Batı Anadolu'da yer kabuğunun yapısı
Structure of the crust at west Anatolia
- Tez No: 21870
- Danışmanlar: PROF. DR. NEZİHİ CANITEZ
- Tez Türü: Doktora
- Konular: Jeofizik Mühendisliği, Geophysics Engineering
- Anahtar Kelimeler: Batı Anadolu, Deprem analizi, Yer kabuğu, West Anatolia, Earthquake analysis, Earth crust
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Bu çalışmada tam (complete) bir yapay sismogram elde etme yoluyla Ege ve Marmara bölgelerinin kabuk yapılarının, yapısal özelliklerinin modellenmesine çalışılmıştır. Çalışmada toplam 5 deprem verisi kullanılmiştır. Her bir deprem için, 3 bileşende (radyal,teğetsel ve düşey) yapay sismogramlar elde edilmiştir. Çalışmadaki sentetik sismogramlar Bouchon (1981) in ayrık dalga sayısı toplama yöntemi kullanılarak frekans ortamında elde edilmektedir. Sismogramlar önce homojen kabuk, daha sonra tabakalı yer modelleri için elde edilmişlerdir. Elde edilen bu kuramsal sismogramlar içinde gözlemsel sismogramlara en iyi uyan kuramsal sismogram o ortam modelini en iyi şekilde belirtmektedir. Önce frekans ortamında hesaplanan kuramsal sismogramlar daha sonra ters fourier dönüşümü ile zaman ortamına dönüştürülmektedir. Fourier dönüşümünden dolayı sismogramlar üzerinde katlanma (aliasing) etkisinin görülmesi kaçınılmazdır. Sismogramlar üzerindeki bu etkiyi gidermek için sentetik sismogramlar kosinüs törpüsü ile törpülenmiştir. Gözlemsel ve elde edilen yapay sismogramlar üzerinde Pn, Pg, Sg, Lg, Rg gibi kabuk fazlan işaretlenmiştir. Yapay sismogramlar üzerinde işaretlenen fazların seyahat zamalan, ışm izleme (Ray tracing) yoluyla zaman-uzaklık grafiğinden belirlenmiştir. Kuramsal kabuk modelleri kullamlararak elde edilen sismogramlar arasında gözlemsel sismogramlara en iyi uyan kuramsal sismogramlar ortamı en iyi temsil eden model değiştirgenlerini vermektedirler. Yapılan çalışmadan elde edilen model sonuçlara göre, Ege bölgesi için bulunan kabuk kalınlığı ortalama 32-36 km arasmda dir. Yüzeyden 10-15 km derinlikte, ince bir düşük hız tabakası bulunmaktadır. Tabakalı ortamın altındaki homojen yan uzayın hızı, kuzey Ege bölgesi için 7.96- 7.98 km/s arasında, güney Ege bölgesi için ise biraz daha düşük, 7.85 km/s dir. Marmara bölgesi için, model çalışmasından elde edilen sonuçlara göre, kabuk kalınlığı bölgenin doğusunda 30-33 km dir. Marmara denizinin içinde, kuzeye doğru kabuk kalınlığı yaklaşık olarak 30 km dir. Marmara bölgesi için, homojen yan uzayın hızı 8.0-8.1 km/s arasında değişmektedir. (vii)
Özet (Çeviri)
STRUCTURE OF THE CRUST AT WEST ANATOLIA SUMMARY In this study it was tried to model out the crustal structure of Aegean and Marmara regions using complete synthetic seismograms. The complete synthetic seismograms are obtained at distances of a few kilometers. Bouchon's discrete wave number summation method (Bouchon, 1981) is used to evaluate Green's functions for elastic layered medium. The principle of the method was previously introduced by Bouchon and Aki (1977) In the original paper the discretization was done in one dimension only. The method is based on the representation of the source radiation by a superposition of plane waves propagating in discrete directions. This discretization is exact and results from a periodic arrangement of sources. Green's functions for an elastic layered medium can be expressed as a double integral over frequency and horizontal wave number. For any time window, the wave number integral can be exactly represented by a discrete summation. Discretization is achieved by adding to the particular point source, an infinite set of specified circular sources centered around the point source and distributed at equal radial interval. (viii)Choice of this interval is dependent on the length of time desired for the point source response and determines the discretized set of horizontal wavenumbers which contribute to the solution. The methods devised for the calculation of the Green's functions are, however, often very complex and, in many cases, only provide approximate solutions. The method that we use in this study is based on an exact discretization of the wave field emitted by the source. Use of complex frequencies is another feature of the discrete wavenumber method. The time response of the source array being infinite, the use of the discrete Fourier transform will result in aliasing in the time domain solution. This effect is removed by giving a negative imaginary part to the frequency, chosen so that the disturbances which do not belong to the time window of interest are attenuated enough for their contribution to be negligible. This condition may be written as exp (w.(t1-t0))<<l. W. is a function of the length of the time window only. The propagation of the discretized source wave field through the layer is carried out by using the Thomson-Haskel matrix formulation ( Thomson, 1950; Haskell, 1953 ). Earthquake records recorded by short period seismometers are used as observational data. The velocity records are converted to displacement records using an integral operator. I used data from 5 earthquakes. Distances and coordinates for these earthquakes were computed with hypocenter program. 3 of the epicenters belong to Ege region and the other two to Marmara region. (ix)The crustal phases Pn, P, S and L, R were - pointed out on the synthetic seismograms. The phases on the synthetic seismogram are found out from the travel time graphics using a ray tracing program. The S wave itself is very weak and hardly visible. The multiple S waves represent energy trapped within the crust. These waves are incident on the Moho at angles beyond the critical angle and their energy cannot be transferred in to the mantle. This effect completely disappears once the critical angle is reached. Synthetic seismograms are computed for 3 components ( Radial, Tangential, Vertical). Each seismogram is made up of 1024 points and the cut off frequency is different for every earthquake. The earthquake is represented by a double couple point source having a ramp function, time dependence defined by [l+tanh(t/tQ)]/2. Each seismogram is normalized to the maximum seismogram amplitude. Seismic moment is equal to 1 unit. Synthetic seismograms are computed for homogeneous and layered crustal model. All vertical synthetic seismograms are compared with the recorded ones. Some traces already show good resemblance with real data. Because of Fourier transformation, we have aliasing in the time domain signal. To get rid of aliasing we use cosine taper in time domain. 00The parameters of the crustal model which satisfies the synthetic seismogram may be summarized as follows: For the Aegean region: The total thickness of the crust is approximately 32-36 km. The sedimentary layer is 5.0 km thick. There is a low velocity layer at the.10-15 km depth. The velocity of the homogenous half space is 7.96-7.98 km/s at the north part of the region and 7.85 km/s at the south part of the region. Camtez(1975), found an average crustal thickness of 30 km and an uppermost mantle P wave velocity of 8.08 km/s for the N.Aegean sea, using the dispersion of surface waves. Makris (1978), using deep seismic soundings, found 32 km thickness but a lower velocity of 7.6 - 7.8 km/s for the North Aegean Sea. However, he suggested that there was a low velocity zone in the uppermost mantle, which was limited in depth. Kalafat, Gürbüz, Üçer (1985) determined the thickness of the crust in western Turkey between 28-32 km. They obtained P wave velocities between 7.7-8.0 km/s beneath the Moho discontinuity. Ezen (1991) found the thickness of the crust 31 km for N-S direction in Western Turkey using Rayleigh wave dispersion. This results are indicated that crustal structure beneath the Western Anatolia in generally shows typical Basin and Range structure. In spite of the great interest in the crust and upper mantle structure of western Turkey almost no information is available from the explosion seismology. (xQFor the Marmara region; The total thickness of the crust is 30-33km. The sedimentary layer is 4.0 km thick. The velocity of the homogenous half space is 8.0-8.1 km/s. C. Gürbüz, S. Püskülcü, and S.B. Üçer (1992) found an average crustal thickness of 27 - 34 km and uppermost mantle Pn wave velocity of 7.5 - 8.0 km/s for the Marmara region, by applying the conventional travel time- distance relation method to arrival times of well located earthquakes recorded at Marnet stations. The thickness of the crust increases to the south of Marmara sea. Minimum thickness is seen around Istanbul. Quarry blast and P wave spectrum studies made previously in Istanbul gave a crustal thickness of 28 km ( Gürbüz and Üçer, 1988; Kenar, 1977). İlkışık (1980) performd some magnetotelluric soundings in Thrace. He found 40-44 km crustal thickness beneath Kırklareli, North Thrace. It increases to 32-35 km beneath Tekirdağ, South Thrace. (xn)
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