In this study, the frequency-magnitude distribution of Eastern Turkey was investigated to determine b-value and Mc parameters. The coordinate range of the region is 360-400 E longitudes and 370-400 N latitudes. In addition, the probabilistic seismic hazard of this region was investigated using the GR and the GPR models and the obtained results were compared. With the use of these models, information about potential earthquakes in this region can be acquired by estimating seismic hazard parameters comprising of return periods and possibilities of their occurrence. For these analyses, a homogeneous and completeness catalogue was used. In order to create a homogeneous catalogue only for this study, the conversion equations between the size of the moment and the other scales (md, ML, mb, Ms and M) were defined by using the general method of orthogonal regression. The established conversion equations were then used to calculate the equivalent moment magnitudes (Mw*) for the entire catalogue. The homogeneous catalogue was also declustered (Reasenberg 1985) for a time-independent seismic hazard assessment (e.g., Arroyo-Solórzano and Linkimer 2021). Therefore, swarms, aftershocks and foreshocks were removed from the homogeneous earthquake catalogue (e.g. Peláez et al. 2007; Amaro-Mellado et al. 2017; Arroyo-Solórzano and Linkimer 2021). This separation between aftershocks and mainshocks done provide to reveal the strength of the faults which is highly influenced by the mode, or style, of faulting (e.g., Simão et al., 2010)
The declustering catalogue was then modelled using the Gutenberg-Richter model to estimate seismic parameters. From this analysis, the seismic parameters of the magnitude scale, consisting of b, a and magnitude of completeness, were estimated at 0.82 ± 0.01, 5.452 and 1.9, respectively. The estimated b-value in the study is similar to the b-value determined by Afegbua1 et al. (2018) in the Middle- Atlantic Ridge (MAR) in the West African region. The estimated b-value was 0.4 to 2.2 (Fig. 7a). An estimated high b-value indicates that small earthquakes predominate over the large ones (e.g., Arroyo-Solorzano and Linkimer 2021). However, in most of the research area, the b-value varying from nearly 0.6 to nearly 0.9 was low to intermediate, which indicates that this study region is one of the seismically active tectonic regions (Mogi 1967; Frohlich and Davis 1993; El-Isa and Eaton 2014; Polat 2022). The observed changes in the b-values measured in this study give information about the differential stress change in the region because it acts as a stress meter. In addition to this, variations in b-values are used to identify the type of faults developed in the study region. There is an inverse relation between b-value and differential stress (e.g. Schorlemmer et al. 2005; Scholz 2015). In the light of the information, b-values are interpreted here.
In addition, the finding from this study seems compatible with the dominant strike-slip system (Paiboon 2006) existing in the region. Furthermore, spatial variations of b values in the whole of Eastern Anatolia provide to find out locally effective stress (Scholz 1968) because variations in b-values depend on tectonic regimes. For example, such study areas have similar underground mines (Urbancic et al. 1992), a subduction slab (Wyss et al. 2001), and fault zones (Wiemer and Wyss 1997) which result in inhomogeneous variations in b-values. The findings are very compatible with the previous studies done in the city of Elazığ and the surrounding area (Polat 2022). In the small part of the region, a relative high b-value was observed between 39.50–400 longitude and 370-37.50 (Fig. 6a). This variation in the estimated b-value from the region to region may be related to a convergence zone between the Arabian and the Anatolian plates. The estimated b-value from this study is also consistent with the b-value calculated by Bayrak et al. (2013) for Ağrı and the surrounding area by using the earthquakes which have occurred between 1900 and 2014. However, the b-value, the a-value and the Mc for this region were quite smaller than the b-value, a-value and Mc to be 1.1, 7.86 and 2.7 for the whole of Turkey (Kalafat 2016), respectively.
The b values lower than 0.8 (Fig. 7a) are related to the area with high-stress regions including the region between the Savrun Fault and the Amonos segment of the EAFZ. However, b-values varying from 1 to slightly smaller than 1.2 were observed in and around the region among the Malatya, Sürgü, Pütürge and Ovacık Faults shown in Fig. 1b. This study clearly reveals that the spatial b-value map for the entire region was not fully homogeneous, while intermediate b-values were mostly dominant in the region except for a few local areas (Fig. 7a). Some main factors consisting of a low degree of heterogeneity, large stress and strain, large velocity of deformation and large faults generally cause a low b value (Manakou and Tsapanos 2000; Hussain et al. 2020). The observed b-low values (Fig. 7a) are probably related to differential crustal stress and strain produced by large faults exhibited in Fig. 1 located in the study region. It means that the region is prone to destructive and massive earthquakes with high magnitudes. In contrast, if a region has high b-values, it indicates that the area is prone to small earthquakes in the near future.
Considering all findings, it can be said that the investigated region has a strong seismic hazard potential. Our study, which shows that the region has a high earthquake risk, is evidenced by recent earthquakes and historical records. Therefore, the seismic hazard analysis for such regions become important. To improve our knowledge about the region with respect to the seismic hazard, statistical methods including the GR and the GPR were also used to calculate occurrence intervals and recurrence periods. As choosing these methods, we considered that previous studies indicated that the GPR mode can be used for seismic risk modelling in Turkey (Kara and Durukan, 2017).
During the seismic hazard analysis, 189 earthquake records with a magnitude of Mw ≥ 4.0 in the time interval of 1905-30 September 2022 years were selected from the complete data set of 28536 earthquakes. For the GR model, a- and b- values were estimated to be equal to 7.406 and 1 for this time period (Fig. 8), respectively. The Poisson relationship for the GPR was found as LnN = 15.430–1.959M. The estimated b- and a-values for the GPR method were 1.959M and 15.430 (Fig. 8), respectively. As seen in Table 4, the seismic parameters of the GPR were slightly more than twice that of the GR method. The probabilities of an earthquake were estimated for the next 10, 20, 30, 50, and 100 years in the study region (Table 7 and Fig. 9). Return period results of both methods were also mapped as shown in Fig. 9 and listed in Table 7. The return periods in the study area were estimated between 0.1064 and 338.0673 years for magnitudes of 4.0-7.5 according to the GR model (Table 7 and Fig. 10). For the model, the probabilities of an earthquake with M = 6.0, 6.5 and 7.0 in the next 100 years were computed as 99.99%, 94.83% and 60.78%, respectively. In terms of the model, the probability of the largest earthquake occurring in the studied area is 25.61% in the next 100 years. On the other hand, the return periods for the GPR model were calculated between 0.26448 and 250.2964 years for magnitudes of 4.0-7.5 (Table 7 and Fig. 10). According to the GPR model (Fig. 9), the probabilities of an earthquake with M = 6.0, 6.5 and 7.0 in the next 100 years were computed as 99.95%, 94.10% and 65.47%, respectively. Return periods for the earthquakes were 13.2726, 35.3282 and 94.0347 years. In the next 100 years, the occurrence probability of the largest earthquake in the region is 32.94%.
As seen in Table 7, the return periods and the occurrence possibility of an earthquake according to the GPR are less than twice than that of the GR method. Finally, taking into account all findings acquired from the analysis, it can be said that the region has the potential for an earthquake larger than 6.0 because the faults developed in the region are very active. The models prove that the occurrence probability of such an event with a magnitude greater than 6 is high and this finding is strongly consistent with the tectonic units. Findings from the study indicated that probabilities of exceedance and return periods calculated for the selected years significantly depend on yearly average occurrence number of earthquakes.
Looking at such studies done in the region, in a recent study carried out by Isik et al. (2021), the earthquake parameters were determined using the earthquake ground motion levels with some probabilities of exceedance. The study indicated that some cities such as Erzincan Malatya are in the first-degree earthquake hazard zone, while other cities such Elazığ and Tunceli are in the second-degree earthquake hazard zone. The findings means that seismic parameters calculated from the frequency-magnitude distribution seem compatible with the earthquake parameters determined using the earthquake ground motion levels with some probabilities of exceedance.
Although this region is not in a ridge region, it shows similar characteristics in terms of producing earthquakes. Although the earthquakes are relating to dyke intrusions and propagation along the Mid-Atlantic Ridge, the orientation of the Arabian plate towards the Anatolian plate and the Dead Sea fault in the Anatolian region play significant role in an increase in the seismic activity of the region. This shows that the cause of seismic activity in both regions is different from each other. It is not correct to make a comparison in terms of seismicity and tectonic structure