برآورد ضریب کیفیت امواج تراکمی وابسته به فرکانس در ناحیه فاریاب (جنوب شرق زون سنندج-سیرجان) با استفاده از پس‌لرزه‌های زلزله 9 اسفند 1384 تیاب

نویسندگان

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

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

چکیده

گستره مورد مطالعه ناحیه فاریاب، در جنوب شرق زون دگرگونی سنندج- سیرجان و در مجاورت گسل معکوس اصلی زاگرس MZRF قرار دارد. این ناحیه جزو معدود نواحی در گستره سنندج- سیرجان است که فعالیت لرزه­ای بالایی از خود نشان می­دهد. زمین‌لرزه 9 اسفند 1384 با بزرگای 0/6 Mw= در این ناحیه رخ داده است. به‌رغم بزرگی قابل‌توجه، آسیب قابل‌ملاحظه‌ای حتی به ساختمان‌های ضعیف خشت و گلی روستایی وارد نشد که می‌تواند نشانگر توان محیط در جذب بالای انرژی امواج باشد‍. پس­لرزه­های این رویداد جهت محاسبه کاهندگی امواج تراکمی مورد استفاده قرار گرفته است. جهت برآورد رابطه وابستگی فرکانسی ضریب کیفیت امواج P به روش نرمال‌سازی کدا، از 431 پس­‏لرزه به‌دقت تعیین محل شده، استفاده شده است. در این روش ضریب کیفیت امواج تراکمی در 5 بازه فرکانسی در منطقه فاریاب تعیین شده است. رابطه وابستگی فرکانسی برای امواج طولی در منطقه مورد مطالعه به‌صورت Qp=23 f 0.78 به دست آمده است. رابطه وابستگی فرکانس برای پرتو­های منتشر شده در دو امتداد شمال غرب- جنوب شرق و شمال شرق- جنوب غرب به‌ترتیب به‌صورت Qp=10 f 0.94 و Qp=25 f 0.75است که مقدار ضریب کیفیت در بسامد مرجع 0/1 هرتز کمتر از 200 می­‏باشد. بنابراین می­‏توان نتیجه گرفت که این منطقه از لحاظ لرزه‌خیزی و زمین­‏ساختی فعال می­‏باشد. کم بودن فاکتور کیفیت که دلالت بر کاهندگی نسبتاً زیاد دارد نشانگر فعالیت لرزه­ای زیاد این ناحیه است و با انتظار ما از ساختار پوسته این ناحیه که زون دگرگونی است چندان انطباق ندارد. این کاهندگی زیاد احتمالاً ناشی از زون خورد شده در اثر زلزله­های متعدد این ناحیه است.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Estimation of compressional waves Frequency Dependence of Quality factor in the Faryab region (SE of Sanandaj-Sirjan zone) using aftershocks of Tiab earthquake of 28 Feb 2006

نویسندگان [English]

  • Sara Reyhani 1
  • Abbas Gholamzadeh 2
1 M.Sc. Student, Department of Physics, Faculty of science, Hormozgan University, Hormozgan. Iran
2 Assistant Professor, Department of Physics, Faculty of science, Hormozgan University, Hormozgan. Iran
چکیده [English]

Seismology has an important role in identifying earth structure using seismic waves. The amplitude and frequency of these waves change when they pass into the earth due to anisotropy and heterogeneity. Seismic waves decay as they radiate away from their sources, partly for geometric reasons because their energy is distributed on an expanding wave front, and partly because their energy is absorbed by the material they travel through. The energy absorption depends on the material properties.
The amplitude of seismic waves decreases with increasing distance from earthquake, explosion, and impact sources. How this amplitude decrease, occurs, how rapidly it occurs, and how it depends on the frequency of the seismic waves is fundamentally important to the efforts to describe Earth structure and seismic sources.
Attenuation of seismic waves is expressed with inverse quality factor (Q-1) and helps us to understand the physical laws governing the propagation of seismic waves in the lithosphere.
The observed seismic-wave amplitudes usually decay exponentially with increasing travel distance after the correction for geometrical spreading. These decay rates are proportional to the Q−1 values which characterize the spatial attenuation of seismic waves.
The study area is located in Fariab region, south-east of Sanandaj-Sirjan metamorphic zone and adjacent to the Main Zagros Reverse Fault (MZRF). This rejoin is among the rare regions located in Sanandaj-Sirjan area which has high seismic activity. Seismicity in this area has a north east- south west trend. Depths of events are relatively low, in the range of less than 40 km.
The 28 Feb. 2006 earthquake with a magnitude of Mw=6.0 has taken place in this region. Despite the remarkable magnitude of this earthquake, no significant damage was reported, even for low strength buildings such as clay buildings. This phenomenon may indicate a significant attenuation of the elastic wave in the area. Aftershocks of this event have been recorded by a temporary network. The well-located aftershocks, with hypocentral distances less than 100 km, have been used for estimation of the P-wave quality factor.
The attenuation of P-waves has been estimated using waveforms of 431 well-located aftershocks. The attenuation of P-wave (QP-1) has been estimated at 5 frequency bands (1.5, 3, 6,12,24 Hz) using extended coda normalization method. The estimated QP values show highly frequency dependency. The frequency dependent relation for longitudinal waves in the study area has been derived as Qp=23f-0.78. The QP values have been estimated by using waveforms with two major directions to investigate the probably existed attenuation anisotropy. The frequency dependent relation in two directions of north west-south east and north east-south west were calculated as Qp=10f-0.94 and Qp=25f-0.75, respectively.
It has been observed that the quality factor value at the reference frequency of 1 Hz is smaller than 200, which is reported for seismically active regions. Therefore it could be concluded that this region is active in terms of seismicity and tectonics.
The small values of the quality factor which demonstrates the relatively high attenuation indicate high seismic activity of this region. This is not so compatible with what we expect for the earth’s crust structure in this area with the metamorphic zone. This high attenuation is probably due to the crushed zone affected by various earthquakes in this region.

کلیدواژه‌ها [English]

  • Attenuation
  • compressional wave
  • Coda normalized method
  • Sanandaj- Sirjan zone
  • Faryab
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