Marmara Denizi'nde Büyük Çekmece-Silivri arsında kalan bölgenin açıklarında toplanmış sığ sismik verilerin yorumlanması

Abstract

Marmara Denizi'nde Büyükçekmece-Silivri Arasında Kalan Bölgenin Açıklarında Toplanmış Sığ Sismik Verilerin Yorumlanması1 isimli bu çalışma, Dz. K. K. Seyir, Hidrografi ve Oşinografi Dairesi Başkanlığı, İstanbul Teknik Üniversitesi ve TÜBİTAK işbirliğiyle sürdürülen proje kapsamında gerçekleştirilmiştir. Bu çalışmada Sparker ünitesiyle toplanan sığ sismik verileri kullanılmıştır. Dolayısıyla en fazla 750 m derinliğe kadar inen veriler elde edilmiştir. Bu durum, Marmara Denizi içinde bulunan yaklaşık 1250 m. derinliğe sahip çukurlukların tabanını görmemizi engellerken, fay atımlarını hesaplayabilmemizi sağlamıştır. Çalışma bölgesinde toplam 35 hat üzerinde sismik veriler toplanmıştır. Bu hatların 32'si N-S (kıyıya dik), geri kalan üçü ise E-W (kıyıya paralel) olarak atılmıştır. Hat aralıkları yaklaşık 1 km'dir. Çalışma bölgesi konumu itibariyle, Marmara Denizi içinde kollara ayrılan Kuzey Anadolu Fay (KAF) zonunun kuzey sınır fayı üzerinde bulunmaktadır. Kesitlerde kuzey sınır fayı görülmekte ayrıca, Marmara Denizi'nin kuzeyindeki geniş şelf ve şelften çukurlara iniş kesitlerde izlenebilmektedir. Toplanan sismik kesitler üzerinde, her fix noktasında ve her fix noktalan arasında 5 noktada olmak üzere derinlik değerleri okunmuştur. Okunan bu değerlere kıyı hattı da birleştirilerek bölgenin batimetrik görüntüsü elde edilmiştir. Ayrıca, kesitler incelenirken, fay tespiti yapılmış ve aynı özelliklere sahip olduğu görünen faylar birleştirilerek fay çizgisellik haritası oluşturulmuştur. Kesitlerde görülen yanıltıcı etkilere de dikkat edilmeye çalışılmış, özellikle hayalet yansımalar (ghost), tekrarlı yansımalar (multiples) ve saçılmalar (diffractions) tespit edilmiştir. Bunların tespitinde, kesitlerdeki görüntüsü dikkate alınmış ve onların bir sinyal olarak değil de gürültü (istenmeyen sinyal) olarak yorumlanması sağlanmıştır. Aslında bu tip istenmeyen sinyaller, çeşitli veri işlem basamaklarıyla giderilebilir. Ancak eldeki verilerin analog olması, bu işlemi gözle ayırdetmemize neden olmuştur. Marmara Denizi İçinde birçok normal faylar bulunmaktadır. Kuzeyde bulunan geniş bir şelf, kuzey sınır fayı tarafından birden kesilmekte ve çukurluklara doğru inen çok dik bir yamaç görülmektedir. Marmara Denizi'ndeki deniz tabanının bu şekilde inişli çıkışlı olması pull-apart mekanizmasıyla açıklanabilir. Yanal atımlı faylar arasında oluşan normal fayların atımları neticesinde çalışma sahasında gözlenen batimetrinin oluştuğu sonucuna varılmıştır..

This thesis which is called 'The Interpretation of Shallow Marine Seismic Data on Offshore Büyükçekmece-Silivri' is a part of a project among the Istanbul Technical University, SHOD and TÜBİTAK. In this study, shallow seismic reflection data were collected by using 'Sparker Seismic System'. This system is able to bring information from maximum depth of 750 m. due to its limited penetration ability. Total of 35 seismic lines were collected. The number of lines perpendicular to the coasts is 32 and the rest of the lines are parallel to the coasts. The distance between the lines perpendicular to the coasts is approximately 1 km. The aim of this study is to interpret the seismic lines in order to investigate the structural elements of the North Anatolian Fault Zone (NAF) in the study area. The NAF zone is separated into three branches in the Sea of Marmara, and the Northern branch is the subject of this study. In Turkey, the general neotectonic regime had begun when the Arabian Plate struck the Anatolian Plate. This age of this collision is Miyosen. After this event, North Anatolian Fault and East Anatolian Fault taken place. So, the Arabian Plate push the Anatolian Plate, The Anatolian Plate began to move towards the west. Finally, the neotectonic of Turkey seperates 3 parts; the graben system in the Aegean Sea at the east, there is a plain regime in the middle of Turkey and there is a compressive regime at the east. The Sea of Marmara is located at the Northwestern part of Turkey, between the Black Sea and the Aegean Sea. Its area is 11350 km2 and its widths in E-W is 275 km and in N-S is 80 km. The topography of the Sea of Marmara indicates a wide shelf area at the south, three depressions as deep as 1250 m. in the middle part, and a wide shelf at the north. The depressions in the middle part correspond to where the northern branch of the North Anatolian Fault Zone pass through. Our study area is just located on this part on offshore Büyükçekmece-Silivri. The bathymetry of the study area was obtained from the seismic sections. To obtain this map, firstly the sea floor depths were measured at every fix points and also three extra measurements were made between the fix points. Then, these measured depths were gridded and contoured to obtain the detailed bathymetry map of the study area. According to this map, a shelf area of about 50-70 m. depth is observed at the north. This shelf suddenly ends at around 100 m. contour by a very high slope. The morphologic investigation of the bathymetric map supports the hypothesis that the shelf area was cut by a normal fault system. In the interpretation of seismic sections, misleading effects must be well recognized and separated from the actual seismic events. For this reason descriptions of misleading effects must be discussed. These misleading events, are multiple reflections, ghost reflections and diffractions. The multiple reflections are caused by the strong reflection coefficient of the sea floor and the sea surface. The propagation waves in the water layer are repeatedly reflected between the sea-floor and the sea surface to generate the multiple reflections. The period of the multiple reflections is the two way travel time (twt time) of the water layer. The ghost reflections result from the source and receiver sides because these units are placed below the sea surface for well coupling. Ghost reflections follow the sea surface very well and most of the time they cover the reflections from the young sediments. Diffractions result from sharp discontinuities and outcrops of magmatic and metamorphic rocks at the sea floor. The data type at hand is an analog data. For this reason, the data processing techniques used in the conventional seismic cannot be applied in the evaluation of this data set. The interpretation is carried out by considering and avoiding the misleading effects of these factors. vui In this thesis, the seismic sections were interpreted from structural point of view. The sections were especially evaluated for fault indicators. Most important fault indicators are * sudden discontinuities of the reflection surfaces, * a collection of diffractions along a vertical or near vertical directions * sudden steps in the sea-floor. The sections were evaluated according to these indicators and a fault map was obtained for the study area. The interpretation of the fault map obtained reveals that the area is a normal fault zone limiting the shelf area at the north. These faults are the result of the stress field due to strike slip fault mechanisms proposed by Barka et all., (1988). The northernmost fault, which has the highest observed slip at the sea-bottom with 110 meters, is called the northern border fault of the depressions developed in the Sea of Marmara. In the seismic sections, the northern border fault is very well defined and followed from section to section. If the Figures 4.3 and 4.4 examine, the bathymetry of this study area is seen as follows. A wide shelf area is seen at the north of the Marmara Sea. Its depth is approximately 70 m. After this depth, the contours are closed. This shows that it is going down to the depressions. Especially, the east and the west parts of the study area in Marmara Sea, the contours are closed. It points where the depressions are. However, the depressions bases can not be seen because of the sparker sismic unit. It brings the information maximum 750 meters. In these study all of the seismic sections were interpreted. However, five of these sections which show typical characteristics, were discussed in detail in the thesis. The explanation of the chosen sections are as follows: A) 1130-1137 Seismic Line: The direction of this line is in West-East and the line is important because it displays a wide shelf and sharp slope at the same time. There is a river valley between 1130-1131 fix numbers. However, it was filled with young sediments. There are also multiple reflections in this seismic sections. Furthermore, on the right side of this seismic section dipping layers were defined and marked. IX B) 1138-1155 Seismic Line: The direction of this line is in North-South and this line also displays the shelf and slope. This line is the longest in the all of the seismic lines. On the north side of the seismic section, there is a wide shelf. This shelf is on the north of the Marmara Sea. Also, the river-bed is seen on the west of the seismic section. However, this river-bed is filled with sediments. Apparent width and apparent depth of this river are calculated as 860 meters and 60 meters. The fault, which is called as northern border fault, is observed in this seismic section. After the fault, the slope of sea-floor is calculated as 2.27°. The slip of this fault is 1 10 meters. This number may be apparent because the seismic lines may not be vertical to the slope. Moreover, the materials are seen as flowing between 1 153-1 154 lines, this may be a fault or land slide or material flow. C) 1285-1294 Seismic Line: The direction of this line is in North-South and this line is located in middle part of the study area. It is possible to see the wide shelf, which is on the north side of the Marmara Sea, and sharp slope which goes down deep depressions. In this seismic sections, the basement rocked, multiple reflections and ghost reflections are observed and marked. The apparent slope is calculated as 2.8°. D) 1317-1322 Seismic Line: This line is at the east in the study area. So, the sharp slope can be seen. In this seismic section, the basement rocked are also observed. Along the seismic line, the calculated slope is 5.45°. There is a fault which is determined between the 1320-1321 fix numbers. The slip of this fault is calculated as 119 meters. E) 1337-1342 Seismic Line: This seismic line is also on the east in the study area. It is possible to see the sharp slope in this seismic section too. This line also contains diffractions. The diffractions occur from rough structure on the sea-floor. These disturb the seismic signals so, they should be eleminated. Furthermore, The faults are observed between 1338-1339 fix numbers and 1341-1342 fix numbers. At same time, in this study structural element map is formed. When this map is formed, firstly all of the seismic sections are examined and the faults are determined in the sections. Then they are marked on the line location map. Then, the faults which have same features are merged. Some of these features are, fault slip, fault location, fault surface slope and etc. Finally the fault lineations are obtained. These normal faults provide going down from wide shelf to depressions in the Marmara Sea. How these normal faults formed, probably depends on the pull-apart mechanism. In this mechanism, normal faults are developed between the strike-slip faults and a deppression area occurs between the resulting normal faults.