1 Introduction

The report explores the potential impact of major earthquakes on the Büyükçekmece district in Istanbul, Turkey, by utilizing stochastic ground motion simulations. The region is situated near the North Anatolian Fault (NAF), which is one of the most seismically active regions globally, making it susceptible to earthquake damage. Due to these characteristics of the area, researchers worldwide have always been drawn to studying the seismicity of the region and assessing earthquake hazard through various methods (including but not limited to Pulido et al. 2004; Sørensen et al. 2007; Tanırcan 2012; Zengin and Çaktı 2012; Paolucci et al. 2018; Infantino et al. 2020; Stupazzini et al. 2020).

Unlike previous studies, this report concentrates on assessing earthquake hazard in a specific district of Istanbul, which is the final in a series of few studies. These studies are developed in collaboration with multiple academic institutions, Büyükçekmece Municipality and Istanbul Metropolitan Municipality. The findings from these studies are anticipated to inform future city-wide planning initiatives.

Initially, we carefully selected five earthquakes that occurred on NAF to validate the EXSIM simulation algorithm and we successfully validated the accuracy of this algorithm by simulating these earthquakes for the wider metropolitan region of Istanbul (Süleyman and Çaktı 2020, 2021, 2022) using data from the Istanbul Early Warning and Rapid Response System (IEWRRS).

To obtain a better insight into the potential impact of a major earthquake, we extensively analyzed the site response of this district in a recent study (Süleyman et al. 2023). We utilized the seismic ambient vibrations recorded in this region to calculate the depth-to-bedrock values, which were used to compute quarter-wavelength amplification factors in this study. These amplification factors were utilized in stochastic ground motion simulations, enabling us to prepare our estimations with expected improved accuracy.

In this study, we aimed to gain a better understanding of the impact of earthquakes with magnitudes ranging from 6.3 to 6.7 and 7.2 to 7.42 on the district. In the first stage, we conducted a total of 50 scenario events of these magnitudes, with a particular focus on the Büyükçekmece district. We thoroughly discussed these simulated scenario events, focusing on the impacts of different magnitude events and various period spectral accelerations on the district. In the study's second stage, we aimed to gain a deeper understanding of the uncertainty stemming from variations in stress parameter and hypocenter location. To achieve this, we significantly increased the number of scenarios examined compared to the first stage. As a result, our analysis shifted towards probabilistic investigations to better investigate the impact of these two key source parameters on the outcomes.

This study highlights the potential for earthquake hazards in this district in the event of medium to large earthquakes. The study emphasizes the importance of understanding the potential impact of earthquakes in Büyükçekmece and taking measures to minimize the impact.

2 Seismotectonics of the Marmara region

The NAF is a significant right-lateral strike-slip fault that stretches over approximately 1500 km and runs along the transform boundary between the Eurasian plate and the Aegean-Anatolian plate. The Marmara fault segments of NAF are located in a seismic gap, and their total length is approximately 100 km (Bohnhoff 2013; Ergintav et al. 2014). According to Ambraseys (2002), the recurrence interval for a major earthquake in this area is between 150 and 400 years, with the last event occurring in 1766. Drab et al. (2015) highlighted the importance of the Çınarcık segment, also known as Princes' Islands segment, which acts as a permanent seismic barrier. The authors identified four earthquake sequences in the Marmara Sea between C.E. 740 and 1999, with only one large earthquake propagating within a single event through the bend between the Çınarcık and the western portion of the İzmit segment.

Although major earthquakes occurred sequentially from east to west along the NAF in the twentieth century, culminating with the 1999 earthquakes, the seismic rupture has not yet broken the Marmara Sea segment. According to Ergintav et al. (2014), Diao et al. (Full size image

The results of this study provide valuable insights into the site-specific parameters of Büyükçekmece district. The estimated site amplification and depth-to-bedrock values can be utilized to improve the accuracy of seismic hazard assessments for the region. More detailed information on the methodology and results of this study can be found in Süleyman et al. (2023).

4.2 Validation of the ground motion simulation algorithm for Büyükçekmece

Ground motion simulations play an essential role in assessing seismic hazard and risk for a given region. In this study, we used the open-source algorithm EXSIM (Motazedian and Atkinson 2005; Boore 2009; EXSIM 2018), which has been validated in previous studies, to generate ground motion scenarios for Büyükçekmece, a region located in the western part of Istanbul, Turkey. The EXSIM algorithm accounts for finite-fault source models and stochastic simulation approaches, critical for accurately simulating realistic earthquake scenarios.

4.2.1 Source and path parameters used in the validation study

The validation exercise for the EXSIM algorithm, by utilizing five selected earthquakes was conducted in Süleyman and Çaktı (2021) and updated in later stages (Süleyman and Çaktı 2022). These five earthquakes considered in the validation process ranged in magnitude from MW 4.0–5.8 and occurred in the Marmara Sea region (Fig. 2). The selection of these events aimed to capture the diversity of the seismic sources in the region–such as, the diversity in epicentral locations, magnitudes, recording locations, and observed ground motion levels were a few of the selection criteria (Süleyman and Çaktı 2020).

Fig. 2
figure 2

The five earthquakes selected for the validation study, their magnitudes, and the dates of their occurrence (Süleyman and Çaktı 2020, 2022)

For the largest earthquake (MW 5.8), a finite-fault source model developed by Karabulut et al. (2021) was used, while for the smaller events, point sources were utilized due to their relatively small rupture sizes. The validated set of parameters used in this study is similar to the ones used in Yenihayat et al. (2023) to validate the stochastic simulation algorithm for the region of Istanbul. Similarly, Infantino et al. 2020 used the same earthquake recordings to validate their 3D velocity model.

Table 1 summarizes the path, site, and source effect parameters used in the validation exercise. These parameters were globally and regionally developed, with particular attention paid to the Istanbul and Marmara region. The bottom lines of Table 1 relate to the source parameters of the five earthquakes included in the validation study.

Table 1 Summary of the parameters used in the validation exercise

In order to accurately model the ground motion in the Büyükçekmece region, a site-specific approach was adopted, which involved the use of amplification factors generated for each of the 42 observation locations. In previous studies, a generic amplification model based on site classes had been used (Süleyman and Çaktı 2022). However, in this updated set of validated parameters, the Square-Root-Impedance (SRI) technique developed by Joyner et al. (1981) is employed for the amplification calculations. Further information on the application of this model can be found in the subsequent section.

Attenuation at high frequencies was measured based on the approach of Anderson and Hough (1984) in a previous study by Süleyman and Çaktı (2018). However, in the current study, to account for frequency-dependent anelastic attenuation, the model proposed by Horasan et al. (1998) was used. This model observes the spectral decay of S-waves at selected central frequencies over Fourier amplitude spectra. To describe geometrical spreading, the model of Akinci et al. (2013) was used. The duration of the simulated ground motions was scaled according to the model suggested by Boore and Thompson (2014). This validated model was then used to conduct scenario ground motion simulations for the metropolitan region of Istanbul in Süleyman and Çaktı (2022).

4.2.2 Ground motion simulation development using site-specific SRI method

The SRI method, commonly referred to as the Quarter-Wavelength Approximation, is a widely recognized technique used for estimating approximate linear site amplifications in simulations of ground motions and for comparing seismic velocity estimates at a single site (Boore and Joyner 1997; Boore 2003; Brown et al. 2002; Stephenson et al. 2005).

The SRI method offers distinct advantages over alternative approaches for calculating site amplifications. Notably, it is characterized by its efficiency, relying solely on a simple velocity model of the site. It assigns an amplification frequency to each depth and effectively smoothens multiple peaks. However, the SRI approach consistently provides lower estimates for the peaks observed in resonant systems and for gradient models lacking significant impedance differences, across a broad spectrum of frequencies. In the case of two gradient models commonly utilized in stochastic ground-motion simulations to derive crustal amplifications, this underestimation is limited to a maximum of approximately 30%. This margin is comparable to the uncertainties associated with several parameters utilized in simulations, and significantly smaller than the typical uncertainties in observed ground motions. Furthermore, Thompson et al. (2011) and Boore (2013) demonstrated that site responses computed using the SRI method exhibit closer alignment with observed site responses compared to results from the “full-resonant” method. Additionally, this study advocates for the adoption of the SRI method in constructing site response functions for average velocity profiles.

The method's core methodology entails estimating average seismic parameters, with a specific emphasis on the quarter-wavelength velocity (VsQWL), up to a depth denoted as z(f), equivalent to one-quarter of the wavelength associated with the frequency (f) of interest. Amplification factors for a given frequency are then computed by deriving the square root of the impedance ratio between the average depth and a reference point. To calculate the quarter-wavelength velocity corresponding to a particular frequency, a process of travel-time averaging across the input profile is employed by minimizing the expression as presented in Eq. 1 and 2.

$$\begin{array}{*{20}c} {\arg \min } \\ {z\left( f \right)} \\ \end{array} \left\|z\left( f \right) - \frac{{V_{s}^{QWL} \left( {z\left( f \right)} \right)}}{4f}\right\|$$
(1)

given that:

$$V_{s}^{QWL} \left( {z\left( f \right)} \right) = z\left( f \right)\left( {\mathop \int \limits_{0}^{z\left( f \right)} \frac{1}{{V_{s} \left( {z\left( f \right)} \right)}}dz\left( f \right)} \right)^{ - 1}$$
(2)

The depth-to-bedrock values and assigned shear-wave velocities were used in conjunction with the 1-D crustal shear-wave velocity model utilized by Karabulut et al. (2021) to generate unique amplification curves for each location. By adopting a site-specific approach and incorporating validated models for amplification, attenuation, and spreading, the simulations presented in this study are expected to offer a more precise representation of ground motion in the Büyükçekmece region.

For comparison, amplification factors computed for amplitudes exceeding 5 Hz at a site with a VS,30 value approximating 260 m/s equate to 4.32, whereas for a site with a VS,30 value approximating 340 m/s, the corresponding factor is calculated as 3.74. While it might be tempting to employ generic amplifications for each site class, this approach could introduce significant disparities in amplification factors, potentially resulting in overestimation or underestimation of stochastically simulated motions. Consequently, this study employs site-specific amplification factors to ensure a closer alignment between the simulated motions and the actual seismicity.

4.3 Scenario ground motion simulation development

As previously indicated, the authors of this study have emphasized seismic gaps along the North Anatolian Fault (NAF), specifically by develo** earthquake scenarios for the eastern portion of the central Marmara segment (Source #1) and the Princes' Islands segment (Source #2) of the NAF. Source #1 and Source #2 have the potential for ruptures spanning approximately 100 km and 35 km, respectively. It is noteworthy that ruptures in either of these sources may trigger ruptures in the other, given that these segments intersect.

A total of 50 simulations were conducted on the two seismic sources mentioned above–as separate rupture locations and jointly. Each of these scenarios represents a unique scenario with different combinations of magnitude, rupture, and hypocenter locations. “Appendix B” summarizes all the source parameters used in these simulations.

Simulations from scenario number 26 to 40 were conducted at Source #1 (Fig. 3), with magnitudes ranging from MW 7.2 to 7.4. Simulations from scenario number 1 to 25 took place at Source #2, (Fig. 3) involving MW 6.3 to MW 6.7 events. The simulations from number 41 to 45 involved combinations of MW 7.4 and MW 6.4 events, and the final five simulations involved combinations of MW 7.4 and MW 6.7 events with—the larger event triggering the latter one, forming larger magnitude events of MW 7.41 and MW 7.42, all taking place at two source locations consecutively. In all these scenarios, the hypocenter location was randomly assigned.

Fig. 3
figure 3

Source locations of the scenario ground motion simulations

At all the ruptures considered slip weights are randomly generated at each sub-source of each simulation. The simulations only involved rectangular rupture plane scenarios due to the limitations of the EXSIM tool, which does not allow for rupture models with irregular shapes. The dip angles for all events were set at 90 degrees, given the predominantly horizontal nature of the faulting in the NAF. Stress values were set at 100 or 130 bars, as previously investigated in Süleyman and Çaktı (2022). The hypocenter depths of the simulated earthquakes range from 5.3 to 17 km, despite their expected shallow depths.

The simulations in this study were conducted for the 0.05–5 s (0.2–20 Hz) range. The simulated mean horizontal S-wave motions for each observation location considered in the Süleyman et al. (2023) study are presented as a result.

4.4 Accounting for variability in hypocenter location and stress drop values

The hypocenter location and stress drop values are recognized as two of the most uncertain parameters in the context of modeling ground motions. However, these parameters can significantly influence the ground motion values observed at near-source locations. The quantification of this uncertainty holds paramount importance in comprehending the range of ground motion levels, thereby enhancing the meaningfulness of outcomes produced through ground motion simulations. Despite the incorporation of diverse scenarios involving varying magnitudes and multiple hypocenter locations within this study, the researchers also undertook a probabilistic assessment of the aforementioned variability in hypocenter and stress drop.

In order to characterize the variability of ground motion at each observation point, a wide range of stress drop values was defined for each of the considered magnitudes. Consequently, for each of the selected stress drop values a hundred randomized scenarios were generated, each involving randomized hypocenter locations. This approach facilitated the quantitative analysis of this variability, leading to a more precise representation of the ground motion models.