps), and arrive at the station within the P wave coda directly after the direct P wave. Ps converted waves are best observed at epicentral distances between 30° and 95° and are contained largely on the horizontal components. The amplitude, arrival time, and polarity of the locally generated Ps phases are sensitive to the S-velocity structure beneath the recording station. By calculating the time difference in arrival of the converted Ps phase relative to the the direct P wave, the depth of the discontinuity can be estimated using a reference velocity model (in this paper, the IASP91 reference velocity model is used). After rotating the coordinate system into a local LQT (P-SV-SH) recording system, in which the L component is in the direction of the incident P wave, the Ps energy is mostly observed on the Q component perpendicular to the L component. The Q components (P receiver functions) contain Ps converted waves as well as related S type multiples. To obtain the P receiver function, the following steps are generally used. Restitution, To utilize data recorded at different types of seismometers, the instrument responses have to be deconvolved. Rotation, Firstly, the two horizontal components N and E are rotated to radial (R) and tangential (T) directions. Most of the energy of the direct P and Ps waves are dominating the Z and R components, respectively. To isolate the converted Ps wave from the direct P wave, the ZRT components are rotated into an LQT (P-SV-SH) ray-based coordinate system, in which the L component is in the direction of the incident P wave; the Q component is perpendicular to the L component and is positive away from the source; the T component is the third component of the LQT right hand system. Deconvolution, To eliminate the influence of the source and ray path, an equalization procedure is applied by deconvolving the Q and T component seismograms with the P signal on the L component (Yuan et al., 2000, 2002). The resulting Q component data are named P receiver functions and are mainly composed of the P-to-S converted energy and contain information on the structure beneath a seismic station. The arrival time of the converted Ps phase in receiver functions depends on depth of the discontinuity, whereas the amplitude of the converted phase depends on the S-wave velocity contrast across the discontinuity. Moveout correction (distance equalization), The converted Ps phases are usually weak and of low amplitude. In order to increase signal-to-noise, it is necessary to align and stack receiver functions from different epicentral distances at each station. However, successful alignment and constructive summation of conversion phases requires that the receiver functions be equalized in terms of their ray parameters. Migration, To improve the spatial resolution and convert the delay times into depths, the Ps amplitudes on each receiver function can be back projected along the ray path onto the spatial locations of the conversion points to their true locations in a process similar to migrating in exploration seismology (Kosarev et al., 1999). The ray paths are calculated using a one dimensional global velocity model (IASP91) with assumption that conversions are produced from planar interfaces. Sometimes a spatial smoothing filter is used to improve the spatial correlation so that the space is gridded and back projected amplitudes originating from adjacent boxes are stacked to improve signal to noise ratio. Estimation of crustal thickness and Vp/Vs ratio, The converted Ps phase and crustal multiples (PpPs, PpSs and PsPs) contain a wealth of information concerning the average crustal properties such as the Moho depth and the Vp/Vs ratio. We compute P receiver functions (PRF) for all stations. We rotate the ZNE-component waveforms into the local LQT ray-based coordinate system and deconvolved the L component from the Q component to isolate the P-to-S conversions on the Q component. Individual and summed PRF for PIR station are presented in Fig 2(Up) and VP/VS ratio base on Zhu and Kanamori (2000) method is plotted in Fig 2(Bottem) as an example. P receiver function analysis of recorded events between 2000 and 2007 by 5 short period stations from the Isfehan Seismic Network shows clear conversions from the crust mantle boundary beneath the Isfehan region and VP/VS ratio. We have been able to present clear images from the Moho at depths ranging from 38.5 to 43 km beneath the Isfahan region and VP/VS ratio ranging 1.71 to 1.79. The average Moho depth and Vp/Vs ratio are achieved 40 km and 1.74 which confirmed previous results obtained by other methods. | ||
| کلیدواژه‌ها [English] | ||
| Moho depth, Receiver function, Teleseismic Waves, VP/VS | ||
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