1 Introduction

The Gulf of Saros is a NE–SW oriented basin that is located at the northeastern part of the Aegean Sea (figure 1a). It is a westward widening and deepening wedge-shaped embayment (\({>}645\hbox { m}\)) (Ustaömer et al. 2008) with a width of 30 km and a length of 55 km. The suggesting ages of the initial opening of the Gulf of Saros change from the Oligocene (Saner 1985; Coşkun 2000) to the Pliocene–Quaternary (Yaltırak 1996; aatay et al. 1998; Tüysüz et al. 1998; Yaltırak et al. 1998; Yaltırak and Alpar 2002). The basinal structure of the gulf is controlled by two main tectonic forces: these are a NE–SW-trending strike-slip fault system at the western termination of the Ganos Fault and N–S extension of western Anatolia. The Ganos Fault (figure 1b) is regarded as the western continuation of the northern strand of the North Anatolian Fault Zone (NAFZ) (figure 1a).

Fig. 1
figure 1

(a) Inset map shows generalised tectonics of Turkey, the seismicity map (1900–2016, koeri.boun.edu.tr) of North Aegean and Gulf of Saros (NAFNS), (b) the locations of the new seismic data superimposed on the geology map (Yaltırak et al. 1998).

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Fig. 2
figure 2

Selected previous fault models proposed for the Gulf of Saros (modified from Ustaömer et al. 2008).

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After the destructive \(M_{\mathrm{w}}=7.3\) earthquake triggered in 1912, the Ganos fault and other secondary faults have been studied and mapped by numerous researchers (Gutzwiller 1923; Ambraseys and Finkel 1987; Barka and Kadinsky-Cade 1988; Tüysüz et al. 1998; Yaltırak et al. 1998, 2000; Armijo et al. 1999, 2002, 2005; Okay et al. 1999, 2004; Kurt et al. 2000; Yaltırak and Alpar 2002; Altınok et al. 2003; Altunel et al. 2004; Seeber et al. 2004; McNeill et al. 2004; Ustaömer et al. 2008). Altınok et al. (2003) estimated the total length of the surface rupture about 56 km, as extending NE–SW from Gaziköy to Gelibolu, with lateral slips ranging between 3.5 and 4.5 m.

The kinematical characteristics of the Gulf of Saros are still under debate and explained by three different tectonic models (figure 2); these are (a) an extension mechanism (Pfannenstiel 1944; Saner 1985), (b) a strike-slip mechanism (Barka and Kadinsky-Cade 1988; Görür et al. 1997; aatay et al. 1998; Tüysüz et al. 1998; Saatılar et al. 1999; Kurt et al. 2000; McNeill et al. 2004) and finally (c) negative flower structure (transtension and transpression domains) model (Yaltırak 1996; Yaltırak et al. 1998; Yaltırak and Alpar 2002).

The discussions on the exact location of the Ganos Fault in the Gulf of Saros also continue. Some authors claim that it follows the southern margin of the main trough, while some others proposed that it is placed on the northern margin. Some other researchers suggested that it is divided into a number of segments at the eastern part of the gulf, so that they control the northern and southern boundaries. A geophysical study was performed by McNeill et al. (2004) who indicated the presence of right-lateral displacements with Riedel-type structures. In addition, Ustaömer et al. (2008) have shown that the Ganos Fault cuts the narrow deformation zone in the eastern gulf. Then it ended up on the slope. From this point onwards, it has been stated that the compressional forces formed two NE and SW oriented faults that cut the centre of the basin and formed Riedel-type faults.

The seismic data covering the Gulf of Saros is limited in order to demonstrate the active faulting within the inner gulf in detail. The available data are mainly distributed on the Aegean Sea exit of the gulf and there are only three seismic profiles (aatay et al. 1998; Yaltırak et al. 1998; Ustaömer et al. 2008; Gökaşan et al. 2012) representing the eastern part. Three seismic profiles are not sufficient to understand the tectonic properties at the inner side of the gulf. The present study focuses on the eastern part of the gulf for determining detailed fault characteristics and types of the deformations that could be observed on shallow seismic data. For this purpose, new high-resolution seismic profiles (figure 1b) were acquired at the eastern part of the Gulf of Saros in May 2016.

1.1 Geological setting

The 1500-km-long NAFZ is an intracontinental transform fault (Şengör 1979) and splits into three strands (Barka and Kadinsky-Cade 1988) at the eastern part of the Sea of Marmara (figure 1a). These branches terminate against the normal faults at the northern Aegean Sea where the westward escape of the Anatolian block turns into anticlockwise rotational wedges (Yaltırak et al. 2012). All strands have different kinematic and seismic character in the region. The Northern Strand North Anatolian Fault Zone (NAFNS) enters the Sea of Marmara from Gulf of İzmit at the east, bounds the northern shelf of the Sea of Marmara, forming three major basins that reach the depth of over 1200 m, and finally connects to the Ganos Fault (Yaltırak and Alpar 2002) at the west (figure 1a).

The most dominant unit in the stratigraphy of the surrounding onshore area of the Gulf of Saros is the Pre-Miocene and young sedimentary volcanic sequences (Ustaömer et al. 2008) (figure 1b). Four major unconformities were defined in the Tertiary–Quaternary evolution, representing dominant tectonic events, erosions and sedimentation periods (Saner 1985; Sümengen et al. 1987; Yaltırak 1996).

These unconformities constitute the bases of the Pre-Miocene, Miocene, and Pliocene and Quaternary sequences. The Gazahandere, Kirazlı, Alçıtepe (Miocene units) formations that are mainly overlain unconformably by younger units, outcrop along the shores of the Çanakkale Strait and between the towns of Enez and Erikli. The Miocene sedimentary sequences show some facies differences in the Thrace and Gelibolu peninsulas (Saner 1985; Sümengen et al. 1987; Yaltırak 1996; Sakin et al. 1999). The sequence was initially deposited as a lacustrine unit in the Gelibolu Peninsula and then it was deposited as a lacustrine and a marine unit at the same time in the peninsula of Thrace (Sümengen et al. 1987). The Pliocene units (Conkbayırı, Ozbek and Truva formations) are observed at the southern part of the Gelibolu Peninsula. The alluvium formations form the current geology of the marine and river basins and the alluvial reservoir work area of the modern river.

Fig. 3
figure 3

(a) The multibeam bathymetry map from Ustaömer et al. (2008) and Gökaşan et al. (2010) and land topography map of the Gulf of Saros; (b) the bathymetry map of the study area (obtained and merged from seismic data used in this study, ETOPO 1 and digitised charter maps data); (c) the slope map of the gulf; (d) the slope direction map of the gulf; and (e) zoomed view of the multibeam bathymetry that shows submarine canyons and offsets (R: Riedel fault; PDZ: principal displacement zone).

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2 Material and methods

A total of 350 km high-resolution shallow seismic reflection data have been collected and interpreted in order to determine the structural signatures of the eastern part of the Gulf of Saros (figure 1b). Chirp data were acquired by using a Bathy \(2010\hbox {P}^{\mathrm{TM}}\) Chirp sub-bottom profiler and bathymetric echo sounder with four transducers, each producing 2–8 kHz Chirp signals. The power level, sweep bandwidth and detection threshold were adjusted automatically during the survey. The transmit pulse repetition rate was dependent on the depth range. The survey speed was fixed to 3.5–4.0 knot during the survey. Gain correction, band-pass filter (10/15–1750/1900 Hz) and predictive deconvolution were applied to Chirp data. These data were processed using Kogeo Seismic Toolkit 2.7 and analysed by ‘The Kingdom Suite’ software.

Some of the available vintage seismic profiles (sparker), navigated with short-range position fixing, were also considered, even though they are expected to have positional errors. In this study, the navigation data were recorded with the Magellan Proflex 500 scientific GPS system with a horizontal error margin of about ±10 cm, and the same positioning information stored in the headers of seismic data files.

The bathymetry map (figure 3b) constitutes three data sets: (a) sea-bottom depth data acquired from seismic profiles; (b) ETOPO 1 data (Amante and Eakins 2009); and (c) digitised charter maps (e.g., Navionics). All the data merged in ARCGIS program as a shape file changed to DEM (Digital Elevation Model) file in ENVI program. The final figure (figure 3b) was shaded in GLOBAL Mapper program.

3 Results

3.1 The morphology of the study area

The Ganos Fault enters into the Gulf of Saros from the eastern termination of the gulf (figure 1b); it cuts the southern part of the gulf and forms a trough deepening westward (figure 3a and b). It is the trough that divides the gulf into two (figure 3a and b). The gentle wall slopes of the trough (i.e., \(1{-}2^{\circ }\)) increases up to \(10{-}12.5^{\circ }\) in front of the Town Mecidiye (figure 3c). The slope of the basin floor is \(0.5^{\circ }\) and ascends to the front of the Mecidiye with \(7.5{^{\circ }}\) slope (figure 3d). The seafloor depths of the trough change from − 100 to − 300 m (figure 3b) with respect to the slope. The most prominent feature of the slope direction map of the gulf is two partial structures and fault controlled lineament (figure 3d).

3.2 Structural interpretation of the high-resolution seismic data

The main fault system that controls the common structural and morphological properties of the gulf can be clearly followed on the high-resolution shallow seismic profiles (figure 4a–c). The main fault system (f1), namely the Ganos Fault, cuts the northern margin (figure 3b) of the basin (figure 4a) at the eastern part of the gulf. The slope gradient of the stratified sedimentary packages in the trough changes \(1{-}9^{\circ }\) (figure 4a). The fault ‘fs’ is parallel to the Ganos Fault to the NW, but contrarily it does not affect the sea floor (figure 4a). The uplift of the acoustic basement at the northern (figure 4a) and southern parts (figure 4a–c) of the study area is noteworthy. The uplift causes normal faulting (fn) (figure 4a) at the northeastern part of the gulf. ‘f2’ fault can be observed roughly parallel to the ‘f1’ fault to the west. These two faults are both affected by the sea floor at the centre and the western part of the study area (figure 4a and b) and cause a small extension that can be traced at the centre of the study area (figure 4b). This is one of the most important proofs for the current activity.

Fig. 4
figure 4

The interpreted–non-interpreted high-resolution seismic profiles and locations. Red dashed lines are multiple reflections. \(1{-}9^{\circ }\) arrows show the real slopes calculated by Kingdom Suite (2.7). The faults fn, fs, f1, f2 and f3 are observed in the seismic profiles. See figure 1(b) for their locations.

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A new fault (f3) appears in the sudden deepening part of the trough where ‘f1 and f2’ vanish westward (figure 4c). It is more deformed if compared to the faults f1 and f2, because it is the most important tectonic element responsible for the abrupt deepening morphology of the gulf (figure 4c). According to the offsets on the submarine canyons (figure 3e) that can be observed at the western outlet of the Gulf of Saros, the fault f3 has left-lateral movement. The step between the faults f1 and f3 possibly occurred due to different oblique movements of the blocks. Therefore, they seem to be different segments depending on their characteristic properties. This fault is covered by sediment at the places where the trough deepens suddenly (figure 4c). The extension and the new fault evaluation (f2) are possibly controlled by the southwestern movement of the southern block.

The terraces observed in the study area (figure 4a–c) are mainly controlled by the faults. The sea-level changes and the sedimentary depositions play some secondary roles. The trough is possibly covered by the sea during the global lowstand period. The sedimentary input into the southern part of the gulf is believed to be relatively lower than that at the northern part, causing the unconformity surfaces observed at the southern margin.

Fig. 5
figure 5

The zoomed seismic lines show active faulting at the Gulf of Saros (GF: Ganos Fault; SF: Saros Fault). The horizontal two-sided arrows show the extension movements. See also figure 1(b) for locations.

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4 Discussion and conclusion

The sea floor topography of the eastern part of the Gulf of Saros is commonly controlled by main structural elements. The slope of the main trough and the trend of its direction increase to the west. The slope and slope orientation maps (figure 4d and e) indicate that the tectonic settings of the northern and southern parts of the gulf exhibit different characteristics. The fault f1 (Ganos Fault) enters the eastern part of the gulf that formed the trough which can be easily outlined from the seismic profiles (figure 5a–e). The fault is also shown by Çağatay et al. (1998, their figure 5). The length of Ganos Fault is about 20 km within the gulf. The fault f2 appears parallel to the fault f1 at the central part of the study area. ‘f1’ and ‘f2’ faults together form a small passage. A new transform fault (f3), which is identified and named as the Saros Fault in this study (figure 5f), is controlling the general morphology of the gulf. The sudden deepening of the trough occurred under the control of the Saros Fault where the Ganos Fault lost its activity (figure 5f). Both the morphological properties of the sea floor and the new steeper and deeper basin formation traced from the seismic sections (figures 4 and 5) indicate an extensional mechanism was dominant in the study area. According to the offsets observed on the submarine canyons at the western outlet of the gulf, the fault SF is left lateral where the fault GF is right lateral. These differential movements cause an extension mechanism at the study area and probably the fault f2 was formed because of these two tectonic forces. According to the morphological view of the small passage that formed between the faults f1 and f2 the initial phase of the extension tectonics dominated in the study area.

Fig. 6
figure 6

Seismic profile showing landslide deposits, potential landslides and the slide plane.

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Fig. 7
figure 7

Fault map of the eastern part of the Gulf of Saros. The fn is a normal fault near the islands, while the fs is a 16-km-long inactive transform fault. GF: Ganos Fault; SF: Saros Fault.

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Although the seismicity map shows seismic gap at the eastern part of the Gulf of Saros (figure 1a), Karabulut et al. (2006) proposed 40–50 km long submarine segment in the Gulf of Saros because the northeastern termination of the aftershock sequence could be accepted as the western termination of the 1912 earthquake rupture. The fault settings obtained in this study, those with evidently traceable surface lineation, support their finding (figures 4a, b and 5a–e). The surface effects cannot be observed at the westernmost part of the study area because of the sedimentary deposits (figures 4c and 5f).

The sedimentary deposits cover the surface fractures of the Saros Fault at the western part of the trough where the basin slope increases suddenly (figures 5f and 6). Their reflection characteristics and terminations indicate that they are landslide deposits (figures 5f and 6), which might be moved by the recent earthquakes. The higher slopes of the gulf (slide planes) observed on the seismic data (figure 6) probably cause mass movements during the earthquakes. It is well known that strike-slip earthquakes may not cause tsunamis, but mass movements occurring during these earthquakes may cause landslide triggered tsunamis (Yalıner et al. 2002). Therefore, the coastal areas near the North Aegean Trough are under the threat of tsunami risk because of the possible submarine landslide areas (see also Altınok et al. 2011, 2013 for Aegean tsunamis). In view of this threat, an estimation of seismic risk of this area should be carried out by modern techniques, i.e., the nowcasting approach (Rundle et al. 2018) which is based on the use of a new time domain termed natural time (Varotsos et al. 2005, 2011).

The inactive faults fs and fn are also determined in the study area (figure 7). The fault fs extends approximately 16 km parallel to the Ganos Fault (figure 7). On the other hand, the fault fn is observed only at the eastern part of the study area near the islands.

The previous fault maps of the gulf were constructed by interpreting the multi-beam bathymetry maps, and confirmed by the analogue seismic sections (aatay et al. 1998; Yaltırak et al. 1998; Ustaömer et al. 2008; Gökaşan et al. 2012). On the other hand, because of the large intervals between the available seismic sections, it was not easy to determine the changes in the fault characters. The new high-resolution seismic data show that the main fault system of the eastern part of the gulf is not made up of a one-piece segment (Ustaömer et al. 2008; Gökaşan et al. 2012) or bounded by two boundary faults (aatay et al. 1998; Yaltırak et al. 1998). The fault energy is carried by two fracture mechanisms (figure 7a). At first, the Ganos Fault affects the northern margin of the trough (figure 7), before the younger Saros Fault passing through the trough’s centre. The Saros Fault causes the active formation of a new basin and the steep deepening of the trough (figure 7). The new basin was possibly formed as the result of the southwestern move of the southern part that creates the Saros Fault, and causes deepening of the basin (figure 7). According to the gravity inversion study derived from the satellite gravity data (Tirel et al. 2004), the North Aegean Trough is related with a two-stage evolution of the Aegean extension.