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İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Jeofizik Mühendisliği Anabilim Dalı / Jeofizik Mühendisliği Bilim Dalı

Adapazarı Havzası'nda Kuzey Anadolu Fayı'nın çok-kanallı sismik yansıma verileriyle araştırılması

Investigation of North Anatolian Fault by multi-channel seismic reflection data in Adapazari Basin

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Özet:

Kuzey Anadolu Fayı (KAF), Türkiye'nin doğusunda Karlıova'dan, Kuzey Ege Denizi'ndeki Saros Körfezine kadar, Güney Karadeniz kıyılarına oldukça paralel bir biçimde yaklaşık olarak 1500 km boyunca uzanım göstermektedir. KAF'ın batı kısmı, Mudurnu yakınlarında, kuzey ve güney olmak üzere kollara ayrılmakta olup, kuzey kolu Sapanca Gölü ve İzmit Körfezi üzerinden Marmara Denizi'ne ulaşmaktadır. Marmara Denizi'nin batı bölgesinde meydana gelen 1912 Ganos depreminden bu yana gerçekleşen ve 1999 İzmit (Mw=7.4) ve 1999 Düzce (Mw=7.2) depremleriyle son bulan bir M>7 deprem sekansı neticesinde, KAF'ın, batıda Marmara Fayı ve doğuda Yedisu Fayı haricinde kalan tüm bölümlerinde kırılma gerçekleşmiştir. 17 Ağustos 1999 İzmit depremi neticesinde batıda Marmara Denizi ile doğuda Düzce arasında yaklaşık olarak 145 km'lik bir yüzey kırığı meydana gelmiştir. KAF Marmara Denizi'nde, özellikle bahsedilmiş olan Mw>7 depremlerden sonra, sismik yansıma yöntemi ile, geniş aralıkta penetrasyon derinlikleri kullanılarak detaylı olarak araştırılmış olsa da İzmit Körfezi'nin doğusunda kalan kara alanlarında ağırlıklı olarak jeolojik ve jeodezik veriler kullanılarak araştırılmalar gerçekleştirilmiştir. Bu nedenle, KAF'ın derine doğru olan geometrisinin ve KAF'a bağlı olarak gelişen fay zonunda yer alan aktif yapıların belirlenmesi amacıyla 117Y130 numaralı TÜBİTAK-1001 Bilimsel Araştırma Projesi kapsamında, Kasım 2018 tarihinde, Sapanca Gölü'nün yaklaşık olarak 15 km doğusunda yer alan Ekinli köyü civarında, yaklaşık olarak 1,3 km uzunluğa sahip ve KAF'ı KD-GB yönünde dik kesen bir hat boyunca çok-kanallı sismik yansıma verileri toplanmıştır. Toplanmış olan çok-kanallı sismik yansıma verilerinin işlenmesi amacıyla İTÜ Jeofizik Mühendisliği Bölümü'nde yer alan Nezihi Canıtez Veri İşlem Laboratuvarı'nda, bölüm adına lisanslı olan Paradigm® ECHOS® sismik veri işlem yazılım paketinden (v15.5) yararlanılmıştır. Verilerin işlenmesi kapsamında gerçekleştirilmiş olan veri işlem adımlarını, başlıca, atış-alıcı geometrisinin tanımlanması, verilerin ayıklanması, süzgeçleme, genlik kazancı uygulaması, CMP sıralama, hız analizi, yığma ve migrasyon olarak listelemek mümkündür. Bahsedilmiş olan sismik yansıma veri işlem aşamaları neticesinde elde edilmiş sonuçlara ek olarak, yakın-yüzey yapısı ile ilgili sonuçların güçlendirilebilmesi amacıyla, toplanmış olan çok-kanallı sismik yansıma verilerindeki ilk varışlar kullanılarak İTÜ Jeofizik Mühendisliği Bölümü'nde lisanslı olarak bulunan SeisImager/2DTM programı vasıtasıyla sismik kırılma tomografisi yöntemi gerçekleştirilmiştir. Çok-kanallı sismik yansıma verilerine uygulanmış olan veri işlem adımları neticesinde elde edilmiş olan final kesitinde KAF'ın yer aldığı kısım net bir şekilde gözlenebilmektedir. Bu kesitte, KAF'ın düşeye oldukça yakın bir şekilde konumlanmış olduğu gözlenmiştir. Kesitte, üç farklı stratigrafik birim gözlenebilmiş ve bu birimler, çalışma alanı jeolojisi ve bu alan için belirtilmiş olan stratigrafik kesitlerle ilişkilendirilebilmişlerdir. Final sismik yansıma kesitinde KAF'ın bu stratigrafik birimlere negatif çiçek yapısı ortaya koyacak şekilde bir geometri kazandırdığı gözlenmektedir. Sismik kırılma tomografisi yöntemi neticesinde elde edilmiş olan hız gradyanı kesitinden de sığ kesimlerdeki arayüzeylerde gözlenen ondülasyonlu yapıların, KAF zonuna ait derindeki faylarla örtüştüğü gözlenmiştir.

Summary:

North Anatolian Fault (NAF) extends for approximately 1500 km from Karlıova in the east of Turkey to Gulf of Saros in the Northern Aegean Sea in a fairly parallel fashion to the shores of southern Black Sea. Western part of the NAF splits into branches as north and south near Mudurnu and the northern branch of the NAF reaches the Sea of Marmara through Sapanca Lake and İzmit Bay. As a result of a M>7 earthquake sequence beginning with 1912 Ganos earthquake, which took place in the western part of the Marmara Sea, and ending with 1999 İzmit (Mw = 7.4) and 1999 Düzce (Mw = 7.2) earthquakes, all parts of the NAF other than Marmara Fault in the west and Yedisu Fault in the east were ruptured. As a result of the 17 August 1999 İzmit Earthquake a surface rupture of approximately 145 km occurred between the Marmara Sea in the west and Düzce in the east. It is extremely important to investigate the long-term activity and behavior of the primary fault present in tectonically active regions to determine the seismic hazard in such regions and also to perform risk analysis. Even though the NAF was investigated in detail in the Sea of Marmara especially after the mentioned Mw>7 earthquakes with seismic reflection method by using wide range of penetration depths, investigations on the land areas to the east of the İzmit Gulf were mainly carried out by using geological and geodetic data. Therefore, in order to determine the geometry of the NAF in the depth and also the active fault structures that are present in the fault zone, which is developing as a result of the NAF, multi-channel seismic reflection data were collected within the context of TÜBİTAK-1001 Scientific Research Project, No: 117Y130 in November 2018, near Ekinli village, which is located approximately 15 km to the east of Sapanca Lake, along an approximately 1.3 km long line that crosses the NAF perpendicularly in the NE-SW direction. Within the scope of multi-channel seismic reflection data collection, a total of 120 P- geophones of 14 Hz, which are produced by Geospace Technologies, were used. Additionally, 5 recorder geodes, which are produced by Geometrics, were used to provide the connections of the receivers. Test shots were performed on the day before the collection of the evaluated multi-channel seismic reflection data, where an electrically fired P-S gun (Multi-Head-Buffalo-Gun) was used as the energy source during these shots. Even though this source is applied on the surface, unlike other impulsive seismic sources that are also applied on the surface, it does not generate any undesirable effects, such as paging effect (the case of uncontrolled impulse repetition of the source and a decrease in the data quality as a result). The signals obtained through the use of this source are rich in high frequency content. On the other hand, high resolution data can be obtained as a result of the use of this source. In addition, similar shot results after different repetitions can be obtained through the use of this source. The source uses 16+16=32 shotgun shells. The pressure generated by every single shell corresponds to ~1050 bar, or in other words ~1071 kg / cm2. In addition to the mentioned energy source, a 500 kg weight drop energy source was also used on the same test shot line. This source is allowed to free-fall from a height of 2 m onto a metal plate, which is placed on the ground. After making an impact on the metal plate, it is ensured that the weight is stopped in the air to prevent it from impacting the plate a second time. Similar shots can be easily repeated through the use of this energy source. This energy source is a fully automated hydraulic system, which is mounted on a 4x4 off-road vehicle. Data groups which are obtained through the use of two mentioned energy sources were compared in terms of their penetration depths and signal qualities. As a result of the comparisons, the use the weight drop method was preferred for the main measurements since the weight drop method is much more practical to use and also generates a cleaner image in the seismic records for the study region. The multi-channel seismic reflection data used in this study were collected in a split-spread system, with a shot interval of 10 meters. A total of 120 receivers were used during the data collection and the receiver interval was chosen as 5 m. The shots were carried out in a roll-along pattern. In this shot pattern, receivers to be used for each shot are determined in the computer environment. As the number of shots increases, receivers to be used are activated in the order of data collection, while previous receivers are deactived. Processing of the collected multi-channel seismic reflection data was carried out in the Nezihi Canıtez Data Processing Laboratory located in ITU Geophysical Engineering Department by using the Paradigm® ECHOS® seismic data processing software package (v15.5), which is licensed for this department. Seismic reflection data processing steps that were carried out within the context of this study can be listed mainly as defining the shot-receiver geometry, editing, filtering, gain application, CMP sorting, velocity analysis, stacking and migration. As an addition to the seismic reflection data processing, seismic refraction tomography method was carried out by using SeisImager/2DTM program, which is licensed for ITU Geophysical Engineering Department, in order to strengthen the results related to the near-surface structures, through the utilization of first arrivals that are present in the collected multi-channel seismic reflection data. Main purpose of the operations carried out within the scope of seismic reflection method is to increase the signal/noise ratio of the data and also obtaining a representative image of the subsurface by performing data processing on multiple channels and additionally stacking. One way to achieve this goal is to edit traces within the data. As an addition, by arranging waveforms in the signal higher resolution can be achieved. Another example to the performed processes is the separation of reflection data from unwanted factors such as multiples, which are repetitive reflections, and surface waves. Moving the reflections to their actual positions through static correction is another processing step that can be carried out within the context of seismic reflection data processing. In addition to all the mentioned processes, information such as the velocity values of the subsurface and also reflection and transmission coefficients can be obtained. The main purpose of seismic refraction tomography method, on the other hand, is to obtain the travel times for source-receiver pairs by raytracing or in other words to determine the shortest path between two points. A velocity gradient of the subsurface can be obtained through the utilization of this method. The region where the NAF is located can be clearly observed in the final section which was obtained as a result of the data processing steps applied to multi-channel seismic reflection data. In this section, it was observed that the NAF has an approximately vertical geometry. Three different stratigraphic units could be observed in the section and these units could be related to the geology of the study area and the stratigraphic sections indicated for this area. It is observed from the final seismic reflection section that the NAF causes these stratigraphic units to have a negative flower structure. On the other hand, from the velocity gradient section, which is obtained through the seismic refraction tomography method, it is observed that the undulating structures within the interfaces at shallower depths are overlapping with faults of the NAF zone at greater depths.