The Manyas fault zone (southern Marmara region, NW Turkey): active tectonics and paleoseismology

The Manyas fault zone (MFZ) is a splay fault of the Yenice Gönen Fault, which is located on the southern branch of the North Anatolian Fault System. The MFZ is a 38 km long, WNW–ESE-trending and normal fault zone comprised of three en-echelon segments. On 6 October 1964, an earthquake (Ms = 6.9) occurred on the Salur segment. In this study, paleoseismic trench studies were performed along the Salur segment. Based on these paleoseismic trench studies, at least three earthquakes resulting in a surface rupture within the last 4000 years, including the 1964 earthquake have been identified and dated. The penultimate event can be correlated with the AD 1323 earthquake. There is no archaeological and/or historical record that can be associated with the oldest earthquake dated between BP 3800 ± 600 and BP 2300 ± 200 years. Additionally, the trench study performed to the north of the Salur segment demonstrates paleoliquefaction structures crossing each other. The surface deformation that occurred during the 1964 earthquake is determined primarily to be the consequence of liquefaction. According to the fault plane slip data, the MFZ is a purely normal fault demonstrating a listric geometry with a dip of 64°–74° to the NNE.

The Manyas earthquake caused extensive liquefaction and lateral spreading around the Salur, Hamamlı, Yeniköy and Kızılköy villages to the south of Lake Manyas. There are different interpretations of the origin of the surface deformation and the seismotectonic features of the earthquake.
The first macro seismic study on the Manyas earthquake was performed by Erentöz and Kurtman (1964). They observed a series of fractures in the Gönen plain, in the area south of Lake Manyas and in the vicinity of the towns Susurluk, Mustafakemalpaşa and Karacabey. These fractures were up to 200 cm long and 5-10 cm wide and indicate that the surface deformation might have occurred due to liquefaction and/or lateral spreading (Erentöz & Kurtman, 1964). The strike of the fractures were reported as N-S in the Gönen plain, E-W in the area south of Lake Manyas, N-S to NNW-SSE partly and WNW-ESE in the Susurluk region and E-W in the regions of Mustafakemalpaşa and Karacabey. Erentöz and Kurtman (1964) stated that during the Manyas earthquake although the northern coastal plain of Lake Manyas was stable, the southern part of the lake subsided. According to Erentöz and Kurtman (1964), the source of earthquake was likely located along E-W-trending faults.
Another macro seismic study on the Manyas earthquake was carried out by Ketin (1969), who mapped a series of NW-trending en-echelon fractures in the vicinity of the villages of Hamamlı, Salur, Kızılköy and Yeniköy. These fractures were interpreted as tension cracks that developed as a result of dextral strike-slip faulting (Ketin, 1969).
There are many different seismological results associated with the source parameters of Manyas earthquake. The first seismological study, which was performed by Canıtez (1968), stated that the vertical component was much larger than the horizontal component associated with the Manyas earthquake. Another seismological study was performed by Öcal et al. (1968) whose focal mechanism solution shows that the Manyas earthquake originated from a WNW-trending reverse fault. According to Öcal et al. (1968) the coordinates of the epicentre were 40.10°N, 27.90°E (south of the MFZ), with a focal depth of 24 km. According to the focal mechanism solution of McKenzie (1972), however the 1964 earthquake originated along a NW-trending normal fault with epicentral coordinates of 40.30°N, 28.23°E (NE of Lake Manyas) and 10-24 km for the focal depth. Kıyak (1986) suggested that the coordinates of the epicentre were 40.18°N, 28.12°E (NE of Lake Manyas) and that the earthquake was sourced by an oblique-slip normal fault. Taymaz, Jackson, and McKenzie (1991) indicated that the Manyas earthquake originated from WNW-trending normal fault and that the coordinates of the epicentre were 40.30°N, 28.23°E (NE of Lake Manyas), with a focal depth of 14 ± 2 km. These differences are primarily associated with the use of different crustal models, initial depths for the moment tensor solutions and the scarcity of avaliable seismological stations.
The structural features and the fault type within the MFZ are controversial in the literature. Şaroğlu, Emre, and Boray (1987) defined the Manyas Fault as an approximately 70-km-long, dextral strike-slip fault extending between the towns of Gönen and Mustafakemalpaşa. Emre, Doğan, Özalp, and Yıldırım (2011) and Emre et al. (2013) described the MFZ as a 38-km-long, dextral strike-slip fault zone also extending between the towns of Gönen and Mustafakemalpaşa and identified a 16-km-long Salur segment that generated a surface rupture during the 1964 Manyas earthquake. Although the MFZ was defined based on its morphological properties, none of those studies presented fault plane slip data.
In this study, we aim to determine the structural and paleoseismological features of the MFZ and identification of the surface deformation structures.
2. Tectonic setting and regional geology
The NAFS is seismologically one of the most active fault systems in the Eastern Mediterranean Region. It begins at the Karlıova Triple Junction and extends to the east of Adapazarı. The NAFS bifurcates into two main branches at the east of Adapazarı as northern and southern branches and forms a large-scale horse tail structure (Barka, 1992;Barka & Gülen, 1988) (Figure 1).
The southern branch of the NAFS extends between the east of Adapazarı and Gemlik Gulf. This branch, which extends parallel to the southern shoreline of the Marmara Sea from the Gemlik Gulf to the Kapıdağ Peninsula, forms a restraining bend towards the southwest. This branch is represented by numerous dextral strike-slip faults such as the Edincik Fault, Sarıköy Fault, Yenice Gönen Fault and Edremit Fault Zone in a wide deformation zone between the Kapıdağ Peninsula and Gulf of Edremit ( Figure 2).
Recent GPS studies demonstrate that plate motions in and around the Marmara Region are principally accommodated by the northern branch of the NAFS, the data for which suggest a velocity of 24 ± 1 mm/year along this branch (McClusky, Reilinger, Mahmoud, Ben Sari, & Tealeb, 2003;Reilinger et al., 2006). The northern branch of the NAFS facilitates approximately four times as much right-lateral motion (~24 mm/year) as the southern branch does (Meade et al., 2002). In the Southern Marmara Region where the NAFS bifurcates into fault segments across a wide zone, the faults are suggested as dextral with extensional slip rates as varying from 0.9-6.8 mm/year to 0.8-5.5 mm/year (Aktuğ et al., 2009;Ergintav et al., 2014;Flerit, Armijo, King, & Meyer, 2004;Meade et al., 2002;Reilinger et al., 2006).

Regional geology
The basement rocks in the region are composed of Kazdağ metamorphics and the Kalabak Group. Kazdağ metamorphics consist of the pre-Triassic Fındıklı formation (Trf) and Altınoluk marble member (Trfa). These units are tec- tonically overlain by the Upper Paleozoic Sazak formation (Pzks) of the Kalabak Group (Duru, Pehlivan, Okay, Şentürk, & Kar, 2012). These metamorphic units are well exposed to the southeast of Manyas and belong to the Sakarya Zone but unconformably overlain by the Karakaya Complex (Trkk) (Figure 3). The Karakaya Complex consists of metaconglomerate, metasandstone, sandy limestone, tuff, metavolcanite and limestone blocks of Devonian -Permian ages (Duru et al., 2012). These units are well exposed to the west of Manyas and the village of Eşen ( Figure 3). It is unconformably overlain by Callovian-Hauterivian Bilecik limestone (Jkb). The basement units are tectonically overlain by the Upper Cretaceous Yayla Melange (Ky) (Figure 3). Basement units are cut by Upper Oligocene -Lower Miocene Hallaçlar Volcanics (Toh), which comprise andesite, dacite and pyroclastic rocks. This formation is comformably overlain by Lower Miocene Şapçı Volcanics (Tmş), which is represented by white tuff and by acidic and andesitic lavas (Genç et al., 2012).
The Hallaçlar Volcanics are unconformably overlain by the Middle-Upper Miocene Göbel Formation (Tmg) and the Mudamköy Volcanic member of this formation (Pehlivan, Duru, Kanar, & Kandemir, 2011     Formation consists of fluvial conglomerate, sandstone and lacustrine claystone in addition to, clayey limestone. The Mudamköy volcanic member is represented by andesite, dacite, basalt, tuff and agglomerate (Pehlivan et al., 2011). The Göbel Formation and Mudamköy volcanite member are exposed in the vicinity of the Eşen segment ( Figure 3). These units are unconformably covered by the Pliocene Bayramiç Formation, which is characterised by the alternation of fluvial conglomerate, sandstone, claystone and siltstone. It is exposed around the Dereköy and Salur segment.

The segment characteristics of the Manyas fault zone (MFZ) and its relation to the North Anatolian fault system
The Southern Marmara Region is primarily deformed by the active dextral strike-slip and normal faults belonging to the southern branch of the NAFS. The Yenice Gönen Fault and MFZ are active fault segments within the southern branch of the NAFS in the Southern Marmara Region. Both the 1953 Yenice Gönen earthquake (Ms = 7.2) and the 1964 Manyas earthquake (Ms = 6.9) were produced by these active faults. A 1-km-long part of the surface ruptures that occurred during both the Yenice Gönen and Manyas earthquakes overlapped each other. The NE-trending Yenice Gönen Fault and WNW-ESE-trending MFZ present a horse tail structure in the Southern Marmara Region (Figure 4).
The MFZ is a 38-km-long normal fault zone that extends in the WNW-ESE direction between the Yenice Gönen Fault to the west and the Mustafa Kemal Paşa fault to the east. The MFZ consists of three en-echelon geometric fault segments, named Dereköy, Salur and Eşen from SW to NE (Figure 3).
The 1964 Manyas earthquake caused surface deformation around the western part of the Dereköy and Salur segments. The Salur segment is a 16-km-long, WNWtrending normal fault segment that extends between Bayramiç in the west and Bölceağaç in the east. In this area, the Salur segment cuts the Bayramiç Formation and Quaternary deposits (Figure 3).

Paleoseismic trenching along the Manyas fault zone (MFZ)
In this study, paleoseismic trenching was performed in two different locations, named the Bayramiç and Hamamlı trench sites. The Bayramiç trench site is located to the west of the Salur segment (UTM Coordinates: 569807 E, 4440636 N) (Figure 3). The Hamamlı trench site is situated north of the Salur segment (NE of Hamamlı village) (GPS Coordinates: 35 0578972 E, 4440811 N) (Figure 3). In this locality, an observation pit was excavated to understand the surface deformation pattern documented by Erentöz and Kurtman (1964) and Ketin (1969).
Six charcoal, organic sediment and bone samples gathered from the Bayramiç trench site were dated at Beta Analytical Laboratories (USA) ( Table 3). Additionally, two Optically Stimulated Luminescence (OSL) samples and two Thermo Luminescence (TL) samples were analysed in the Luminescence Dating Research Laboratory of Ankara University (Turkey) ( Table 4). Trench walls were logged with a photomosaic technique using GIS software.

Bayramiç Trench site
Bayramiç Trenches were excavated on old fluvial deposits (Q1) (Figure 3). The fault trace is characteristic with a several-hundred-meter long linear fault scarp. Bayramiç trenches were excavated on local depressions formed by back-tilting.
Three trenches parallel to each other were excavated across to the Salur segment. They were named Bayramiç Trench 1, 2 and 3 ( Figure 3). Bayramiç Trench 1 is 26 m-long, 3 m-deep and 5 m-wide, Bayramiç Trench 2 is 14 m-long, 3 m-deep and 4 m-wide and Bayramiç Trench 3 is 26 m-long, 3 m-deep and 5 m-wide.     Kürçer et al. (2015) Whereas the paleoseismological results were obtained from Bayramiç Trench 1, fault plane slip data were collected and interpreted from the other two trenches. Three different stratigraphic packages were identified at Bayramiç Trenches 1 and 2. The oldest units are Late Pleistocene-Holocene fluvial sediments ( Figure 5, Unit 1 -9). This sequence was incised by late Holocene fluvial chan-nel deposits comprising shards (Figures 5 and 6, Unit 10a and 10b). All of these units are unconformably overlain by modern soil (Figures 5 and 6, Unit 11).
Based on paleoseismological criteria such as stratigraphic and structural relations and upwards termination of fault strands, at least three events in the last 4000 years that resulted in a surface rupture, including the 1964 earthquake were identified and dated at Bayramiç trenches.

Event 3
Event 3 is well identified between 21 and 22 meters in the west wall of Bayramiç Trench 1 ( Figure 5). The earthquake horizon for this event was identified as the base of Unit 10a because Unit 2 was cut by the fault and covered by Unit 10a. Based on the OSL method, Unit 2 was dated to 19100 ± 2500 BP years. The radiocarbon age of Unit 10a is 2300 ± 200 BP. Because approximately 17 000 years of the sediment package (Units between 3 and 9) were eroded by fluvial channel Unit 10a, the age of Event 3 is unclear.

Event 2
Event 2 can be seen in both walls of Bayramiç trench 1 ( Figure 5 and 6). Because the upward termination of the fault strand is covered by Unit 11, the earthquake horizon is defined as the base of Unit 11. The youngest radiocarbon age, obtained from Unit 10b below the earthquake horizon, is 820 ± 30 BP. The age of Unit 11 covering the earthquake horizon is dated to 320 ± 30 BP. According to these data, Event 2 should have occurred between 820 ± 30 BP and 320 ± 30 BP. Based on historical records, earthquakes occurred in both 1323 AD and 1556 AD (Ambraseys & Finkel, 1991) (Figure 2). Because of the epicenteral location of the 1323 AD earthquake, Event 2 seems to be associated with the MFZ. Therefore, Event 2 could be correlated with the 1323 AD earthquake.
Event 1 (6 October 1964. Manyas Earthquake -Ms=6.9) The 1964 earthquake surface rupture is clearly seen on Bayramiç trench walls and is represented by normal fault characteristics in a narrow deformation zone between 2 and 4 mtrs on the east and west walls of Bayramiç Trench 1 (Figures 5 and 6). The age of Unit 11 is 320 ± 30 BP, which is the youngest unit. Because this unit is cut by a fault, this fault is interpreted as a 1964  (Allmendinger, Cardozo, & Fisher, 2012;Marrett & Allmendinger, 1990;V 6.6.3). Arrows show tensional axis. surface rupture. An interpretation of the earthquakes identified from Bayramiç Trenches is presented in (Figure 7).

Hamamlı Trench site
The most intensive surface deformation occurred around the Salur segment during the 1964 earthquake. However, the features of the surface deformation, particularly between the villages of Salur and Hamamlı are controversial. Erentöz and Kurtman (1964) suggested that the fractures occurred due to liquefaction and/or lateral spreading. Conversely, Ketin (1969) interpreted the NW-SE trending fractures as tension cracks resulting from, dextral strike-slip faulting.
We excavated a cross trench to the northeast of Hamamlı village (GPS Coordinates: 35 0578972 E,4440811 N) to understand the origin of these surface deformational features (Figure 3).
The Hamamlı Trench site is located on the flood plain of the Manyas River (Qfp) (Figure 3). The groundwater table lies approximately 2 m below the surface. According to trench microstratigraphy, there is yellowish, water-saturated, fine sand at the bottom (approx. −4 m) Just above this unit, there is a yellowish-green, water-saturated, fine sand and silt alternation. Above the fine sand and silt alternation, there is yellowish clay, which is relatively more plastic. The recent soil level covers all of the units (Figure 8).
The crosscutting paleoliquefaction structures were observed at Hamamlı Trench (Figure 8). The model of earthquake-induced paleoliquefaction structures is illustrated in Figure 9.
The relatively older sand blow determined in Hamamlı Trench at −2 m is interpreted as a paleoliquefaction structure related to the penultimate event of 1964 earthquake. This event can be correlated with the 1323 AD earthquake.

Kinematic features of the Manyas fault zone (MFZ)
The MFZ consists of WNW-ESE-trending and northward-dipping normal faults. The general structural features of the fault zone are presented in Figure 10.
Kinematic data of the MFZ are limited. We were able to collect fault plane slip-data of the Eşen segment approximately 1.5 km northeast of the Eşen Village  (Allmendinger et al., 2012;Marrett & Allmendinger, 1990;V 6.6.3). Arrows show tensional axis.
( Figure 11). According to the fault plane slip-data, the Eşen segment is a normal fault with a minor dextral strike-slip component.
The structural data, which reveals the recent kinematic features of the MFZ, was gathered from Bayramiç Trenches 2 and 3 (Table 5; Figures 12-15). According to the fault plane slip-data measured from the trench walls, the MFZ is a pure normal fault trending WNW-ESE and dipping 65°-75°towards to north.

Discussion and conclusion
The deformation of the Southern Marmara region is primarily controlled by the westernmost segments of the NAFS. To the east of Adapazarı, the NAFS splits into two main branches that extend towards the Marmara Sea and the Southern Marmara Region, forming a horse-tail structure (Barka, 1992;Barka & Gülen, 1988) (Figure 2). The northern and southern branches of the NAFS form an approximately 100-km-wide dextral shear zone in noertwestern Turkey (Figure 4). This shear zone is bordered by the Ganos and Saros segments to the north and the Havran-Balıkesir fault zone (HBFZ)  and Edremit Fault Zone (EFZ)  to the south (Figure 4).
The southern branch of the NAFS consists of several dextral strike-slip faults (e.g. the Yenice Gönen Fault and Sarıköy Fault), reverse faults (e.g. the Bekten fault; Özalp, Kürçer, Özdemir, & Duman, 2016) and normal faults (e.g. the Manyas Fault Zone and Uluabat Fault) associated with the dextral strike-slip faulting mechanism in the area of study. The 1944The , 1953 were produced by the EFZ, YGF and MFZ, respectively (Figure 4).
The YGF is one of the most active dextral strike-slip faults in the Southern Marmara Region. A 70-km-long surface rupture occurred between Yenice and Gönen during the 18 March 1953, Yenice Gönen earthquake (Mw = 7.2). Eleven years after the Yenice Gönen earthquake, the Manyas earthquake (Ms = 6.9) occurred along the MFZ, on 6 October 1964. An 18-km-long surface rupture occurred as a result of the 1964 event along the Dereköy and Salur segments of the MFZ. 1-km-long part of surface rupture that was occurred both during the Yenice Gönen and Manyas earthquakes overlapped each other.
The MFZ is a WNW-ESE-trending 38-km-long normal fault zone that consists of three en-echelon fault segments, named Dereköy, Salur and Eşen from SW to NE. The MFZ is interpreted as a normal fault zone associated with dextral Equal-angle lower-hemisphere plots of fault slip data collected from the trenches. Fault and striae are superposed on a fault plane solution derived using the FaultKin program (V 6.6.3, Allmendinger et al., 2012;Marrett & Allmendinger, 1990). Arrows show tensional axis. strike-slip faulting (see Figure 4). The NE-trending YGF and WNW-ESE-trending MFZ presents as horse-tail structure in the Southern Marmara region (Figure 4).
On 6 October 1954, an earthquake (Ms = 6.9) occurred along the MFZ. Although some surface deformation occurred around the Salur segment, there is
In this study, paleoseismologic trench studies performed on the Salur segment of the MFZ resulted in the gathering of new and substantial data to overcome the discussions outlined above. At least three large earthquakes within the last 4000 years that resulted in surface ruptures, including the 1964 earthquake, have been identified and dated.
The ante-penultimate event was dated to between 3800 ± 600 BP and 2300 ± 200 BP, but there are no archaeological data or historical records associated with this earthquake.
To understand the origin of the surface deformations that occurred during the Manyas earthquake along the Salur segment a trench was excavated to the north of Hamamlı village. Paleoliquefaction structures crossing each other were observed in the Hamamlı trench. We concluded that the surface deformations that occurred during the 1964 earthquake are primarily the consequence of liquefaction.
Based on the fault plane slip-data, the MFZ is a purely normal fault that trends WNW-ESE and dips 64°-74°to the NNE. The Eşen segment of the fault zone has a minor dextral strike-slip component.
The seismotectonic features of the 1964 Manyas earthquake (Ms = 6.9) are controversial in the literature. There are substantially different views about the epicenter, focal depth and the faulting type of the earthquake.
We suggest a new seismotectonic model for the Manyas earthquake (Figure 16). In this model, a surface rupture is assumed to have occurred along the Salur segment of the MFZ during the Manyas earthquake. Fault plane slip-data collected from Bayramiç Trenches show that the MFZ is a purely normal fault trending to the WNW-ESE and dipping 64°-74°NNE (Table 5). We assume that the focal depth of the Manyas earthquake was 14 ± 2 km and that the dip of the fault plane at the hypocenter was 40°NNE (Taymaz et al., 1991).
According to these data, the MFZ should have a listric geometry. The epicentral location for the Manyas earthquake should be somewhere within Manyas Lake ( Figure 16).
Because of the local geological conditions and the proximity of the epicentre, the surface deformations, including liquefaction and lateral spreading, are concentrated between the villages of Salur and Hamamlı.