Seismic Activity of the Manisa Fault Zone in Western Turkey Constrained by Cosmogenic 36 Cl Dating

: This study reports on the cosmogenic 36 Cl dating of two normal fault scarps in western Turkey, that of the Manastır and Mugırtepe faults, beyond existing historical records. These faults are elements of the western Manisa Fault Zone (MFZ) in the seismically active Gediz Graben. Our modeling revealed that the Manastır fault underwent at least two surface ruptures at 3.5 ± 0.9 ka and 2.0 ± 0.5 ka, with vertical displacements of 3.3 ± 0.5 m and 3.6 ± 0.5 m, respectively. An event at 6.5 ± 1.6 ka with a vertical displacement of 2.7 ± 0.4 m was reconstructed on the Mugırtepe fault. We attribute these earthquakes to the recurring MFZ ruptures, when also the investigated faults slipped. We calculated average slip rates of 1.9 and 0.3 mm yr − 1 for the Manastır and Mugırtepe faults, respectively. 36 Cl concentrations already measured on the Mugırtepe fault surface We show that the MFZ experienced numerous ground-rupturing earthquakes during Holocene. We also provide a comparison and interpretation of our results with respect to paleoseismological data in our effort to better constrain the seismic history for the western MFZ. We ﬁnd that this region experienced clustered earthquakes during late Holocene.


Introduction
Although earthquakes are one of the most hazardous natural disasters, seismic records from instrumental and historical earthquake data cover only a limited time frame [1][2][3][4]. Therefore, the forecasting of future earthquake events and disaster mitigation design are based upon short and incomplete seismic records (e.g., [1]). The dearth of such data limits our understanding of the spatial extent of deformation and magnitude of future earthquakes, which may lead to a misevaluation of high seismic risk areas [5][6][7].
Numerous fault studies have been conducted worldwide using different techniques (e.g., [8][9][10][11][12][13][14]). One of the possible tools for tracking the pace of earthquakes on individual faults over timeframes that exceed those included in the existing seismic records, is fault scarp dating. This is a valuable tool that directly date episodic exposures of normal fault scarps produced by large magnitude earthquakes and was first proposed by Zreda and Noller [15]. This tool has been used and progressively improved by many other researchers over the last two decades . The investigation of fault scarp exposure using cosmogenic 36 Cl allows for the reconstruction of the timing, vertical displacement, recurrence interval, and magnitude of earthquakes as well as the fault slip rate. Thereby, the aforementioned technique offers the opportunity to extend the timeframe of slip histories on individual faults providing additional knowledge with respect to regional seismic behavior. The overall dating concept is as follows. On a fresh fault surface, exposed by aforementioned technique offers the opportunity to extend the timeframe of slip histories on individual faults providing additional knowledge with respect to regional seismic behavior. The overall dating concept is as follows. On a fresh fault surface, exposed by a dipslip component of rupture, cosmogenic 36 Cl begins to accumulate along the newly exposed segment at a uniform distribution and at a higher rate than the unexposed part of the scarp under the colluvium. Periods of earthquake activity are then disentangled based on: (1) cosmogenic 36 Cl concentrations measured along a continuous strip on the fault scarp; and (2) differences in 36 Cl accumulation rates on the exposed and covered surfaces during the quiescence times [15][16][17][18][19]33].
Strong earthquakes in extensional tectonic regimes may cause surface ruptures and deformation, principally documented as normal fault scarps that juxtapose Quaternary alluvium or colluvium against bedrock at a variety of scales [39,40]. An example of such a setting is Western Anatolia, Turkey, which includes approximately E-W-trending graben systems as a result of roughly N-S extension evidenced by the occurrence of large normal fault scarps occurring in the limestone bedrock ( Figure 1).  In addition, the active zone of the Izmir-Balıkesir Transfer Zone (IBTZ) extended between Izmir and Balıkesir cities, generally consists of N-S and NNE-SSE trending strike slip faults, and acts as the western border of the E-W-trending grabens i.e., Gediz (Figure 1). IBTZ was demonstrated to be a deep crustal transform fault zone during Late Cretaceous, which acted as a transtensional transfer zone in the Neogene period ( [43][44][45]). Recent seismicity with focal mechanism, Global Navigation Satellite System (GNSS) measurements and several geological studies indicate that IBTZ is undergoing an E-W shortening as well as N-S extension (e.g., [43,44,[46][47][48][49]). Here, recent investigations confirm a very close connection between the normal surface ruptures and large magnitude 6 or higher earthquakes (e.g., [33][34][35][50][51][52]). Earthquakes are considered imminent in this intensively active region, with the most recent destructive event occurring offshore Samos Island (south of Izmir region) on 30th October 2020, with Mw 7.0 [53]. However, the association of historic earthquakes with individual normal faults in Western Anatolia continues to be very limited (e.g., [54][55][56]).
In this study, we focus on one of the fault zones in the western sector of the Gediz Graben (western Turkey) to obtain a broader insight into the seismic behavior of active faults beyond the historical and instrumental earthquake archives ( Figure 2). We focus on two fault scarps within the western part of the Manisa Fault Zone (MFZ), documented as one of the most seismically-damaged regions in history [3,48,55,57,58]. Specifically, we applied the Fault Scarp Dating Tool (FSDT) computation code [37] to recover rupture histories of the Manastır and Mugırtepe faults (MAN and MUG in Figure 3, respectively) in the western segment of the active MFZ. We analyzed 87 samples from the Manastır fault surface and remodeled the cosmogenic 36 Cl concentrations already measured on the Mugırtepe fault surface [24]. We show that the MFZ experienced numerous groundrupturing earthquakes during Holocene. We also provide a comparison and interpretation of our results with respect to paleoseismological data in our effort to better constrain the seismic history for the western MFZ. We find that this region experienced clustered earthquakes during late Holocene. In addition, the active zone of the Izmir-Balıkesir Transfer Zone (IBTZ) extended between Izmir and Balıkesir cities, generally consists of N-S and NNE-SSE trending strike slip faults, and acts as the western border of the E-W-trending grabens i.e., Gediz ( Figure  1). IBTZ was demonstrated to be a deep crustal transform fault zone during Late Cretaceous, which acted as a transtensional transfer zone in the Neogene period ( [43][44][45]). Recent seismicity with focal mechanism, Global Navigation Satellite System (GNSS) measurements and several geological studies indicate that IBTZ is undergoing an E-W shortening as well as N-S extension (e.g., [43][44][46][47][48][49]). Here, recent investigations confirm a very close connection between the normal surface ruptures and large magnitude 6 or higher earthquakes (e.g., [33][34][35][50][51][52]). Earthquakes are considered imminent in this intensively active region, with the most recent destructive event occurring offshore Samos Island (south of Izmir region) on 30th October 2020, with Mw 7.0 [53]. However, the association of historic earthquakes with individual normal faults in Western Anatolia continues to be very limited (e.g., [54][55][56]).
In this study, we focus on one of the fault zones in the western sector of the Gediz Graben (western Turkey) to obtain a broader insight into the seismic behavior of active faults beyond the historical and instrumental earthquake archives ( Figure 2). We focus on two fault scarps within the western part of the Manisa Fault Zone (MFZ), documented as one of the most seismically-damaged regions in history [3,48,55,[57][58]. Specifically, we applied the Fault Scarp Dating Tool (FSDT) computation code [37] to recover rupture histories of the Manastır and Mugırtepe faults (MAN and MUG in Figure 3, respectively) in the western segment of the active MFZ. We analyzed 87 samples from the Manastır fault surface and remodeled the cosmogenic 36 Cl concentrations already measured on the Mugırtepe fault surface [24]. We show that the MFZ experienced numerous ground-rupturing earthquakes during Holocene. We also provide a comparison and interpretation of our results with respect to paleoseismological data in our effort to better constrain the seismic history for the western MFZ. We find that this region experienced clustered earthquakes during late Holocene.  [59] along Mugırtepe fault are shown as T1 and T2. Yellow stars locate fault scarp dating sampling sites of Manastır (MAN) and Mugırtepe (MUG) faults, respectively. GFZ: Gürle Fault Zone, TAF: Taşlıburun fault, KEF: Keçiliköy fault; Inset is a simple sketch to show Taşlıburun fault, which its location is beyond the map frame. Geological cross section shows stratigraphic and structural relationships of the units. Note that offset of Quaternary deposits by several instances of synthetic Holocene faulting in the hangingwall of the Manastır fault is the most direct evidence for their activity (modified from [24,59]).

Study Area
The approximately WNW-ESE-trending MFZ within the Gediz Graben extends for 35 km at the southern margin of the Manisa Basin [44,51,60] and includes a large number of Quaternary fault scarps [41,44,51,[61][62] (Figures 1 and 2). The MFZ is considered to be a northeastward-arcuate structure of the graben [59]. Different groups of kinematic indicators, including sinistral strike-slip, dextral strike-slip, and normal-slip denote three phases of activity in the MFZ since the early Miocene, respectively [44,51]. In the  [59] along Mugırtepe fault are shown as T1 and T2. Yellow stars locate fault scarp dating sampling sites of Manastır (MAN) and Mugırtepe (MUG) faults, respectively. GFZ: Gürle Fault Zone, TAF: Taşlıburun fault, KEF: Keçiliköy fault; Inset is a simple sketch to show Taşlıburun fault, which its location is beyond the map frame. Geological cross section shows stratigraphic and structural relationships of the units. Note that offset of Quaternary deposits by several instances of synthetic Holocene faulting in the hangingwall of the Manastır fault is the most direct evidence for their activity (modified from [24,59]).

Study Area
The approximately WNW-ESE-trending MFZ within the Gediz Graben extends for 35 km at the southern margin of the Manisa Basin [44,51,60] and includes a large number of Quaternary fault scarps [41,44,51,61,62] (Figures 1 and 2). The MFZ is considered to be a northeastward-arcuate structure of the graben [59]. Different groups of kinematic indicators, including sinistral strike-slip, dextral strike-slip, and normal-slip denote three phases of activity in the MFZ since the early Miocene, respectively [44,51]. In the investigated area (westernmost part of the MFZ), at least six WNW-ESE-trending and NNE-dipping normal fault scarps displaced Late Cretaceous-Paleocene carbonate footwall, against the Late Pleistocene-early Holocene sediments of Emlakdere Formation, occasionally covered by colluvium deposits on the hangingwall (Figures 3 and 4). This group of faults comprise the Manastır, F1, F2, F3, Mugırtepe, and F4 form separate scarp terraces and defines the southwestern boundary of the Manisa Basin. The Manastır fault was probably initiated as a master fault with the onset of graben system formation during the Early-Miocene or later in western Anatolia. While the other faults are interpreted to be formed as a consequence of basinward migration evidenced by back-tilting of the Emlakdere Formation and the Neogene volcano-sedimentary rocks [59]. The strata are parallel in these rock units separated by unconformity, indicating synchronized tilting [59] (Figures 3 and 4). Gradual deposition and rotation of hangingwall deposits caused by slip on the Manastır master fault is evidenced by a clear angular unconformity recorded in the upper part of the Emlakdere Formation showing dissimilar dip of strata of similar lithology. Based on the radiocarbon ages, this syn-depositional tilting was considered to occur between ca. 19 and 9 cal kyr BP [59]. In an extensional tectonic setting, shallow antithetic and synthetic faults within the hangingwall of the larger master fault are typically prevalent; these parallel/subparallel faults are refractions of the master fault dip and maintain evolution towards the basin and can move in synchrony with the master fault they are linked to (e.g., [63][64][65]). Secondary faults are normally incapable of producing significant earthquakes with magnitudes exceeding 5.5 and are considered as non-seismogenic faults [64]. Among this set of faults, the approximately 4.5-km long and 140-m-high Manastır fault is considered as the master fault. The Manastır fault is connected to the approximately 3-km long Taşlıburun fault through an N-S-trending relay ramp ( Figure 3). The Taşlıburun fault is, in turn, linked to the Keçiliköy fault with a similar length on its northeast side (Figures 2 and 3). These three faults constitute an en échelon structure linked to MFZ at its westernmost end [44].  (Figures 3 and 4). This group of faults comprise the Manastır, F1, F2, F3, Mugırtepe, and F4 form separate scarp terraces and defines the southwestern boundary of the Manisa Basin. The Manastır fault was probably initiated as a master fault with the onset of graben system formation during the Early-Miocene or later in western Anatolia. While the other faults are interpreted to be formed as a consequence of basinward migration evidenced by back-tilting of the Emlakdere Formation and the Neogene volcano-sedimentary rocks [59]. The strata are parallel in these rock units separated by unconformity, indicating synchronized tilting [59]  Based on the radiocarbon ages, this syn-depositional tilting was considered to occur between ca. 19 and 9 cal kyr BP [59]. In an extensional tectonic setting, shallow antithetic and synthetic faults within the hangingwall of the larger master fault are typically prevalent; these parallel/subparallel faults are refractions of the master fault dip and maintain evolution towards the basin and can move in synchrony with the master fault they are linked to (e.g., [63][64][65]). Secondary faults are normally incapable of producing significant earthquakes with magnitudes exceeding 5.5 and are considered as non-seismogenic faults [64]. Among this set of faults, the approximately 4.5-km long and 140-m-high Manastır fault is considered as the master fault. The Manastır fault is connected to the approximately 3-km long Taşlıburun fault through an N-S-trending relay ramp ( Figure 3). The Taşlıburun fault is, in turn, linked to the Keçiliköy fault with a similar length on its northeast side (Figures 2 and 3). These three faults constitute an en échelon structure linked to MFZ at its westernmost end [44].  The Manastır fault activity is expressed by sets of screes, landslides, and at least two generations of triangular facets [59]. The overprinting of the strike-slip slickenlines by the dip-slip ones indicate reactivation of the Manastır fault. Accordingly, the exposure of the sampled fault surface is attributed to an approximately N-S trending extensional tectonic regime that started in Quaternary. The Manastır fault extends to the NE side of Manastır Hill and intersects the NW end of the approximately 1.5 km wide strike-slip Gürle Fault Zone [59] (Figure 3). Gürle Fault Zone, a segment of IBTZ, is characterized by segmented parallel-subparallel faults, which extends to the north of Paşadegirmeni Hill and bounds MFZ on its west end. All the synthetic faults located at the western part of the MFZ are assumed to be linked in depth to the master Manastır fault and there are five secondary faults to the north (F1, F2, F3, Mugırtepe, and F4), which run parallel to the Manastır fault [59]. Faults F1, F2, and F3 are approximately 1, 3, and 2 km in length, respectively. Their dip ranges between 45 • and 65 • NNE with an average scarp height of 3 m, whereas the Mugırtepe fault has a maximum of 4 m height and is approximately 3 km long. To the northwest, the Mugırtepe and the similar-sized F4 faults merge at the foot of Paşadegirmeni Hill and cut across the Gürle Fault Zone [59] (Figure 3).
Six strong earthquakes have been recorded in Manisa and the surrounding region historically ( Figure 2). The oldest occurred in Lydia in 17 AD and had an intensity of IX. It caused significant damage in 13 or 16 ancient cities, mostly located in the Manisa Basin [1,48,55,57,58]. This earthquake is an example of the discrepancies in the geographic locations and intensity/magnitudes of ancient earthquakes recorded by different sources. The succeeding destructive earthquake dates back to 44 AD and likely damaged the ancient Greek cities of Magnesia, Samos, Militus and Ephesus, with an intensity of VIII [1,3,58,66]. In addition, [1] reported an earthquake in 926 (925) AD, in the province of the Thraceseans, caused traverse of the region by the Gediz and Menderes rivers. An earthquake in 1595 was documented ca. 60 km to the east of Manisa [1,55,67]. Another major earthquake with an intensity of VII was reported to have occurred in Izmir in 1664 [1,3,66,67], although [55] claim this event to have occurred near Izmir, and perhaps towards Manisa. The last recorded historical earthquake occurred in 1845 in Lesvos, and felt in Manisa [3,66], with a reconstructed intensity of VIII [3] or M = 6.7 [68]. The earthquake of 28th January 1994, with Mw 5.2 or 5.4, is the largest instrumental earthquake recorded close to Manisa with estimated focal depth of 5 to 10 km [48,53,69] and epicentered about ten kilometers northeast of Manisa ( Figure 2).
The rupture history of the Mugırtepe fault is reconstructed [24] using a Matlab ® code developed by Schlagenhauf et al. [22]. They proposed two scenarios. The first one yielded two seismic events at 13.7 ± 0.8 ka and 7.8 ± 0.5 ka with a displacement of 0.5 ± 0.2 m and 2.15 ± 0.35 m, respectively. The second scenario resulted in a single seismic event of 8.5 ± 0.6 ka with a vertical displacement of 2.65 ± 0.35 m. Here, we note that this event, similar to most of the events recovered by FSDT, consists of clusters of earthquakes that occurred close in time. In addition to the abovementioned documented and reconstructed earthquakes, three palaeoearthquakes defined during the last 1 kyr using radiocarbon dating of palaeosol samples collected inside two paleoseismic trenches dug across the Mugırtepe fault [59]. These three events were tentatively linked to the historical earthquakes of 926 AD, 1595/1664 AD, and 1845 AD.

Sampling
To select appropriate sampling sites, we explored the fault surfaces in several outcrops along the MFZ. We considered the most suitable site as the well-preserved surface with negligible evidence of weathering ( Figure 3). The site is close to the Mugırtepe fault scarp studied [24,59]. The scarp of the Manastır fault was sampled in summer 2008 following a similar sampling strategy by Mitchell et al. [16] ( Figure 5). Two parallel vertical slots spaced approximately 12 cm apart were cut to a depth of 3-4 cm into the scarp surface using a hand-held circular saw with a diamond blade. The rock strip was then divided into 5-10-cm slabs perpendicular to the vertical slots. Finally, the sample slabs were broken off with a chisel and hammer. Being vital factors for earthquake modeling, the scarp geometry elements including scarp dip, scarp height, top surface dip, and colluvium dip were determined in the field. Rock density and water content of the bedrock and colluvium were also estimated, and colluvium density was measured in the field. In addition, top and bottom positions of each sample were documented for modeling.
Geosciences 2021, 11, x FOR PEER REVIEW 7 of 23 surface using a hand-held circular saw with a diamond blade. The rock strip was then divided into 5-10-cm slabs perpendicular to the vertical slots. Finally, the sample slabs were broken off with a chisel and hammer. Being vital factors for earthquake modeling, the scarp geometry elements including scarp dip, scarp height, top surface dip, and colluvium dip were determined in the field. Rock density and water content of the bedrock and colluvium were also estimated, and colluvium density was measured in the field. In addition, top and bottom positions of each sample were documented for modeling. Along the Manastır fault scarp, three sampling strips (MAN-A, MAN-B, and MAN-C) that were spaced a few meters apart, were cut to cover the maximum height along the scarp surface ( Figure 5). We collected 87 samples, which covered approximately 7 m of Along the Manastır fault scarp, three sampling strips (MAN-A, MAN-B, and MAN-C) that were spaced a few meters apart, were cut to cover the maximum height along the scarp surface ( Figure 5). We collected 87 samples, which covered approximately 7 m of the 12-m scarp surface height from the ground level; 36 samples were obtained from both MAN-A and MAN-B, and 15 samples from MAN-C. In addition, the fault scarp geometry parameters (i.e., scarp dip, scarp height, top surface dip, and colluvium dip, Figure 5) were precisely measured in the field (e.g., [22,27,37]. In comparison, Akçar et al. [24] collected 44 samples along a ca. 2.7-m sampling profile of the 4-m high Mugırtepe fault scarp ( Figure 6).  Figure 5) were precisely measured in the field (e.g., [22,27,37]. In comparison, Akçar et al. [24] collected 44 samples along a ca. 2.7-m sampling profile of the 4-m high Mugırtepe fault scarp ( Figure 6). The sampling has been done by [24]. Schematic sketch shows fault scarp with used parameters for modeling. White dashed line represents the sampled surface.

Cosmogenic 36 Cl Analysis
The samples collected from the Manastır fault were processed at the Surface Exposure Laboratory of the Institute of Geological Sciences, University of Bern, following the procedure reported by Stone et al. [70] and Ivy-Ochs et al. [71,72], and the isotope dilution method [72,73]. A full description of the standard protocol of the laboratory for 36 Cl extraction from limestone samples is presented in previous publications (cf. [24,[33][34]). The total Cl and 36 Cl of the Manastır samples were measured at the TANDEM accelerator mass spectrometry (AMS) facility at ETH Zurich. The calcium concentrations of individual samples from Manastır as well as major and trace elements of five proxy samples were also measured using inductively coupled plasma mass spectrometry (ICP-MS) at SGS Mineral Services, Canada. In addition, we determined the calcium concentrations of the Mugırtepe fault scarp from the study conducted by Akçar et al. [24].

Fault Scarp Dating Tool
To analyze the distribution of 36 Cl concentrations accumulated on the Manastır fault scarp (MAN) and reanalyze the dataset for the Mugırtepe fault scarp (MUG), we applied the computation code based on the Monte-Carlo method, which allows the reconstructions of the time-slip histories of normal fault scarps through two separate stages of Figure 6. Mugırtepe fault scarp, view towards south with two sampling strips of MUG-A and MUG-B. The sampling has been done by [24]. Schematic sketch shows fault scarp with used parameters for modeling. White dashed line represents the sampled surface.

Cosmogenic 36 Cl Analysis
The samples collected from the Manastır fault were processed at the Surface Exposure Laboratory of the Institute of Geological Sciences, University of Bern, following the procedure reported by Stone et al. [70] and Ivy-Ochs et al. [71,72], and the isotope dilution method [72,73]. A full description of the standard protocol of the laboratory for 36 Cl extraction from limestone samples is presented in previous publications (cf. [24,33,34]). The total Cl and 36 Cl of the Manastır samples were measured at the TANDEM accelerator mass spectrometry (AMS) facility at ETH Zurich. The calcium concentrations of individual samples from Manastır as well as major and trace elements of five proxy samples were also measured using inductively coupled plasma mass spectrometry (ICP-MS) at SGS Mineral Services, Canada. In addition, we determined the calcium concentrations of the Mugırtepe fault scarp from the study conducted by Akçar et al. [24].

Fault Scarp Dating Tool
To analyze the distribution of 36 Cl concentrations accumulated on the Manastır fault scarp (MAN) and reanalyze the dataset for the Mugırtepe fault scarp (MUG), we applied the computation code based on the Monte-Carlo method, which allows the reconstructions of the time-slip histories of normal fault scarps through two separate stages of database building and data simulation [37]. In the database-building stage, the chemical composition of the bedrock, sample positions, shielding of the scarp and colluvium are considered to calculate the production of the isotope in every sample depending on fixed slip step and rock erosion. Creation of the database improves time efficiency via the approximation of pre-calculated isotope production during each round of simulation. The maximal erosion rate was set to 15 cm kyr −1 to provide some flexibility in the current analysis. In the simulation stage, exposure histories are generated within an earthquake scenario based on the number of earthquakes, earthquake ages, slip values, and erosion rates (cf. [33,34]). Simulated 36 Cl concentration of the samples are statistically compared with measured concentrations taking into account the measurement errors of 36 Cl, parent elements and production rates (Tables 1-4). Following the preliminary simulation of the fault dataset, we began our main simulations by entering an excessive number of earthquakes. After identifying the most accurate number of events in terms of the lowest statistical criteria, we modeled the time-slip histories using minimum 100,000 simulations to achieve the best fit scenario based on one and two earthquake scenarios for the Manastır fault and one to three earthquake scenarios for the Mugırtepe fault. In the FSDT code [37] "Beginning of exposure" indicates the time when the 36 Cl starts to accumulate in the analyzed section of the fault scarp at depth, but it does not refer to any exposure and/or any seismic event. Thus, the analyzed strip is assumed to be still underground (e.g., covered by the colluvium) at the beginning of cosmogenic 36 Cl accumulation. The thickness of the overburden can theoretically be in the order of several meters. To avoid any confusion, in this paper we use the term "beginning of accumulation". It is important to note that the fault scarp dating process only allows for the detection of large earthquakes with considerable displacement values, thereby yielding a lower estimate of earthquake frequency (cf. [20,21,33,34]). Furthermore, episodic earthquakes occurring within the uncertainty of the analysis are not identified as a series of earthquakes but rather as a single event, which cannot be disentangled by any code. This causes lower resolution of the older ages and larger slips [37]. The simulation output is given as a plot of measured 36 Cl concentrations against the sample height along the sampled profile. Following a comparison of the measured and modeled 36 Cl concentrations, the most realistic scenario is selected based on the lowest statistic criteria.  [76] Epithermal neutrons from fast neutrons: 760 ± 150 n/g −1 yr −1 [77] Scaling scheme [78]      We reanalyzed the Mugırtepe fault data reported by Akçar et al. [24] using the FSDT [37] by applying relatively more precise bottom and top positions of the samples rather than their heights along the sampling profile. Moreover, the calcium concentrations of the individual samples were used in the simulation. In this study, as we remodeled the Mugırtepe fault, which was formerly examined by Akçar et al. [24] using the Schlagenhauf et al. [22] code, it is useful to outline the main differences between the two modeling strategies. With respect to cosmic ray shielding by the fault scarp, while the Schlagenhauf code [22] applies scarp shielding only to neutron spallation, the FSDT code considers all cosmogenic particles producing 36 Cl; that is neutron spallation, fast muons, and thermal and epithermal neutrons (cf. [26,37]). In addition, in the Schlagenhauf code, one exponential simplification of muon attenuation is considered, whereas the FSDT approach uses the full model by Heisinger et al. [79,80]. Moreover, considering the exact position of bottom and top of the samples along the fault surface in FSDT is required to obtain more accurate results in terms of distributions of particles at nodes of three-dimensional mesh. This provides coverage of all possible positions of the sample strip to calculate the theoretical 36 Cl, which might have been produced. Dimensions of mesh are considered as the depth of sample perpendicular to scarp surface, the position of the samples along the fault surface and the relative position of footwall and colluvium wedge. These differences affect the model outputs with differences of a few percentage points. The FSDT code applies a broad-ranging search for the optimal solution using the Monte-Carlo method. In addition, despite both codes apply forward modeling, the FSDT method uses a two-step modeling process whereby a database is created during the first step, which has the advantage of reducing the simulation running time.

Cosmogenic 36 Cl Concentration Analysis
The fault scarp parameters used for the database and default rates of 36 Cl production are presented in Table 1. The samples positions, thicknesses, cosmogenic 36 Cl and natural Cl values and uncertainties, calcium, oxygen, and carbon contents are provided in Tables 2 and 3 for the Manastır and Mugırtepe faults, respectively. The average compositions of major and trace elements of the bedrock and colluvium are listed in Table 4.

Time-Slip Histories of the Manastır and Mugırtepe Fault Scarps
Our best-fit model for the Manastır and Mugırtepe faults yields two and one earthquake(s), respectively ( Table 5). The best-fit solution resulting from the simulation of the Manastır dataset indicates seismic events at 3.5 ± 0.9 ka and 2.0 ± 0.5 ka, with the beginning of accumulation at 8.8 ka (Figure 7). The modeled slips for these events are 3.3 ± 0.5 m and 3.6 ± 0.5 m, respectively. The Akaike information criterion (AICc) of this simulation was 444.46, the weighted root-mean-square (RMSw) was 2.12, and the chi-square (χ2) value was 4.91 ( Table 5). The incremental slip rate of 2.2 mm yr −1 is calculated for the time interval between the first and second modeled earthquakes, and 1.8 mm yr −1 between the second earthquake and the present. The average slip rate of 1.9 mm yr −1 is estimated based on a 6.7-m cumulative throw since the oldest modeled earthquake (Figure 8). Note: Slip rates are calculated using the maximum vertical displacement (e.g., throw).
The re-analysis of the Mugırtepe fault data from Akçar et al. [24] showed a single seismic event at 6.5 ± 1.6 ka, with the beginning of exposure at 27 ka and a vertical slip of 2.7 ± 0.4 m (Figure 9). For this single-earthquake scenario, the best-fit (AICc) analysis yielded a value of 178.31, RMSw a value of 1.31, and χ 2 a value of 1.88 (Table 5). A slip rate of 0.3 mm yr −1 was calculated based on maximum vertical displacement divided by the age of the modeled seismic event (Figure 10). Because the upper parts of both the Manastır and Mugırtepe faults were not suitable for sampling, it should be noted that the number of reconstructed seismic events is minimum.

Plausibility of Earthquake Modeling
The modeling of seismic events associated with the Mugırtepe and Manastır faults indicates that both faults slipped during the Holocene. As mentioned above, these faults are elements of the MFZ, therefore they must have moved in response to slip on this fault zone at 6.5 ± 1.6, 3.5 ± 0.9 and 2.0 ± 0.5 ka. The modeled seismic event at 6.5 ± 1.6 ka is close to that of the youngest earthquake modeled by Akçar et al. [24]. Based on the assumption

Plausibility of Earthquake Modeling
The modeling of seismic events associated with the Mugırtepe and Manastır indicates that both faults slipped during the Holocene. As mentioned above, these are elements of the MFZ, therefore they must have moved in response to slip on th

Plausibility of Earthquake Modeling
The modeling of seismic events associated with the Mugırtepe and Manastır faults indicates that both faults slipped during the Holocene. As mentioned above, these faults are elements of the MFZ, therefore they must have moved in response to slip on this fault zone at 6.5 ± 1.6, 3.5 ± 0.9 and 2.0 ± 0.5 ka. The modeled seismic event at 6.5 ± 1.6 ka is close to that of the youngest earthquake modeled by Akçar et al. [24]. Based on the assumption that the Manastır fault is the principal slip surface of the fault zone, we suggest that this event (and probably the older one(s)) was recorded in the upper 5 m of the Manastır fault scarp. We support this argument by comparing measured the cosmogenic 36 Cl concentrations and timing of modeled seismic events of Manastır and Mugırtepe faults. The chemical compositions of Mugırtepe and Manastır samples, especially 40 Ca concentrations as the main target of cosmogenic 36 Cl production, are very similar (Tables 2 and 3) and the longer is the fault surface exposed, the more concentration of cosmogenic 36 Cl is expected. We assume that the accumulation pattern of the measured cosmogenic 36 Cl concentrations on the Manastır fault (Figure 7) is similar to the accumulation pattern on the Mugırtepe fault ( Figure 9). Therefore, we expect that the cosmogenic 36 Cl concentrations on the higher unsampled surface of the Manastır fault (>6.5 m), if it was possible to measure, should increase upscarp surface and be relatively close to those on the Mugırtepe fault scarp. However, no chronology can be attributed to this unsampled section with a high degree of certainty, owing poor surface preservation for sampling.
Here, we discuss the fault parameters that arise from our modeling by applying empirical relationships ( Table 6) that link the modeled earthquake magnitudes to the surface rupture lengths and displacements [50,81,82]. Theoretically, the instantaneous rupture of the entire 35-km-long MFZ would have required an earthquake with a magnitude of approximately 6.9 and an average slip amount of 1-1.7 m regardless of modeling [50,81,82] ( Table 6). By considering Equation (6) in Table 6, the maximum slip of 3.1 m for MFZ was calculated, which fits to the lower bound of modeled slip, though is the most appropriate approach in this case. Therefore, our modeled slips of over 3 m can probably be explained by at least two large-magnitude earthquakes (>6) occurring over a short time span within the uncertainty of modeled ages. However, such concurrent earthquakes cannot be differentiated by the FSDT or any other code or recognized as separate events (cf. [22,37]). We assert the occurrence of clustered earthquake in a close time, because in addition to theoretical calculations above, basically the amount of displacement close to tips of the normal faults is smaller than that around the fault's center (e.g., [83][84][85]). We propose that the earthquakes that occurred in the MFZ triggered the synchronous displacement of all or some of the main segments of the fault zone, including the Manastır master fault, which in turn resulted in the exposure of secondary fault scarps such as the Mugırtepe.  The time span of the modeled seismic events covers a part of the activity of the MFZ during the Holocene, extending the seismic archives significantly beyond the historical records. Indeed, the youngest modeled seismic event at 2.0 ± 0.5 ka temporarily coincides with the most devastating historical earthquake in this region. Many ancient cities within the Manisa Basin and its environs were damaged or completely destroyed in 17 AD by an earthquake with an intensity of IX [3] and a reconstructed Mw of 7.4 (e.g., [57]). Shortly after, in 44 AD, another event with an intensity of VIII damaged the ancient cities of Magnesia and Ephesus [66]. Considering the rough magnitude value of an earthquake possibly sourced by the 35-km-long MFZ (Mw 6.9) and slip (1-3.1 m), we argue that the 17 AD earthquake is a reliable candidate for the event at 2.0 ± 0.5 ka. The earthquake of 44 AD is mainly attributed to rupture the southern faults close to Izmir and Kemalpaşa ( Figure 2); Thus, the 3.6 ± 0.5 m rupture of the Manastır fault scarp is triggered by earthquake of the 17 AD event and probably a smaller unrecorded earthquake.
Our modeling did not yield any seismic event younger than 2.0 ± 0.5 ka. At least three additional historical destructive earthquakes, which caused damages in the region, have been reported: the 926 AD, 1595/1664 AD, and 1845 AD events (Figure 2). The epicenter location of the earthquake of 926 AD similar to that of 44 AD appears to be associated with the faults in the south. Moreover, the reconstructed epicenters of the 1595/1664 AD events are located on the eastern part of the southern main boundary of the Gediz Graben and the Izmir fault, respectively ( Figure 2) [3,48,58]. In addition, the 1845 AD earthquake, with an epicenter to the east of Manisa is not a definitive earthquake and considered as extreme exaggeration of June 5, 1845 AD Izmir earthquake. The evidence of this event is missing in the Church Missonary Society archives for 1845-46 AD damages of the Izmir-Manisa region [1]. Ambraseys [1] states that this earthquake is only reported by Perrey [86], which claims several weeks before July 23, Manisa was completely destroyed by an earthquake, and this was accordingly reported in modern earthquake catalogues. The abovementioned earthquakes must have initiated significant rupturing of nearby faults but, most likely, negligible or zero rupturing of more distal faults, such as MFZ. However, these events left evidence of liquefaction and lateral spreading in the colluvium in front of the Mugırtepe fault; but their impact in terms of surface rupture or dip-slip displacement of the fault is unclear owing to a lack of field data [59]. These findings could be directly related to availability of organic material, however further discussion regarding the possibilities is beyond the scope of this study.

Evolution of the Western Manisa Fault Zone
There is a dearth of information about the timing of the initiation of surface rupture of the Manastır fault, which is the main and longest fault in the western MFZ. According to radiocarbon dating of charcoal samples in palaeosol and bulk sediment samples from the Emlakdere Formation, the progressive accumulation and tilting of the Emlakdere Formation in front of the Manastır fault in the hanging-wall began ca. 19 cal kyr BP [59], which can be considered as the lower bound for the initial surface rupture of the Manastır fault ( Figure 11). Deposition and progressive tilting of hanging-wall deposits are indicated by diverse dip of bedding planes of the Emlakdere Formation. This syn-sedimentary faulting continued until ca. 9 cal kyr BP based on the 14 C age of a palaeosol sample collected from the uppermost part of the Emlakdere Formation within the hanging-wall of F1, where the dip-slip offset of Emlakdere block is a minimum of 12 m (Figure 11) [59]. The time span of sedimentation (from ca. 19 to 9 ka) in this area correlates with the timing of Last Glacial Maximum (LGM) and Termination-I in the northern hemisphere [87][88][89], when the rate of sedimentation is assumed to be rapid. This implies that the rupture of the secondary faults in the Manisa basin (F1 to F4 including the Mugirtepe Fault) should have initiated after ca. 9 ka. Based on these lines of evidence, we argue that activity of F1, similarly to all the other secondary faults, are younger than surface rupture of the Manastır fault.
Late Holocene alluvial fan is displaced by F4 (Figures 3 and 4). Though, we interpret that F4 was broken by a younger activity than that of responsible for Mugırtepe fault rupture, presumably synchronized with Manastır fault activity either at ca. 3.5 or 2 ka. The deformation of the alluvial fan by F4, which overlies the Mugırtepe fault, assures that F4 ruptured later than the Mugırtepe fault. This sequence of events might be a hint for the basinward migration of the faulting. However, the evolution of the faulting in the Manisa Fault Zone remains obscure and needs to be explored by additional dating studies. Nevertheless, we propose that the Manastır and Mugırtepe faults could underwent number of earthquakes between ca. 9 cal kyr BP and 6.5 ± 1.6 ka. These should have resulted in associated slips, which are obscured today in the poorly preserved upper 5 m and 1.3 m of these faults. The seismic event at 6.5 ± 1.6 ka displaced the Mugırtepe secondary fault by 2.7 ± 0.4 m, as revealed by our modeling (Figure 11). This event might have occurred as two clustered earthquakes in the Manastır master fault that caused the simultaneous displacement of the Mugırtepe fault, if this is true, these ruptures should presumably be recorded in the current upper 5 m of the Manastır fault. At 3.5 ± 0.9 ka, the Manastır fault moved by 3.3 ± 0.5 m as a result of several subsequent earthquakes, which appear not to cause any movement of the Mugırtepe fault. The Manastır fault experienced another seismic event at 2.0 ± 0.5 ka with a significant displacement of 3.6 ± 0.5 m ( Figure  11), which we attribute to the destructive earthquakes of 17 AD and a probable smaller event missing in historical records ( Figure 11).

Timing of Seismically Active Periods in Western Anatolia
Using fault scarp dating, we reconstructed the oldest discovered seismic event in MFZ at 6.5 ± 0.5 ka followed by the subsequent event at 3.5 ± 0.9 ka. The subsequent modeled earthquake at 2.0 ± 0.5 ka temporally coincides with the 17 destructive earthquakes recorded in the historic records. Back-tilting of bedding planes in the hangingwall of the faults, in general decreases towards the basin. Accordingly, we plead that the closest faults to the Manastır fault experienced most likely more earthquakes and higher subsequent slip than those close to the basin. Among those, hangingwall of F2 accommodates the highest backtilting, while the MUG and F4 are characterized by sub-horizontal bedding planes in their hangingwalls. This reveals that most likely not all secondary faults are affected by seismic event simultaneously. Although F4 was not dated in this study, there is field evidence that a Late Holocene alluvial fan is displaced by F4 (Figures 3 and 4). Though, we interpret that F4 was broken by a younger activity than that of responsible for Mugırtepe fault rupture, presumably synchronized with Manastır fault activity either at ca. 3.5 or 2 ka. The deformation of the alluvial fan by F4, which overlies the Mugırtepe fault, assures that F4 ruptured later than the Mugırtepe fault. This sequence of events might be a hint for the basinward migration of the faulting. However, the evolution of the faulting in the Manisa Fault Zone remains obscure and needs to be explored by additional dating studies.
Nevertheless, we propose that the Manastır and Mugırtepe faults could underwent number of earthquakes between ca. 9 cal kyr BP and 6.5 ± 1.6 ka. These should have resulted in associated slips, which are obscured today in the poorly preserved upper 5 m and 1.3 m of these faults. The seismic event at 6.5 ± 1.6 ka displaced the Mugırtepe secondary fault by 2.7 ± 0.4 m, as revealed by our modeling (Figure 11). This event might have occurred as two clustered earthquakes in the Manastır master fault that caused the simultaneous displacement of the Mugırtepe fault, if this is true, these ruptures should presumably be recorded in the current upper 5 m of the Manastır fault. At 3.5 ± 0.9 ka, the Manastır fault moved by 3.3 ± 0.5 m as a result of several subsequent earthquakes, which appear not to cause any movement of the Mugırtepe fault. The Manastır fault experienced another seismic event at 2.0 ± 0.5 ka with a significant displacement of 3.6 ± 0.5 m (Figure 11), which we attribute to the destructive earthquakes of 17 AD and a probable smaller event missing in historical records (Figure 11).

Timing of Seismically Active Periods in Western Anatolia
Using fault scarp dating, we reconstructed the oldest discovered seismic event in MFZ at 6.5 ± 0.5 ka followed by the subsequent event at 3.5 ± 0.9 ka. The subsequent modeled earthquake at 2.0 ± 0.5 ka temporally coincides with the 17 destructive earthquakes recorded in the historic records.
We showed that MFZ was active during Holocene similar to other faults in the region (cf. [33,34]). The 2.0 ± 0.5 ka earthquake is highly concordant with the timing of the youngest earthquakes discovered using fault scarp dating on the Yavansu, Priene-Sazlı, and Ören faults (Figure 1). In addition to the Rahmiye fault, all of these faults are considered to have been activated in a close time by the modeled Manastır fault earthquake at 3.5 ± 0.9 ka. Similarly, the timing of the reconstructed earthquake at 6.5 ± 1.6 ka for the Mugırtepe fault is compatible with the age of the reconstructed earthquakes of the Priene-Sazlı and Ören faults. Overall, our fault scarp dating shows that regional seismic activity in Western Anatolia has a rhythmic pattern and is broadly characterized by clusters of surface rupturing earthquakes with phases of high seismic activities with a recurrence interval of ca. 2000 yr.

Conclusions
Fault scarp dating in the western MFZ has been observed to be a means of exploring major earthquake events. We documented the occurrence of two and one seismic events, respectively, for the Manastır and Mugırtepe faults as a component of the MFZ during the Holocene. Each of these events is considered to result from clustered earthquakes with the modeled displacements representing the cumulative slip due to these events. The youngest of these events coincides with earthquakes documented in the historic record at 17. The reconstructed earthquakes associated with the Mugırtepe fault are interpreted to have occurred as a consequence of activity on the Manastır fault. While both the Manastır and Mugırtepe faults are tectonic, the former is considered to be seismogenic and the latter non-seismogenic. Our results together with the geological and paleoseismological investigations [59] demonstrate that in the western MFZ, the hangingwall of the master Manastır fault experienced syn-depositional rotation during the Late Pleistocene-early Holocene. Thereafter, secondary faults developed during the Early-late Holocene as a consequence of repeated earthquakes. Our results can unfortunately not solve the growth of the secondary faults. Whether they display a migration pattern or irregular rupture pattern remains to be explored. Our findings are consistent with previous fault scarp dating results from western Turkey [33][34][35]. This demonstrates the significant potential of this method for deriving the critical parameters required for precise evaluations of seismic risk.