Helen Yayı üzerindeki derin depremlerin odak mekanizma çözümleri
Focal mechanism solutions of the deep earthquakes at Hellenic Arc
Yazar: NİLSU GEÇİMOĞLU
Danışman: DOÇ. DR. GONCA ÖRGÜLÜ
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Jeofizik Mühendisliği Ana Bilim Dalı / Jeofizik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Jeofizik Mühendisliği = Geophysics Engineering
Dizin: Deprem analizi = Earthquake analysis ; Litosfer = Lithospher ; Sismoloji = Seismology ; Tektonik analiz = Tectonic analysis ; Zonlaşma = Zonation

Özet:
Dalma batma zonları sismoloji açısından oldukça önemli tektonik yapılardır. Dünya üzerindeki diğer dalma batma zonlarına kıyasla daha küçük bir alana sahip olan Helen Dalma Batma Zonu, ülkemizin sismolojik haritasını tamamlamak açısından incelenmesi ve anlaşılması gereken oldukça önemli bir yapıdır.Türkiye ile Yunanistan arasında kalan ve Ege Bölge'sinin tektoniğini birinci derecede etkileyen Helen Dalma Batma Zonu, Kuzeyde Adriyatik Denizi'nden başlayıp Güney Akdeniz'e kadar devam eder. Kuzey Afrika litosferinin, okyanusal Ege litosferi altına dalmasıyla oluşur. Kuzey Afrika litosferinin kuzeye doğru yılda 5 - 10 mm ilerlemesine karşın Ege litosferi güneybatıya doğru yılda ortalama 30 - 35 mm ilerler.
Geçmişten günümüze bir çok çalışmanın konusu olmuş olan Helen Yayı detaylı olarak incelenmek istendiğinde genellikle batı, merkez veya doğu kısmı olarak ele alınmıştır. Helen Yayı'nın batı ve merkez kısımları ile ilgili bir çok çalışma olmasına karşın doğu kısmı ile ilgili araştırmalar nispeten daha az ve bulunan sonuçlar oldukça tartışmalıdır. Bu yüksek lisans çalışması Helen Yayı'nın doğu kısmına ilişkin bir yorum getirmek amacıyla hazırlanmıştır. Yapılan çalışmada, Helen Yayı bölgesinde 2010 – 2020 yılları arası gerçekleşmiş olan, deprem büyüklükleri 3.9 – 5.9 arası değişen depremlerin odak mekanizma çözümleri yapıldı. KOERİ ve diğer sismolojik ağlardan alınan veriler SAC (Seismic Analysis Code) formatına çevrildi. Odak mekanizmalarını hesaplamak için basit yöntemlerden biri olan P dalgası ilk hareketleri kullanıldı. SAC formatına çevrilen 3 bileşen sayısal verilerine P dalgası ilk hareket yönü işlendi ve FOCMEC programı kullanılarak olası odak mekanizma sonuçları hesaplanıldı.
Elde edilen sonuçlarda Helen Yayı'nın doğu kısmının da kendi içinde çeşitli faylanma türlerine sahip olduğu görüldü. Yay'ın doğu kısmının derinlerde bariz bir sol yanal harekete maruz kaldığı gözlemlendi. Bu sol yanal hareketin yayın doğusunda kuzey batıya doğru dalan litosferin uzantısında olduğu düşünülmektedir.

Abstract:
Subduction zones are very important tectonic structures in terms of seismology. The Hellenic Subduction Zone, which has a smaller area compared to other subduction zones in the world, is a very important structure that should be examined and understood in order to complete the seismological map of our country. Since Mesozoic times, the seafloor of the Mediterranean Sea has subducted northward under Crete due to the rapid southwestern movement of the Southern Aegean relative to Eurasia. Therefore, the Aegean region has had an active tectonism since ancient times. A prime example of this is the great Cretan Earthquake, estimated to have a magnitude of about 8.5 in AD 365. The Hellenic Subduction Zone starts from the Adriatic Sea in the north and continues to the southern Mediterranean. It is formed by the subduction of the North African lithosphere under the oceanic Aegean lithosphere. While the North African lithosphere moves northward by 5-10 mm per year, the Aegean lithosphere moves southwestward by an average of 30 - 35 mm per year. The Hellenic Arc, which has been the subject of many studies from the past to the present, is generally considered as the western, central or eastern part when it is desired to be examined in detail. The western part of the arc is under the influence of the Adriatic plate and the Ionian plate. The continental Adriatic plate is separated from the oceanic Ionian plate by the Kefalonia Transform Fault (KTF). In the eastern part, Hellenic Arc divides into at least three branches south of Crete, drawing an arc between Peloponnese and Crete, and continues east towards Rhodes as the branches of Ptolemy, Pliny, and Strabo. Although there are many studies on the western and central parts of the Hellenic Arc, the studies on the eastern part are relatively few and the results are highly controversial. This master's study has been prepared mainly to provide an interpretation of the eastern part of the Hellenic Arc. Today, ground movements are recorded digitally by seismographs. By using these digital records (seismograms), the source parameters such as the location, size and depth of the ground movements can be determined with various methods. In this study, as method we used first motion polarity of P waves. The first motion polarity of the P wave is the most useful information in determining the focal mechanisms of earthquakes. For a strike-slip fault, the P wave defines four quadrants in total, with the first motion directions being two compression and two dilatation. The separation between the quadrants is provided by the fault plane and the auxiliary plane perpendicular to it. When an earthquake record is examined, if it is observed that the first motion polarity of the P wave is upward, this indicates a compression region. If it is observed that the first motion polarity of the P wave is downward, this indicates the dilatation region. There are different types of forces that represent the effect of earthquake waves on the earth's crust. The most commonly used type of force is the double couple. This force couple creates perpendicular fundamental stresses at the source of the earthquake, which are defined as pressure (P), tension (T), and null (B). In its simplest terms, if the P-axis is vertical and the T-axis is horizontal from these two axes, it represents the normal faulting mechanism. If the T-axis is vertical and the P-axis is horizontal, this indicates a thrust faulting mechanism. If it is horizontal on both axes, it indicates a strike-slip faulting mechanism. In the study, earthquake data with magnitudes ranging from 3.9 to 5.9 between 2010 and 2020 in the Hellenic Arc region were examined. Since it was observed that the earthquakes with a magnitude below 3.9 were not recorded by a sufficient number of stations, earthquakes below this magnitude were not included in the study. Data from KOERI, EIDA and other seismological networks were converted into SAC (Seismic Analysis Code) format. P wave initial motions, one of the primitive methods, were used to calculate focal mechanisms. The first motion polarity of P wave was signed into the 3-component data in SAC format, and possible focal mechanism results were obtained using the FOCMEC program. In the results obtained, it was seen that the eastern part of the Hellenic Arc also had various types of faulting within itself and exposed to an obvious strike slip faulting mechanism at depth. These strike-slip faults, located in the west of Rhodes Island, are thought to be the extension of the subducting plate that dips to the NW. According to Myhill (2013), during the subduction of the subducting plate into the mantle, the bends that occur in the slab geometry cause different stress fields to form in the slab. Different stress fields naturally occurring in the subduction can lead to different types of faulting. An example of this is the strike-slip faulting observed in the extension of the branch. This strike slip mechanism was though to be in the extention of the subducted slab, dipping to the northwest at the east of the arc.