بررسی ساختار سطحی ناحیه فاریاب با استفاده از توموگرافی دو بعدی سرعت گروه امواج لاو

نوع مقاله : پژوهشی

نویسندگان

1 دانشجوی کارشناسی ارشد، گروه فیزیک، دانشکده علوم، دانشگاه هرمزگان، بندرعباس، ایران

2 استادیار، گروه فیزیک، دانشکده علوم، دانشگاه هرمزگان، بندرعباس، ایران

چکیده

در این مطالعه ساختار سطحی پوسته در ناحیه فاریاب، استان کرمان، با استفاده از توموگرافی دو بعدی امواج لاو، مورد بررسی قرار گرفته است. با استفاده از روش تک ایستگاهی 437 پس­لرزه ثبت شده در 9 ایستگاه شبکه موقت محلی، مورد تجزیه و تحلیل قرار گرفت. منحنی پاشندگی برای هریک از رویدادهای ثبت شده در ایستگاه‌ها محاسبه شده است. پس از تعیین منحنی پاشندگی، سرعت گروه امواج لاو، در بازه 1/0 ثانیه تا 10 ثانیه با استفاده از وارون­سازی خطی، به‌دست آمد. در پریودهای کمتر از 3 ثانیه محدوده سرعت گروه امواج لاو از 5/0 تا 3 کیلومتر بر ثانیه است در حالی‌که برای پریودهای بیشتر از 3 ثانیه سرعت گروه امواج لاو بین 2/0 تا 5/1 کیلومتر بر ثانیه است. بر این اساس به‌نظر می‌رسد در این منطقه با دو نوع پوسته اقیانوسی و قاره‌ای مواجه باشیم. وجود پوسته اقیانوسی با شواهدی از حضور سنگ‌هایی در سطح زمین شامل پریدوتیت و گابرو و افیولیت تقویت می‌شود. با توجه به کاهش سرعت گروه در پریودهای بیشتر از 3 ثانیه، به‌نظر می‌رسد در زیر پوسته اقیانوسی با رسوبات نرم پوسته قاره­ای مواجه باشیم. بر این مبنا به‌نظر می‌رسد امواج لاو که دارای پریود بین 3 تا 5/9 ثانیه هستند از سنگ‌هایی عبور کرده‌اند که زیر این پوسته اقیانوسی قرار دارند. یک توده پر سرعت با سرعتی در حدود 5/2 کیلومتر بر ثانیه در بخش میانی منطقه مورد مطالعه دیده می­شود. این آنومالی از پریود 1/0 ثانیه قابل مشاهده است اما در پریودهای بیشتر وسعت و محدوده آن واضح‌تر دیده می­شود.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Shallow structure of Faryab region using two-dimensional Love wave group velocity tomography

نویسندگان [English]

  • Soudabeh Kargar Ghanat Alnooji 1
  • Abbas Gholamzadeh 2
  • Mohsen Farrokhi 2
1 M.Sc. Student, Department of Physics, Faculty of science, University of Hormozgan, Bandar Abbas, Iran
2 Assistant Professor, Department of Physics, Faculty of science, University of Hormozgan, Bandar Abbas, Iran
چکیده [English]

The Sanandaj-Sirjan Zone (SSZ) extends ~1500 km from the northwest (Sanandaj) to southeast (Sirjan) parallel to the Zagros Fold Thrust belt with average width of 150–200 km. This zone is a metamorphic–magmatic belt, associated with the Zagros Orogen and part of the Alpine-Himalayan orogenic system in Iran. Its limits on either side are marked with discontinuously preserved ophiolites including the following: (1) the Neyriz-Kermanshah ophiolite situated on the northern edge of the Zagros Mountains and (2) the Khoy and Nain-Baft ophiolite complexes to the northeast (Stöcklin, 1981).
The rocks in this zone are the most highly deformed of the Zagros belt and share the NW–SE trend of surrounding structures. The zone is dominated by Mesozoic rocks; Palaeozoic rocks are generally rare but are common in the southeast (Berberian, 1995). The SSZ is characterized by metamorphosed and complexly deformed rocks associated with abundant deformed and undeformed plutons, as well as widespread Mesozoic volcanic. The ophiolites are generally regarded as preserving a record of an ocean basin or basins that lay between these elements in Mesozoic through mid-Cenozoic time, as a whole referred to as the Neotethys Ocean, with the Eurasian continent to the north, and Gondwana-land to the south (e.g., Stöcklin, 1974; Sengör, 1979; Berberian and King, 1981; Stampfli and Borel, 2002; Agard et al., 2011).
Faryab region in the Sanandaj-Sirjan zone is located in a very tectonically active zone, materialized by highly deformed metamorphic rocks, colored melange and ultramafic-mafic complexes.
An earthquake with magnitude Mw 6.0 occurred on the Faryab region, on the southeastern part of Sanandaj-Sirjan, on February 28, 2006. Aftershocks of this earthquake were used to study Love wave's group velocity. Seismic surface wave tomography of short-period dispersion curves is a useful method for studying the shallow structures of the Earth.
The main aim of this study is to apply the group velocity dispersion to Faryab region, southeast of Sanandaj-Sirjan zone, to calculate the two-dimentional Love wave group velocity tomography.
We have analyzed surface wave dispersion curves of 2616 waveforms of 437 aftershocks, (figure 1). These aftershocks were recorded by a local temporary network including 9 short period station that were installed by International Institute of Earthquake Engineering and Seismology (IIEES) during 28 Feb. 2006 to 30 Mar. 2006. The temporary stations were equipped with Guralp CMG-6TD velocity seismometer with flat frequency response between 0.1 – 50 Hz. The epicentral distance and magnitude of earthquakes were less than 50 km and larger than 1.5, respectively. The dispersion curves were calculated in the period range between 0.1 seconds to 10 of seconds, which corresponded to the shallow structure of upper crust including sedimentary layers. Surface wave tomography was also performed to estimate the two-dimensional group velocity maps of Love waves in the Faryab region. The isolated surface wave fundamental modes (and group velocity dispersion curves) have been analyzed using linear inversion method for estimation of 2D tomography maps (Yanovskaya-Ditmar; 1990). Based on the ray coverage inside the 2 × 2 km cells in the region, the estimated minimum dimension of distinct heterogeneities was about 5 km.
There are numerous anomalies in tomography maps. The range of Love waves velocity has two part: in periods shorter than 3 seconds, the velocity ranges from 0.5 to 3 km/s, and in periods above 3 second, the velocity ranges are 0.2- 1.5 km/s. It seems that in this area we are faced with two different crusts: oceanic and continental crust. Waves with a period shorter than 3 seconds pass shallower part of the crust, seem to be related to the oceanic crust that confirmed by evidence of some rocks such as Gabbro, peridotite, and ophiolites, that are exposed on the surface (figure 4). Under this oceanic layer, there is some soft sediment of continental crust. Waves with a period more than 3 seconds travel through these soft materials.

کلیدواژه‌ها [English]

  • Love wave
  • 2D tomography
  • shallow structure
  • Sanandaj-Sirjan zone
  • Faryab region
رحیمی، ح.، 1389، ساختارهای الاستیک و غیرالاستیک منطقه‌ای برای پوسته و گوشته بالائی ایران، رساله دکتری، پژوهشگاه بین المللی زلزله‌شناسی و مهندسی زلزله.
علوی، ص.، غلام‌زاده، ع. و فرخی، م.، 1397، بررسی انتهای شمالی گسل زندان-میناب با استفاده از توموگرافی دو بعدی امواج ریلی، هجدهمین کنفرانس ژئوفیزیک ایران.
Alavi, M., 1980, Tectonostratigraphic evolution of the Zagrosides of Iran, Geology, 8, 144–149.
Alavi, M., 1994, Tectonics of the Zagros Orogenic belt of Iran: new data and interpretations, Tectonophysics, 229, 211–238.
Backus, G. E. and Gilbert, J. F., 1986, The resolving power of gross Earth data, Geophysics R. Astr. Soc. 16, 169–205.
Berberian, M., 1995, Master blind thrust faults hidden under the Zagros folds: active basement tectonics and surface morphotectonics. Tecto-nophysics 241, 193 – 224.
Cho, K. H., Herrmann, R. B., Ammon, C. J. and Lee, K., 2007, Imaging the Upper Crust of the Korean Peninsula by Surface-Wave Tomography., Bulletin of the Seismological Society of America., 97(1B), 198–207. doi: 10.1785/0120060096
Ditmar, P. G. and Yanovskya, T. B., 1987, Generalization of Backus-Gilbert Method for 63 Estimation of Lateral Variations of Surface Wave Velocities, Phys. Solid Earth, Izvestia Acad. Sci. USSR, 23, 470–477.
Fang, L., Wu, J., Ding, Z. and Panza, G. F., 2009, High resolution Rayleigh wave group velocity tomography in North-China from ambient seismic noise, Gephys. J. Int., 181, 1-171-1182.
Gholamzadeh, A., Rahimi, H. and Yaminifard, F., 2013, Spatial and temporal variation of coda‐wave attenuation in the Faryab region, southeast of the Sanandaj–Sirjan zone, using aftershocks of the Tiab earthquake of 28 February 2006. Bulletin of the Seismological Society of America, 104(1), 529-539.
Gholamzadeh, A., Yamini-Fard, F., Hessami, K. and Tatar, M., 2009, The February 28, 2006 Tiab earthquake, Mw 6.0: implications for tectonics of the transition between the Zagros continental collision and the Makran subduction zone. Journal of Geodynamics, 47, 280-287.
Hassanzadeh, J. and Wernicke, B. P., 2016, The Neotethyan Sanandaj‐Sirjan zone of Iran as an archetype for passive margin‐arc transitions. Tectonics, 35(3), 586-621.
Herrmann, R. B. and Ammon, C. J., 2002, Computer Programs in Seismology, Surface Waves, Receiver functions and Crustal structure, Department of Earth and Atmospheric Sciences, Saint Louis University, St Louis.
Lay, T. and Wallace, T. C., 1995, Modern Global Seismology, Academic Press.
Pamic, J., Sestini, G. and Adib D., 1979, Alpine magmatic and metamorphic processes and plate tectonics in the Zagros range, Iran, Geol. Soc. Am. Bull., 90, 569–576
Petrosino, S., 2006, Attenuation and velocity structure in the area of Pozzuoli-Solfatara (Campi Flegrei, Italy) for the estimate of local site response (Doctoral dissertation, Università degli Studi di Napoli Federico II).
Shafaii Moghadam, H. and Stern R. J., 2011, Geodynamic evolution of upper Cretaceous Zagros ophiolites: Formation of oceanic lithosphere above a nascent subduction zone, Geol. Mag., 148, 762–801.
Udias, A., 1999, Principles of Seismology, Cambridge University Press.
Vernant, P., Nilforoushan, F., Hatzfeld, D., Abassi, M., Vigny, C., Masson, F., Nankali, H., Martinod, J., Ashtiani, A., Bayer, R., Tavakoli, F. and Chery, J., 2004, Contemporary crustal deformation and plate kinematics in Middle East contrained by GPS measurements in Iran and northern Oman, Geophys. J. Int. 157, 381– 398.
Yanovskaya, T. B. and Ditmar, P. G., 1990, Smoothness Criteria in Surface Wave Tomography. Geophys. J. Int., 102, 63–72.