پديد آورندگان :
راستگو، مهدي دانشگاه تهران - موسسه ژئوفيزيك - گروه فيزيك زمين , رحيمي، حبيب دانشگاه تهران - موسسه ژئوفيزيك - گروه فيزيك زمين , حمزه لو، حسين پژوهشگاه بين‌المللي زلزله‌شناسي و مهندسي زلزله، تهران
كليدواژه :
البرز شرقي , پوسته و ليتوسفر , توابع گيرندۀ موج P , ساختار سرعت موج برشي , منحني‌هاي پاشش امواج ريلي
چكيده فارسي :
كمربند كوهستاني البرز واقع در شمال ايران، به عنوان يك ناحيۀ فعال زمين‌ساختي و لرزه‌خيز شناخته مي‌شود كه تعيين ساختار سرعت اين ناحيه به منظور تفسير فعاليت‌هاي زمين‌ساختي حائز اهميت است. در اين پژوهش با استفاده از 12 ايستگاه لرزه‌نگاري در البرز شرقي، براساس روش وارون‌سازي همزمان تابع گيرندۀ موج P و منحني‌هاي پاشش امواج ريلي، ساختار يك‌بعدي سرعت موج برشي در محدودۀ هر ايستگاه و نيز ساختار دو بعدي آن در طول دو پروفايل (يكي در راستاي روند شرق البرز و ديگري عمود بر اين روند) تعيين مي‌شود. طبق نتايج به‌دست‌آمده، عمق موهو و مرز ليتوسفر-استنوسفر در اين ناحيه به ترتيب 2±47 و 6±86 كيلومتر است. همچنين طبق ساختارهاي دوبعدي سرعت موج برشي، يك لايۀ آنومالي پرسرعت در گسترۀ عمقي 120 تا 180 كيلومتر مطابق با زيرراندگي خزر به زير البرز مشاهده مي‌شود. با توجه به توپوگرافي سطحي البرز شرقي، ضخامت پوسته در اين ناحيه جبران‌كنندۀ ارتفاعات كوهستاني در مقياس پريودبلند است و در عين حال وجود لايۀ آنومالي سرعت بالا در زير ليتوسفر، توجيه‌كنندۀ توپوگرافي سطحي در مقياس پريودكوتاه است.
چكيده لاتين :
Alborz mountain belt in the North of Iran is known as a tectonically and seismically active region. Determination of shear wave velocity structure is important to interpret the tectonic activities. In this study, we determine 1D shear wave velocity structure beneath 12 seismic stations in the Eastern part of Alborz and also 2D shear wave velocity structure along to two profiles (one is along to the trend of Eastern part of Alborz and another one is perpendicular to its trend), based on the joint inversion of P-wave receiver function (PRF) and dispersion curves of Rayleigh waves. To obtain the PRFs of each seismic station, we lonsider three-component body wave seismograms of 177 teleseismic earthquake events with magnitude Mw>5.2 and epicentral distance range 30° to 95°, related to the study region. Also the dispersion curves of Rayleigh waves in the vicinity of each station are extracted from surface wave tomographic study reported by Rahimi et al. (2014). Then these two group data are regarded as the input data for the joint inversion process using “joint96” program (Herrmann and Ammon, 2007). ). In this study, the initial models are taken from shear wave velocity models reported by Rahimi et al. (2014), based on tomographic inversion of Rayleigh wave dispersion for various tectonic region of Iran. We regard the maximum depth of investigation about 300 km (upper mantle) in this joint inversion process based on sensitivity kernels of the dispersion curves of the Rayleigh wave fundamental mode with respect to the shear wave velocity at different periods (Rahimi et al., 2014). To find the most robust final velocity model for each station, we regard two stability tests: first, searching for the optimal parameterization for the joint inversion process; second, simplify of the representative solution of the joint inversion process (Motaghi et al., 2015). According to the obtained results, the depth of Moho boundary beneath the eastern part of Alborz mountain range is relatively uniform and following 47±2 km. By attention to the absolute shear wave velocity structure along the two profiles, depth of lithosphere-asthenosphere boundary beneath covered area is roughly constant and mainly varies around 86±6 km. Also there are high velocity anomalies in depth range 120-180 km. These high velocity anomalies in the upper mantle are consistent with the presence of under thrusting of Caspian lithosphere beneath Alborz. This observation is reported previously by Jackson et al., 2002. These observations may support the remaining question about higher surface topography in the study region without enough supporting crustal thickness. Maggi et al. (2000), using the admittance between topography and gravity in frequency domain mentioned that the only very short period topography could be supported by the flexure of the layer, whilst any longer period topography must be supported by an isostatic response. This result supports our observations, which shows an isostatic compensation for much of the long period topography. On the other hand, for short period topography, the mechanism of elastic flexure layer beneath Alborz, allowing high topographies to be supported by thin crust. We observed almost well correlation between the thickness of high velocity under thrusted layer and surface topography and also our observation could support higher surface topography in study region without enough supporting crustal thickness.
