ساختار پوسته‌ شمال‌غرب ایران با استفاده از توموگرافی منحنی پاشش امواج لاو

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

موسسه ژئوفیزیک دانشگاه تهران

چکیده

برای به‌دست‌آوردن توزیع سرعت گروه امواج لاو در شمال‌غرب ایران و نواحی اطراف آن در دوره‌های تناوب 7، 10، 20 و 25 ثانیه، از شکل‌موج 241 زلزله‌ محلی و منطقه‌ای در توموگرافی سرعت گروه منحنی پاشش امواج لاو استفاده شد. نتایج این بررسی نشان می‌دهد ساختار پوسته در حوضه‌ خزرجنوبی و ناحیه‌ فروافتاده کورا تقریباً یکسان است اما با پوسته شمال‌غرب‌ ایران تفاوت چشمگیری دارد. ناهنجاری‌های کم‌سرعت در دوره‌های تناوب کمتر از 10 ثانیه، در رسوبات ضخیم و کم‌سرعت حوضه خزرجنوبی مشاهده شد. در دوره‌های تناوب کمتر از 25 ثانیه، برای Aمجموعه به‌هم‌پیوسته شرق آناتولی@ که فعالیت‌های آتشفشانی بسیار بیشتری نسبت به سایر مناطق دارد، سرعت کمتری نسبت به دیگر مناطق مطالعه شده به‌دست‌آمد. نتایج نشان می‌دهد که ساختار پوسته در سه ناحیه تکتونیکی اصلی رشته کوه­های زاگرس با یکدیگر متفاوت است. در دوره‌های تناوب کمتر از 25 ثانیه، پوسته بالایی خرد شده و فعال Aزاگرس رورانده و چین‌خورده@ - که همراه با گسل‌های معکوس و رورانده فعال و کم‌عمق و نرخ لرزه­خیزی زیاد در این ناحیه است - به‌صورت یک بی‌هنجاری کم­سرعت مشاهده می­شود که مرزی واضح با بی‌هنجاری پُرسرعت در ناحیه Aسنندج‌- سیرجان@ دارد. نتایج این تحقیق نشان می‌دهد ساختار پوسته در شمال‌غرب‌ ایران تقریباً یکنواخت و با کمترین تغییرات است و با ساختار پوسته در نواحی حوضه خزرجنوبی، شرق ترکیه و کوه‌های زاگرس کاملاً متفاوت است.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Crustal structure of NW Iran investigated by Love waves dispersion curve tomography

نویسندگان [English]

  • Gholamreza Mortezanejad
  • Habib Rahimi
Institute of Geophysics, University of Tehran
چکیده [English]

To obtain the distribution of the Love wave group velocities in northwest Iran and its surrounding areas for 7, 10, 20 and 25 second periods, the waveform of 241 local and regional events used in a dispersion curve tomography of the Love waves. The evnets occurred within NW Iran and surrounding areas between 2005 and 2015 and were recorded by 39 permanent and temporary medium and broad band seismic stations belonging to national and international seismic networks. In order to reduce the effect of non-uniform distribution of events, we selected the more uniform list of events out of an initial number of 1734 non-uniform distributed events. We applied the Frequency-Time Analysis method to each event-station path for estimation of Love wave group velocities, then we used a tomographic method to compute the distribution of local group velocities throughout the region. From the analysis of the surface wave tomography, we concluded that crustal structure in the South Caspian Basin and the Kura Depression is almost the same, but it is significantly different from that of the northwest Iran. In the presence of thick and low shear velocity sediments in the South Caspian Basin, we observed low velocity anomalies at periods less than 10 seconds that are surrounded by relatively higher velocities along Alborz, Talesh and Lesser Caucasus. In the “Eastern Anatolian Accretionary Complex” where volcanic activities are much higher than in rest of the region, less group velocities were observed for periods less than 25 seconds. The main reason for this observation can be related to the presence of partial melting zones inside the crust as a result of intensive volcanic activities in this region. In Zagros, for the periods of 7 and 10 seconds, a relatively high velocity anomaly along the “Sanandaj-Sirjan” metamorphic zone was observed, which was trapped by two low velocities along the “Zagros Folding and Thrust Belt” in one side and “Urmieh-Dokhtar Magmatic Arc” in another side. The active and broken crust, reverse, active and shallow thrust faults and high seismicity of Zagros Folding and Thrust Belt is characterized by low group velocities in our results for periods of less than 25 seconds. In most areas of central Iran and Urmieh-Dokhtar Magmatic Arc, local sedimentary basins are characterized by low velocities for periods of 7 and 10 seconds. Our results indicate that crustal structure in the northwest of Iran is almost uniform and has the minimum changes, but it is completely different from the crustal structure in the South Caspian Basin, eastern Turkey, and the Zagros mountains.

کلیدواژه‌ها [English]

  • NW Iran
  • Love waves
  • Group velocity
  • Tomography
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