Bull Earthquake EngDOI 10.1007/s10518-013-9443-6

ORIGINAL RESEARCH PAPER

Probabilistic seismic hazard assessment of EasternMarmara Region

Zeynep Gülerce · Soner Ocak

Received: 19 October 2012 / Accepted: 10 March 2013© Springer Science+Business Media Dordrecht 2013

Abstract The objective of this study is to evaluate the seismic hazard in Eastern MarmaraRegion using an improved probabilistic seismic hazard assessment methodology. Two signif-icant improvements over the previous seismic hazard assessment practices are accomplishedin this study: advanced seismic source characterization models in terms of source geome-try and recurrence relationships are developed, and improved global ground motion models(NGA-W1 models) are employed to represent the ground motion variability. Planar faultsegments are defined and a composite magnitude distribution model is used for all seismicsources in the region to properly represent the characteristic behavior of the North AnatolianFault without the need for an additional background zone. Multi-segment ruptures are con-sidered using the rupture model proposed by the Working Group on California EarthquakeProbabilities (2003). Events in the earthquake catalogue are attributed to the fault zones andscenario weights are determined by releasing the accumulated seismic energy. The uniformhazard spectra at 10 % probability of exceedance in 50 years hazard level for different soilconditions (soil and rock) are revealed for specific locations in the region (Adapazarı, Düzce,Gemlik, Izmit, Iznik and Sapanca). Hazard maps of the region for rock site conditions atthe selected hazard levels are provided to allow the readers perform site-specific hazardassessment and develop site-specific design spectrum for local site conditions.

Keywords North Aanatolian Fault · Probabilistic seismic hazard assessment ·Ground motion prediction equations

1 Introduction

Eastern Marmara Region is located around one of the most active fault systems in theworld, the North Anatolian Fault (NAF), which extends along Northern Turkey for morethan 1,500 km. Strain energy accumulating along the NAF due to the westward-moving

Z. Gülerce (B) · S. OcakCivil Engineering Department K1-308, Middle East Technical University, 06531 Ankara, Turkeye-mail: zyilmaz@metu.edu.tr

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Anatolian Block is the main source of seismic activity in this region. The NAF systemruptured progressively by eight large and destructive earthquakes in the last century; eventsbetween 1939 and 1967 had broken approximately 900 km of a uniform eastern trace whereasthe Kocaeli and Düzce Earthquakes in 1999 ruptured a total fault spray of approximately200 km on the west where the NAF system is divided into a number of branches. EasternMarmara, the industrial center of Turkey, is a large region bounded by Marmara Sea on thewest and Abant Lake on the east including the fault segments of the NAF that ruptured during1957 Abant, 1967 Mudurnu, 1999 Kocaeli, and 1999 Düzce earthquakes as shown in Fig. 1.High seismic activity and large population in the region requires comprehensive evaluationof the seismic hazard and risk considering the important local source characteristics such ashighly different rupture episodes of each fault segment, recency of ruptures, and strain rates,which makes the application of probabilistic seismic hazard assessment (PSHA) frameworkquite challenging. The objective of this study is to re-evaluate the seismic hazard in EasternMarmara Region using novel PSHA tools for seismic source characterization and groundmotion prediction. An improved seismic hazard assessment is intended to be accomplishedby utilization of advanced seismic source models in terms of source geometry and recurrencerelationships and application of new global ground motion models (Next Generation Attenu-ation Models, NGA-W1, Abrahamson and Silva 2008; Boore and Atkinson 2008; Campbelland Bozorgnia 2008; Chiou and Youngs 2008; Idriss 2008) to represent the ground motionvariability.

The PSHA studies for the region were limited (Erdik et al. 1985; Gülkan et al. 1993)before the 1999 events. Several researchers published estimates of seismic hazard and riskfor the Marmara Region and for Istanbul after these events (Atakan et al. 2002; Erdik etal. 2004; Crowley and Bommer 2006; Kalkan et al. 2009). Seismic source characteriza-tion was typically based on earthquake catalogue data using areal sources in early seismichazard assessment studies (Erdik et al. 1985; Gülkan et al. 1993; Atakan et al. 2002) andthe magnitude distributions of these areal sources were modeled with truncated exponentialfrequency-magnitude relationship. In recent studies (Erdik et al. 2004; Crowley and Bommer2006; Kalkan et al. 2009), seismic sources were modeled by defining linear fault segmentswith the assumption that the seismic energy along these fault segments was released by char-acteristic events. The magnitude distribution functions of these linear sources were consideredto be fully characteristic (truncated normal distribution). In addition, a background sourcerepresenting the small-to-moderate magnitude earthquakes (magnitudes between 5 and 6.5–7depending on the study) were added to the source model and the earthquake recurrence ofthe background source was modeled using a truncated exponential magnitude distributionmodel. Either the Poisson (Erdik et al. 2004; Crowley and Bommer 2006; Kalkan et al. 2009)or time dependent renewal (Brownian Passage Time Model, Ellsworth et al. 1999) model(Erdik et al. 2004) was preferred to model the earthquake recurrences for linear segments,whereas the Poisson distribution was used to model the recurrence rates of the backgroundsource in the previous studies.

Due to the lack of local predictive models, global ground motion prediction equations(GMPEs) such as Boore et al. (1997), Campbell (1997), and Sadigh et al. (1997) were usedin earlier studies (Erdik et al. 1985, 2004; Gülkan et al. 1993; Atakan et al. 2002) to representthe ground motion variability in the region. Erdik et al. (1985) showed that the peak groundaccelerations (PGAs) recorded in Turkey are safely located within the observed dispersionof the Western US data and the empirical response spectra of the ground motions at severallocations in Turkey can be predicted, within engineering tolerances, by the Western USbased attenuation relationships. A more recent study by Kalkan et al. (2009) used NGA-W1ground motion prediction models and the GMPE developed for Turkey after the 1999 events

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by Kalkan and Gülkan (2004). Erdik et al. (2004) and Kalkan et al. (2009) both generated thehazard maps of the region for PGA, T = 0.2 s and T = 1.0 s spectral accelerations for genericrock site conditions (Vs30 = 760 m/s) at different hazard levels and the hazard values werefound to be comparable to other highly seismic regions such as San Francisco or Tokyo. Thelargest PGA in the region was found to be around 1.5 g for 10 % chance of exceedance in50 years by Erdik et al. (2004). Kalkan et al. (2009) computed the maximum PGAs as 1.5and 0.8 g for return periods of 2,475 and 475 years, respectively.

Probabilistic seismic hazard assessment (PSHA) methodology and the main componentsof the PSHA framework are rapidly evolving with the increasing number of special projects(such as nuclear power plants, bridges and high-rise structures) and awareness of earthquakerisk reduction around the world. The applicability of the advances in these fields, especially thestate-of-the-art methods practiced in California (US), for PSHA studies conducted in Turkeyis a controversial topic mainly due to the lack of local information on parameters used inthese models [e.g. depth to the top of the rupture plane, fault width, and depth to the bedrock(Z1.0 and Z2.5)]. In this study, geometry of the fault segments (especially the length of seg-ments and segmentation points) are determined and incorporated with the help of availablefault maps and traced source lines on the satellite images from recent information available(Cambazoglu et al. 2012). Planar fault segments are defined and a composite magnitudedistribution model (Youngs and Coppersmith 1985) is used for all seismic sources in theregion to properly represent the characteristic behavior of NAF without an additional back-ground zone. Fault segments, rupture sources, rupture scenarios and fault rupture modelsare determined using the WGCEP-2003 terminology and multi-segment rupture scenariosare considered. Events in the earthquake catalogue are attributed to the individual seismicsources and scenario weights are determined by releasing the accumulated seismic energyby the catalogue seismicity on each source.

Next Generation Attenuation Models (NGA-W1) models are improved in terms of addi-tional prediction parameters (such as depth of the source, basin effects, magnitude dependentstandard deviations, etc.), statistical approach, and a well-constrained global database whencompared to the early stage GMPEs proposed by the developers. The use of NGA-W1 modelsin this study is expected to reduce the uncertainty in the total hazard incorporated by regionalor older models based on smaller datasets. Details of the source characterization method-ology and discussions on GMPE selection and weights of selected GMPEs are provided inthe following sections. Results of the study are presented as uniform hazard spectra at 10 %probability of exceedance in 50 years hazard level for soil and rock site conditions at main citycenters of the region. In addition, hazard maps of the area for rock site conditions at selectedhazard levels are generated to provide input for future site-specific hazard assessment studiesand development of site-specific design spectra for any site conditions.

2 Seismic source characterization models

2.1 Assessment of fault segments

Our study adopts the same approach as the WGCEP-2003 SF Bay Area Model for seismicsource characterization, which is primarily based on characterized faults that are dividedinto non-overlapping segments. These segments are considered as the basic building blocksfor earthquake ruptures on each fault. Following this approach, fault segments for all pri-mary faults in the study area are defined to characterize the seismic sources for the study,which are North Anatolian Fault Northern Strand (NAF_N), Düzce Fault, North Anatolian

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Fig. 1 Eastern Marmara Region and the segments of NAF System (sketch modified from the Active FaultMap of Turkey, MTA, Saroglu et al. 1992). Numbers in circles represent: (1) rupture zone of 1999 KocaeliEarthquake, (2) rupture zone of 1999 Düzce Earthquake, (3) rupture zones of 1957 Abant and 1967 MudurnuEarthquakes, (4) Geyve–Iznik Fault. Numbers in squares show the analyzed sites. Bold red lines define theborders of the study area where the PSHA analyses were conducted

Fig. 2 Segmentation diagram of 1999 Kocaeli and Düzce Earthquakes (modified from Figure 4a of Barka etal. 2002).

Fault Southern Strand (NAF_S), and Geyve–Iznik Fault denoted by 1, 2, 3, and 4 in Fig. 1,respectively.

Location, geometry and slip distribution of NAF_N and Düzce Faults has been studiedextensively after the 1999 earthquakes (Barka et al. 2002; Langridge et al. 2002; Akyüz etal. 2002). The surface rupture of the 1999 Kocaeli Earthquake extended for almost 165 kmand 4 distinct segments were ruptured (Hersek Segment, Gölcük-Karamürsel-Izmit Segment,Sapanca-Akyazı Segment, and Karadere Segment in Fig. 2) (Barka et al. 2002). A segmentof NAF_N located on the boundary between the Marmara Sea and Çınarcık Block (ÇınarcıkSegment, shown by broken lines in Fig. 2) did not rupture during 1999 Kocaeli Earthquake.Cambazoglu (2012) performed lineament analysis using satellite images of the region toaccurately determine the segmentation points consistent with the existing information. Wedefined the NAF_N fault zone based on the surface rupture lengths delineated by Cambazogluet al. (2012) with minor changes; 6-segment model developed for NAF_N includes Çınarcık,Hersek, Gölcük-Karamürsel-Izmit, Sapanca-Akyazı, and Karadere segments in addition toHendek segment parallel to Düzce Fault (Fig. 3). Düzce Earthquake formed a 40-km-longsurface rupture zone; however, there is a 4-km releasing step-over around Eften Lake (Akyüzet al. 2002), therefore a 2-segment model is developed for Düzce Fault as shown in Fig. 3.

Two segments are defined for NAF_S considering the rupture zones of the two previousearthquakes (1957 Abant and 1967 Mudurnu Earthquakes). 80 km long Mudurnu segmentstarts from Sapanca Lake and extends up to Mudurnu. Abant segment, starts from Abant Lakeand extents 40 km to Arpaseki (Barka 1996) as shown in Fig. 2. Only a few small magnitudeevents occurred on the Geyve–Iznik Fault between years 1905 and 2005 and the lack of

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Fig. 3 Seismic source model used for this study (modified from Figure 3.1 of Cambazoglu et al. 2012).Segment names given in squares are provided in Table 1

Table 1 Segment geometry, assigned slip rate and characteristic magnitude for each segment

Source name Segment Segment name Length (km) Width (km) Slip rate(mm/year)

Characteristicearthquake(Mchar)

NAF_N W1 Çınarcık 46.4 18 19 7.0

NAF_N W2 Hersek 12.4 18 19 6.4

NAF_N C Gölcük-Karamürsel-Izmit

47.0 18 19 7.0

NAF_N E1 Sapanca-Akyazı 21.6 18 10 6.6

NAF_N E2 Karadere 26.6 18 10 6.7

NAF_N H Hendek 45.2 18 10 7.0

Düzce D1 Düzce_1 10.7 35.8 10 6.6

Düzce D2 Düzce_2 29.4 35.8 10 7.1

NAF_S M Mudurnu 64 12 12 6.9

NAF_S A Abant 40 12 15 6.7

Geyve–Iznik I Iznik 111.6 12 6 7.2

Geyve–Iznik G Geyve 34.5 12 3 6.7

moderate-to-large magnitude earthquakes on this fault zone makes it difficult to determinethe exact location of segmentation points. Again, the fault segments and segment lengthsproposed by Cambazoglu et al. (2012) with minor changes are used for Geyve–Iznik Fault.Fault segments and corresponding lengths used in the study are provided in Table 1.

Widths of fault segments are back-calculated using empirical Wells and Coppersmith(1994) magnitude-rupture area relationships. Back-calculated width values are found to becompatible with the depths of previous large magnitude events, except for the Düzce Faultwhich has a significantly small surface rupture length for a Mw = 7.2 earthquake. Character-istic magnitudes for each segment are calculated by the relationship proposed by Wells andCoppersmith (1994) for strike-slip faults (Eq. 1) and are listed in Table 1.

MChar = 3.98 + 1.02 × log(R A) (1)

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Fig. 4 Composite magnitudedistribution model proposed byYoungs and Coppersmith (1985)

where RA is the rupture area. Planar sources are defined considering the variation of therupture plane location along the fault width. Final seismic source model used in this study isshown in Fig. 3.

2.2 Magnitude recurrence models and partitioning of slip-rates

We used composite magnitude distribution models combining the truncated exponentialmodel for small-to-moderate size earthquakes and characteristic model for the characteristicevents for all sources to represent relative rates of different magnitude events. The well-known composite model proposed by Youngs and Coppersmith (1985) is used for NAF_N,NAF_S, and Düzce Faults (Fig. 4) as given by:

f m (Mw)

=

⎧⎪⎨

⎪⎩

β exp(− β(Mw−Mmin))1−exp(− β(Mmax−�M2−Mmin))

× 11+c , Mw ≤ Mmax − �0.5M2

β exp(− β(Mmax−�M1−�M2−Mmin))1−exp(− β(Mmax−�M2−Mmin))

× 11+c , Mw > Mmax − �0.5M2

(2)

where, M1 = 1.0, M2 = 0.5 and c is defined by:

c = β exp (− β (Mmax − �M1 − �M2 − Mmin))

1 − exp (− β (Mmax − �M2 − Mmin))× �M2 (3)

The key feature of this model is the relative sizes of released seismic moments for small-to-moderate and large magnitude events. Due to the constraints of the model, 94 % of theaccumulated seismic moment is released by characteristic events and the rest of the seismicmoment is released by small-to-moderate magnitude earthquakes on the exponential tail.For Geyve–Iznik Fault, Youngs and Coppersmith (1985) composite model is modified torepresent the weaker seismicity of the source. Since the events attributed to Geyve–Iznik Faultare smaller in number and magnitude, the seismic moment released by the exponential tail ofthe composite model is increased to 10 % by modifying the model constraints. The magnitudedistribution function for each source is bounded with a minimum magnitude considering theengineering application. Except for the Geyve–Iznik Fault, the minimum magnitude is setto Mw = 5 for all sources. For Geyve–Iznik Fault the minimum magnitude is loweredto 4 given the catalogue seismicity of the source, which mostly contains small magnitudeevents as mentioned above. The upper bounds for the magnitude distribution functions (inother words the maximum magnitudes) are determined by adding 0.25 magnitude units to

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the characteristic magnitude for each source (Youngs and Coppersmith 1985). The b-valueis calculated as 0.74 for the study area.

The annual seismic moment rate (M0) accumulating on each source (single or multi-segment sources) is calculated by Eq. 4:

M0 = μ × A × s (4)

where μ is the rigidity of the crust (taken as 3.3 × 1010Pa, from Barka 1996), A is thearea of the fault zone, and s is the annual slip rate of the source. Past studies based on GPSmeasurements and field research performed for the region (McClusky et al. 2000; Reilinger etal. 2000) showed that the total annual slip rate of North Anatolian Fault Zone is approximately25 mm/year. However, the seismic moment accumulating on the NAF system is shared byparallel fault segments; therefore slip rates should be assigned to individual strands. For thesegments that formed the west and center parts of NAF_N (W1, W2 and C segments inFig. 3), the total slip rate of 25 mm/year is shared with Geyve–Iznik Fault. The slip rate of19 mm/year is assigned to these segments of NAF_N and 6 mm/year is assigned to Izniksegment based on the values proposed by Stein et al. (1997) with slight modifications dueto catalogue seismicity. Similarly, the total slip rate of 25 mm/year is distributed over theeastern segment of NAF_S (Abant, A) and Düzce Fault (D1 and D2). Since contribution ofDüzce Fault to the total slip is around 33–50 % (Ayhan et al. 2001), a slip rate of 15 mm/yearis assigned to NAF_S Abant Segment and 10 mm/year is assigned to Düzce Fault. Sameslip rate (10 mm/year) is assigned to the segments of NAF_N that are connected to (E1 andE2) and parallel to (H) the Düzce Fault for consistency (10 mm/year is associated to eitherE1-E2 rupture or Hendek rupture alternatively). The slip rate of 3 mm/year is assigned to theGeyve segment (G) of the Geyve–Iznik Fault since this segment meets with the Mudurnusegment (M, 12 mm/year) and then joins to the Abant segment (A, 15 mm/year) to formthe Southern Strand (see Fig. 3). Compatibility of the assigned slip rates with the ratesof events attributed to each source is cross-checked while forming the recurrence modelsusing cumulative rates of events vs. magnitude plots (an example is provided in Figure 6 forDüzce Fault).

To determine the magnitude recurrence relationship for a seismic source, activity rate ofthe source, which is defined as the rate of earthquakes above the minimum magnitude, shouldbe estimated in addition to the magnitude distribution model. The activity rate of a source iscalculated by integrating the moment release per earthquake times the relative frequency ofearthquakes as given in Eq. 5:

N (Mmin) = μ × A × s∫ Mmax

MminfM (M) × 101.5M+16.05

(5)

where N(Mmin) is the activity rate, fM(M) is the magnitude distribution model and101.5M+16.05 represents the seismic moment released during an earthquake (Hanks andKanamori 1979). The Integrated Homogeneous Turkey Earthquake Catalogue (KandilliObservatory and Earthquake Research Institute 2007) including the events with Mw > 4that occurred between 1900 and 2005 was utilized and the source-epicenter matching foreach individual fault zone was performed using the GIS tools and style-of-faulting data ofthe events (Cambazoglu et al. 2012). The activity rate N(Mmin) is combined with the magni-tude distribution function to develop the recurrence model N(M) for the source:

N (M) = N (Mmin)

Mmax∫

Mmin

fM (M) (6)

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Fig. 5 Representation of segments, rupture sources and rupture scenarios

2.3 Rupture sources and rupture scenarios

In WGCEP-2003 methodology, rupture source is defined as a fault segment or a combinationof multiple adjacent fault segments that may rupture and produce an earthquake in the future.For an arbitrary fault with three segments (as A, B and C in Fig. 5) six different rupture sourcescan be defined; single segment sources (1, 2 and 3), two adjacent segment sources (4 and 5)and a three-segment source (6) (Fig. 5). Any possible combination of sources that describes apossible failure mode is defined as the rupture scenario. Four rupture scenarios for this faultwould be; (1) rupture of the three segments individually (A, B, C), (2) rupture of the first twosegments together and the third segment individually (A + B,C), (3) rupture of the last twosegments together and the first segment individually (A,B + C), and (4) rupture of the threesegments together (A+B+C) (Fig. 5). The rupture model includes the weighted combinationof rupture scenarios of the fault. Rupture models for Düzce Fault, NAF_S, and Geyve–IznikFault involve only two segments, three rupture sources, and two scenarios. The six segmentsdefined for NAF_N forms a rupture model with 19 rupture sources and 24 rupture scenarios(the table of rupture sources and scenarios is included in Ocak 2011). While generating thescenarios, the eastern segments (E1 and E2) and Hendek (H) segment are assumed to behavedependently; since they are parallel segments of the same source, simultaneous rupture ofthese segments is not taken into account. For a complete fault model, a weight is assignedto each rupture scenario and the cumulative rates of events attributed to that particular faultare plotted with the weighted average of rupture scenarios to calibrate the assigned weights.Rupture model for Düzce Fault is provided as an example for this procedure in Fig. 6,illustrating that the best fit between the cumulative rate of events and weighted average ofrupture scenarios is established by modifying the weights of the rupture scenarios. The blackdots in Fig. 6 stand for the cumulative annual rates of earthquakes from the catalogue assignedto Düzce Fault and the error bars represent the uncertainty introduced by unequal periodsof observation for different magnitudes (Weichert 1980). Figure 6 also indicates that therecurrence rates of maximum magnitude events of the recurrence model (0.0024 eq/year forthe magnitude 7.2 earthquake for Düzce Fault) are in agreement with the rate of multiplecharacteristic events associated to this fault. The average recurrence rate of large magnitudeevents for Düzce Fault is approximately 320–390 years which corresponds to a rate of 0.0025–0.003 eq/year (Pucci et al. 2009). The weights assigned to rupture scenarios are used in the

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Fig. 6 Cumulative rates ofcatalogue events attributed toDüzce Fault and rupture modelproposed for this fault. Theweighted average line (brokenblack line) is adjusted bymodifying the weights of rupturescenarios (gray and black lines)to achieve the best fit

Fig. 7 Logic tree used for Düzce Fault in the PSHA framework

logic tree (an example is provided for Düzce Fault in Fig. 7). No uncertainties related to thefault parameters (length, depth, dip, width and slip rates) other than the maximum magnitudesare included in the logic tree as indicated by Fig. 7.

3 Ground motion prediction equations

In PSHA, the GMPEs are used to estimate the ground motion parameters for the earth-quake scenarios from each source. These equations use statistical models based on physicalcharacteristics of ground motions to predict the ground motion intensities given the source(magnitude, depth, style-of faulting, etc.), path (distance, etc.) and site (site conditions, basineffects, etc.) parameters. There are major uncertainties associated with both the seismicsource characterization model and the GMPE in the PSHA framework, but the uncertaintiesassociated with the latter will generally have the larger impact on the results (Bommer etal. 2005). Many GMPEs are available in the literature; global ground motion models rep-resenting the shallow crustal regions or local ground motion models developed for Turkeyduring the last decade (Özbey et al. 2004; Kalkan and Gülkan 2004; Akkar and Cagnan2010). Choosing the ground motion model from one of these groups is a controversial topicsince each has its own advantages and disadvantages. Local GMPEs are developed from theregional datasets therefore they may reflect the regional tectonic differences better than theglobal models. However the uncertainties introduced by these models are higher than those

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of the global models since they are based on smaller databases. Since the total hazard issignificantly affected by the ground motion variability, we preferred to use the global groundmotion models for this study. The NGA-W1 models published in 2008 are selected amongmany available global models and equal weights are assigned to each model in the logictree to capture the epistemic uncertainty since the range of applicability of these modelsare indifferent (Fig. 7). Please note that the Idriss (2008) model is applicable to stiff sites(Vs30 > 450 m/s) only, therefore this GMPE is excluded from the analysis at average soilsite conditions (Vs30 = 270 m/s).

4 Probabilistic seismic hazard assessment results

The probabilistic seismic hazard assessment methodology defined by Cornell (1968) andMcGuire (2004) is used for this study. The hazard integral for a single point source is givenby:

ν(A > z)= Nmin ·∫

M

∫

R

∫

ε

fM (M) fR(M, R) fε(ε)P(A > z |M, R, ε)×d M×d R×d ε (7)

where R is the distance from the source to site, M is the earthquake magnitude; Nmin is theannual rate of earthquakes with magnitude greater than or equal to the minimum magnitude,fM(M) and fR(M, R) are the probability density functions for the magnitude and distance, ε isthe number of standard deviations above or below the median, fε(ε) is the probability densityfunction for the epsilon (given by a standard normal distribution), and P(A > z|M,R, ε)is either 0 or 1. In this formulation, P(A > z|M,R, ε) selects those scenarios and groundmotion combinations that lead to ground motions greater than the test level z. The numericalintegration of the hazard integral is performed by the computer code HAZ39 developedby N. Abrahamson (Pacific Gas & Electric Company 2010). HAZ39 treats the epistemicuncertainties in the source characterization and the GMPEs through use of logic trees. Foreach source, all combinations of the logic tree branches are evaluated. For the total hazard,Monte Carlo sampling of source characterization uncertainty is used to combine the epistemicuncertainty for each source and full sampling of the GMPEs is used to develop fractals onthe total hazard.

The results of the study are presented initially for six selected city centers in EasternMarmara; Adapazarı, Düzce, Gemlik, Izmit, Iznik and Sapanca (locations of these city centersand main seismic sources are shown in Fig. 1). The hazard curves at these sites assumingrock site conditions (Vs30 = 760 m/s) for PGA, T = 0.2 s and T = 1 s spectral accelerationsare shown in Fig. 8a, c, e, respectively. Similarly, the hazard curves at the same locationsassuming average soil site conditions (Vs30 = 270 m/s) for PGA, T = 0.2 s and T = 1 sspectral accelerations are given in Fig. 8b, d, f. Highest level of seismic hazard is observedin Sapanca for all spectral periods as expected, since Sapanca is closer than the other sites toall seismic sources in the region. Izmit also has higher seismic hazard compared to the othersites as a result of high accumulation of seismic energy at NAF_N. Hazard at Düzce is lowerthan that at the other sites due to the fact that the hazard is underestimated for the sites in theeastern part of the region by ignoring the contribution of NAF Bolu-Gerede segment (1944rupture). This segment is out of the study area so it is not included in the PSHA calculations.The PGAs for 2, 10, and 50 % probability of exceedance levels in 50 years at selected sitesare presented in Table 2. The study area is located in the first seismic zone according toTurkish Earthquake Code (TEC-2007) and the 475 years return period design peak groundacceleration is 0.4 g for regular buildings and the design peak ground acceleration increases

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Fig. 8 Hazard curves at Adapazarı, Düzce, Gemlik, Izmit, Iznik, and Sapanca for rock site conditions (Vs30 =760 m/s) for a PGA, c T = 0.2 second spectral period, e T = 1 second spectral period and for average soil siteconditions (Vs30 = 270 m/s) for, b PGA, d T = 0.2 second spectral period, f T = 1 second spectral period

to 0.6 g for emergency facilities (hospitals, fire and police stations, etc.). The analysis resultsare higher than the TEC-2007 requirements at each site, due to the close proximity of selectedsites to the NAF system. Lower PGA values should be expected for sites at distances biggerthan 30 km.

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Table 2 PGA for different hazard levels at six selected cities (Vs30 = 270 m/s)

Hazard Level Adapazarı (g) Düzce (g) Gemlik (g) Izmit (g) Iznik (g) Sapanca (g)

2 % in 50 years 0.92 0.78 0.82 1.08 1.04 1.22

10 % in 50 years 0.60 0.48 0.51 0.70 0.66 0.80

50 % in 50 years 0.26 0.22 0.24 0.30 0.28 0.36

The hazard curve gives the combined effect of all magnitudes and distances on the prob-ability of exceeding the specified ground motion level. Since all of the sources, magnitudes,and distances are mixed together, it is difficult to understand what is controlling the hazardfrom the hazard curve by itself. To provide an insight into which events are most important forthe hazard at a given ground motion level, the hazard curve is broken down into its contribu-tions from different earthquake scenarios and this process is called deaggregation (Bazzurroand Cornell 1999). The deaggregation plots of PGA hazard for 10 % chance of exceedancein 50 years hazard level for rock site conditions are presented in Fig. 9a–f for six locations;Adapazarı, Düzce, Gemlik, Izmit, Iznik, and Sapanca, respectively. For sites close to theNAF_N strand, the hazard is dominated by this source as Fig. 9a, d indicates for Adapazarıand Izmit. For both of these sites, the dominating scenario has the magnitude between 6.5 and7.5 at 5–10 km distance. For Gemlik and Iznik, the dominating source becomes Geyve–IznikFault (black bars in Fig. 9c, e), however the effect of NAF_N is still present (gray bars in thesame figures). Similarly, the main contributor to the hazard is Düzce Fault for Düzce (Fig. 9b)but the effect of sources within 10–20 km distance from the site (NAF_N and NAF_S) is alsosignificant. It is difficult to obtain the main contributing source to the hazard from Fig. 9ffor Sapanca but the dominating scenario for Sapanca has a magnitude range of 6.5–7.5 anddistance range of 0–10 km.

A common method for developing design spectra based on the probabilistic approachis to use the uniform hazard spectrum (UHS). UHS is developed by computing the hazardindependently at a set of spectral periods and then computing the ground motion for a specifiedprobability level at each spectral period. The term “uniform hazard spectrum” is used becausethe spectral acceleration value at each period has an equal chance of being exceeded. UHSof the selected sites (Adapazarı, Düzce, Gemlik, Izmit, Iznik and Sapanca) for rock siteconditions (Vs30 = 760 m/s) at 10 % probability of exceedance in 50 years probability levelare presented in Fig. 10. Similarly, UHS of the selected sites for average soil site conditions(Vs30 = 270 m/s) are provided in Fig. 11. The TEC-2007 design spectra for rock or soilsite conditions are plotted with the UHS to allow the comparison of the results with the codespecifications. Soil class is selected as Z1 and Z3 to represent rock and soil site conditions forTEC-2007 design spectrum. UHS developed for rock site conditions for all sites, except forDüzce and Gemlik, are significantly higher than the TEC-2007 design spectrum between the0.2 and 1 s spectral periods (Fig. 10). Higher ground motion levels would be also observed inDüzce if the Bolu-Gerede segment was added to the hazard calculations. The UHS and theTEC-2007 spectrum are in good agreement for long spectral periods (longer than 2 s). Whencompared to the rock site curves, the differences between the 0.2 and 1 s plateau of the designspectrum and the UHS are smaller however, the misfit between the TEC-2007 spectrum andthe UHS increases in the long period range for soil site condition curves at all sites (Fig. 11).

The seismic hazard maps of the region for 2 and 10 % chance of exceedance at 50 yearsare generated for PGA, T = 0.2 s and T = 1 s spectral accelerations for the generic rocksite conditions (NEHRP B/C boundary, Vs30 = 760 m/s). For mapping purposes, 260 grid

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Fig. 9 Deaggregation of the PGA hazard for rock site conditions at a Adapazarı, b Düzce, c Gemlik, d Izmit,e Iznik, and f Sapanca for rock site conditions (Vs30 = 760 m/s).

nodes (0.1◦ intervals) were defined in the study area and the PSHA was performed at eachgrid node. The density of grids between the fault lines is increased (0.1◦ by 0.05◦ intervals)for accuracy. The seismic hazard maps for PGA at 2 and 10 % chance of exceedance at 50years are shown in Fig. 12a, b, respectively. Similarly, the seismic hazard maps for 0.2 and 1 sspectral accelerations at 2 and 10 % chance of exceedance at 50 years are provided in Fig. 13and Fig. 14. Generally the contours of hazard follow the fault lines as expected and larger

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Fig. 10 Uniform hazard spectra for selected sites and Turkish Earthquake Code (TEC 2007) spectrum forrock site conditions

Fig. 11 Uniform hazard spectra for selected sites and Turkish Earthquake Code (TEC 2007) spectrum forsoil site conditions

values are observed along the vicinity of the fault lines. Ground motion values increase at theintersection points of the seismic sources and at the defined segmentation points on the faults.The maximum value of peak ground acceleration in the area is 1.62 g for 2475 years returnperiod, however the maximum PGA value decreases to 1.05 g at 475 years return period.Similarly, high spectral accelerations at 0.2 and 1 s spectral period are observed at 2475 yearsreturn period for sites very close to the active faults in Figs. 13 and 14.

5 Summary and conclusions

The objective of this study was to evaluate the seismic hazard in Eastern Marmara Regionusing an improved PSHA methodology. New hazard maps of the region for rock site con-ditions (VS30 = 760 m/s) at 2 and 10 % chance of exceedance in 50 years hazard levelsfor PGA, 0.2 and 1 s spectral periods are provided to allow the readers perform site-specific

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Fig. 12 Hazard maps of the region for PGA for rock site conditions (Vs30 = 760 m/s) a for 2 % chance ofexceedance in 50 years, and b for 10 % chance of exceedance in 50 years

hazard assessment and develop site-specific design spectrum for local site conditions. Thesenew maps introduce significant improvements over the previous seismic hazard maps of theregion in terms of seismic source characterization models. Initially, major faults sources inthe region are identified (NAF_N, NAF_S, Düzce Fault, and Geyve–Iznik Fault) consider-ing the surface ruptures of previous large earthquakes and each zone is defined by planarsource models. Geometry of the planar sources (segmentation points, length, orientation ofthe segments, etc.) is determined with the help of lineament analysis of satellite imagesof the region by Cambazoglu et al. (2012). This recent information is adopted with minorchanges and no uncertainty on these source parameters is included in the logic tree. However,building planar source models was still challenging since the segment widths and partitionof slip rates for parallel fault segments were not available. The Kandilli Observatory andEarthquake Research Institute (2007) earthquake catalogue is employed and the events inthe catalogue are attributed to individual planar sources by defining buffer zones and usingsource geometry. Slip rates assigned to parallel segments are validated by the cumulativerates of catalog events attributed to each source. Segment widths are estimated using empir-ical Wells and Coppersmith (1994) equations. A full rupture model is built for each faultzone considering single-segment and multi-segment rupture sources consistent with the def-initions of WGCEP-2003. Despite the use of linear source models in the last two attempts todevelop the hazard maps of the region (Erdik et al. 2004; Kalkan et al. 2009), these demandingtasks were avoided by assigning only larger (or characteristic) earthquakes to the fault seg-ments and small-to-moderate magnitude events to the background zone. Instead of defining a

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Fig. 13 Hazard maps of the region for 0.2 s spectral acceleration for rock site conditions (Vs30 = 760 m/s)a for 2 % chance of exceedance in 50 years, and b for 10 % chance of exceedance in 50 years

background source to represent the small-to-moderate magnitude events, composite recur-rence models combining the truncated exponential model for small-to-moderate magnitudeevents and characteristic model for larger events are preferred for each zone. Additionally,improved global ground motion models (NGA-W1) are employed to represent the groundmotion variability and equal weights are assigned to each NGA-W1 model.

Proposed hazard maps are comparable with the previous efforts; the maximum PGA valueat 2475 years return period was found as 1.5 g both by Kalkan et al. (2009) and Erdik et al.(2004) and the same value is found as 1.62 g in this study. On the other hand, the distributionof the 2475 year PGA values in the region are quite different, larger values are concentratedaround the major fault zones and segment nodes but the hazard values diminish significantlyas the distance to the major faults increased. These apparent hazard contours are not observedin previous maps that used background sources and smoothed-seismicity approach. Largespectral acceleration values of 0.2 s (up to 3.8 g) and 1 s (up to 1.5 g) spectral accelerations areestimated for the sites right on top of the fault lines, but hazard values reduce quickly at theend of the near-field zones. Hazard estimations in this study are similar to the hazard levelsproposed by Kalkan et al. (2009) but larger than that of Erdik et al. (2004) indicating thatthe difference might be the end results of using NGA-W1 predictive models. The maximumPGA value in the region is found as 1.05 g at the probability level included in TEC-2007(475 years return period) showing that the design PGA recommended by TEC-2007 for theregion (0.4 g) is inadequate, especially for near-fault sites. The uniform hazard spectra at 10 %probability of exceedance in 50 years hazard level for different soil conditions (soil and rock)

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Fig. 14 Hazard maps of the region for 1 s spectral acceleration for rock site conditions (Vs30 = 760 m/s)a for 2 % chance of exceedance in 50 years, and b for 10 % chance of exceedance in 50 years

are provided for six specific locations in the region (Adapazarı, Düzce, Gemlik, Izmit, Iznikand Sapanca). The UHS developed for rock site conditions for all sites are significantly higherthan the TEC-2007 design spectrum between the 0.2 and 1 s spectral periods, the deviationbetween the 0.2 and 1 s plateau of the design spectrum and the UHS being smaller for soilsites. The rock UHS and the TEC-2007 spectrum are in good agreement for long spectralperiods (longer than 2 s) but the soil UHS are quite higher than the TEC-2007 spectrumfor longer periods. Even if it is not explicitly mentioned in TEC-2007, the PGA values at10 % probability of exceedance in 50 years were adopted from Gülkan et al. (1993) for eachseismic zone (Kalkan et al. 2009). Gülkan et al. (1993) utilized one of the early-stage GMPEsby Joyner and Boore (1981) and truncated the ground motion variability with approximately±1 σ (Yılmaz-Öztürk 2008). The main reasons driving the observed difference lie in thecompletely diverse approaches adopted for the truncation of ground motion variability andmodeling of seismic sources in this study and in Gülkan et al. (1993). We believe that theuncertainty in the hazard estimates reduced significantly by properly modeling the seismicsources and selecting suitable ground motion models.

Acknowledgments This work was partially funded by METU: “Seismic Source Modeling and ProbabilisticSeismic Hazard Assessment of Marmara Sea and Adapazarı Region” (BAP Award Number: BAP-03-03-2009-01). We are thankful to Prof. Dr. Yener Özkan for his support. The authors wish to acknowledge Prof. Dr.Haluk Akgün, Dr. Mustafa Koçkar, and Selim Cambazoglu for their contribution on seismic source modelsof the region. We appreciate the help and support of Norman A. Abrahamson and Nick Gregor regarding theprobabilistic seismic hazard assessment calculations.

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