The Black Sea is an oval basin with an area of 423.000 km2, a volume of 534.000 km3 and a maximum depth of 2206 m. It is connected to Atlantic Ocean by Mediterranean Sea-Aegean Sea-Turkish Straits System (The Sea of Marmara and the straits of Bosphorus and Dardanelles) and it is also connected to Sea of Azov by the Kerch strait. The mean sea level at the Black Sea is about 55 cm higher than the level at the Aegean Sea. The maximum development of the Black Sea shelf is at the west of Crimean peninsula, where it is more than 190 km wide. Along the Turkish coast, Russian coast and south of the Crimean peninsula, the shelf exceeds 20 km in width in only a few places. The widths of about 40 km are found off the Bulgarian coast and south of the Sea of Azov.
The Black Sea opened as a back-arc and/or intra-arc basin to the north of Pontide magmatic belt. It is seperated by the mid-Black Sea Ridge (comprising Andrusov and Archangelsky ridges). Western Black Sea Basin (WBSB) and Eastearn Black Sea Basin (EBSB) have different time and mechanisms of opening.
The study area is located on the southern margin of the WBSB, on the fragment called Istanbul Zone (which is delimited by the Intra-Pontid suture in the south and by the WBSB to the north) and its offshore area. WBSB opened as a back-arc basin behind the southwards-moving Istanbul Zone and collided with the Sakarya Zone along the Intra-Pontide Suture during the Eocene. Istanbul Zone rifted off from its original location around the present Odessa Shelf during the early Cretaceous, and started to move southwards along the West Black Sea and West Crimean transform faults. The emplacement of the Istanbul Zone into its present position was possibly during the late Albian.
Geophysical data have become increasingly important for rapidly developed tectonic research since 1960s. Seismic reflection method is the most effectively used geophysical method for investigation of the deep structure and the geological features beneath the sea floor. From the middle of 1990's to present, in spite of well-rounded but non-utilizable data of commercial two dimensional (2D) and three dimensional (3D) seismic studies of petroleum companies, the number of scientific studies and publications for academic purposes, revealing the geological structure beneath Turkish sector of the WBSB using marine seismic reflection data, has been increasing. Especially, there is a considerable amount of scientific articles focused on various subjects (such as mass wasting, sedimentation, sea-level fluctuations, BSR, Messinian events etc.) presenting detailed seismic sections and providing a good definition of the structural properties beneath the shelf and the continental slope of the southern part of the basin along Turkish margin.
In tectonics, researchers search for ways to predict the time and location of damaging tectonic events such as earthquakes and tectonically induced events. To be neotectonically useful, the predictions must be precise enough to alleviate the loss of life and property. This quest for prediction techniques also involves the investigation of active tectonics. The earthquake occurred on October 15, 2016 (Ml=5.0) re-attracted attention to the tectonic activity of WBSB. This earthquake took place at 11:18 local time and lasted 7-8 seconds. It was felt in the cities of Istanbul, Kocaeli, Duzce, Sakarya, Zonguldak and even in the Bulgarian cities Varna and Bourgas. The epicenter of the earthquake was at a point 195 km north-east of Istanbul and 124 km north-west of Zonguldak. This earthquake with a shallow epicenter (11.4 km below the surface) was one of the rare instrumentally recorded strong earthquakes occured at the southern part of the WBSB.
The Bartın Earthquake of September 3, 1968 (MS=6.6) is the strongest instrumentally recorded earthquake along the Turkish margin of Black Sea. This earthquake was the first seismological evidence of active thrusting occuring at the southern margin of the Black Sea and its fault-plane solution indicated a thrust mechanism which emphasized a different behaviour from the right-lateral strike-slip North Anatolian Fault. Focal mechanism derived from the solution of the moment tensor for the October 15, 2016 Black Sea earthquake, revealed another clue for the active thrusting at the WBSB with a very similar focal mechanism solution of Bartin Earthquake.
The central and the deepest parts of the Black Sea was believed to be aseismic but, with the recent studies, the researchers showed that there are a significant number of earthquakes in the Black Sea, mostly of magnitude Mw=4.0 or smaller. They also revealed that the seismicity increases towards the margins of the Black Sea, with the largest events at the margins. The researchers also emphasized that, in southern Black Sea and the middle part, especially offshore of Bartın and Samsun, the earthquakes which have the reverse faulting component dominated solutions, are related with the compressional tectonic regime and the strikes of these earthquakes generally lie parallel to the basin. These earthquakes show that the region is not aseismic and produces earthquakes from time to time, though not very frequently.
The folds are commonly accompanied by reverse faulting and many of these reverse faults are low angle and are called thrust faults. Some of the thrust faults and high-angle reverse faults may break the surface, but many others remain hidden within the cores of anticlines and are termed buried reverse faults. Buried active faults present a significant earthquake hazard-that large damaging earthquakes can occur on faults located entirely beneath or within folded rocks. The tips of such faults may be buried at depths of several kilometers, and when they rupture during earthquakes, uplift and folding occur at the surface. In this study, the fault structures that are considered to be formed by the effect of compressional tectonic regime and the structures formed by the activities of these faults beneath the shelf and slope areas between the region offshore Akçakoca-Cide at the southern part of the WBSB, revealed by using marine seismic reflection data.
The marine seismic reflection data used in this study were collected by the collaboration of General Directorate of Mineral Research and Exploration (MTA), Cambridge University and Istanbul Technical University (ITU) in September, 1998. R/V MTA Sismik-1 was used to collect data on 14 seismic lines with a total length of 460 km. All necessary and some optional processes of typical marine seismic data processing sequence were applied to raw data to obtain time-migrated seismic sections. From the time-migrated seismic sections, to give examples for thrust fault structures and folds formed by the activities of these faults became possible. These images were beneficial to clarify the presence of the compressional regime effecting the study area.
Available onland and offshore geological exploratory well data of Akçakoca-1, Akçakoca-2, Ereğli-1, Filyos-1, Bartin-1, Ulus-1, Amasra-1, Cakraz-1 and Gegendere-1 are obtained from General Directorate of Petroleum Affairs (PIGM) and used to correlate with seismic sections. Seismic markers of the time-migrated seismic sections were dated by referencing to Akcakoca-1 and Akcakoca-2 wells.
Also in this study, the land-offshore geological sections prepared by means of the geological sections given by previous geological studies and the information about the continuation of the geological features from land to offshore in the study area was presented. These geological sections also give quite beneficial new evidences for the presence of the compressional tectonic regime in the study area.
The important findings of this study were to give another clue for the presence of the compressional tectonic regime effecting the basin by using the outcomes of seismic reflection studies and to reveal the continuation of the thrust related geological features from land to offshore. The results obtained, agrees with the opinion that Black Sea is being compressed in N-S direction.