كليدواژه :
نوفه , همبستگي متقابل , منحني پاشندگي , موج برشي , توموگرافي لرزه‌اي , لرزه‌نگار درون‌چاهي
چكيده فارسي :
روش توموگرافي نوفه محيطي در سال‌هاي اخير به منظور مطالعه ساختارهاي سرعتي در عمق‌هاي مختلف و در مناطقي با لرزه‌خيزي پايين به‌طور گسترده استفاده شده است. در مطالعه حاضر، نوفه لرزه‌اي ثبت شده در آرايه بلند دوره ايران (ILPA) واقع در ناحيه جنوب غرب استان تهران با به‌كارگيري روش توموگرافي نوفه لرزه‌اي مورد مطالعه قرار گرفته است. با استفاده از تكنيك همبستگي متقابل، توابع گرين تجربي در دوره تناوب بين 3 تا 10 ثانيه با فواصل بين ايستگاهي حداقل 24 كيلومتر محاسبه و سپس منحني‌هاي پاشندگي سرعت گروه اين توابع براي انجام فرآيند توموگرافي لرزه‌اي محاسبه شده‌اند. به دليل استفاده از حس‌گرهاي درون‌چاهي در عمق 100 متري در شبكه ايلپا (ILPA) و همچنين استفاده از روش انتخاب نوفه گوسي، توابع گرين به‌دست‌آمده بين همه جفت ايستگاه‌هاي ممكن داراي نسبت سيگنال به نوفه قابل قبول بودند. در ادامه با استفاده از نقشه‌هاي سرعت گروه براي هر دوره معين، منحني پاشندگي محلي در هر سلول جغرافيايي تعيين و با انجام فرآيند وارون سازي غير خطي، مدل عمقي سرعت موج برشي محاسبه و ارائه شد. اين روش امكان برآورد مدل شبه سه‌بعدي موج برشي تا عمق 15 كيلومتر را براي پوسته فوقاني ناحيه جنوب غرب استان تهران فراهم نموده است. با بررسي پروفيل‌هاي سرعت موج برشي در راستاي شمال شرق - جنوب غرب، يك تغيير روند سرعت در راستاي پروفيل‌ها ديده مي‌شود كه مي‌تواند نشانگر وجود يكي از شاخه‌هاي گسل ايپك و يا وجود تاقديسي با سطح محور در اين راستا و شيب به سمت شمال شرق در پوسته بالايي منطقه مورد مطالعه باشد.
چكيده لاتين :
The crustal structure study based on ambient noise measurements has become a popular, fast and reliable method in earthquake seismology in recent years around the world. Generally, not only in seismology but also in other applications which deal with signals, accept noise as an undesired component of the signal. It is believed that noise obscures data and does not contain useful information. Ambient noise measurements promise significant improvements in the resolution and accuracy of crustal and upper mantle images. Traditional dispersion analysis, however, does not yield reliable estimates of the structure in the shallow crust because of strong scattering at short-periods (T<30). Recent advances in surface-wave ambient noise tomography (e.g., Shapiro et al., 2005; Sabra et al., 2005; Yao et al., 2006; Yang et al., 2007; Lin et al., 2007) greatly enhance our ability to resolve the shallow crustal structure. In this study, we apply ambient noise tomography to image and investigate the shallow shear velocity structure of the upper crust beneath south west of Tehran area. Data from seven stations of Iranian Long Period Array (ILPA) are acquired from IRIS free data center. These data were recorded during 1975 to 1977 in corporation of the FDSN with the Institute of Geophysics at University of Tehran. At the moment, all continuous seismic waveforms are available for researchers. After obtaining the continuous waveforms, we preprocessed and segmented the data into one hour time windows. Hourly cross correlation of ambient noise between all station pairs were calculated and group velocity of Rayleigh waves dispersion curve in periods between 3 to 10 seconds are measured from the Green’s function resulting cross correlations. To determine dispersion curves of surface waves we have used the Frequency-Time Analysis technique (FTAN). Because of using borehole seismometer in ILPA array and our new Gaussian noise selection proposed method; all Green functions had acceptable SNR ratio and greater than 10. Therefore, we predict the suitable and reliable result of Green’s functions in comparison with ambient noise of free surface seismometers. Then using dispersion map in each period, we extracted a local dispersion curve for each grid point. Finally, the quasi-3D shear wave velocity model in the study area provided using nonlinear inversion procedure for each grid point of local dispersion curves by means of Shapiro et al., (2005) technique. By preparing different shear wave velocity profiles in the direction of NE-SW, of the studied area, we try to image the changing velocity variations and trends along the profiles, which can indicate the existence of one of the branches of the IPAK fault, or the existence of an anticline with the axis in this direction and the slope to mards the northeast direction. The results indicate a reliable image from upper crust of south east of Tehran region in consistent with the results of Doloei and Roberts (2003) from teleseismic P-waveform time domain receiver function (RF) method. Moreover, the upper crustal structural model proposed for this area is in agreement with surface geological setting. Therefore, we suggest that for isolating the ambient noise from temporary and very local conditions, the digging and covering the seismic stations prepares a suitable noise level for the crustal structure studies.
