The a and b parameters, which are associated with the physics of earthquake occurrence, the tectonics of the region, and the total seismic activity, were determined specifically for the SFZ as a = 6.33 and b = 0.92 (Figure 5). The a-value, which characterizes the degree of seismic activity or indicates the total seismic activity level of the studied region, was classified by Scholz [120] as low seismic activity if it is less than six. However, subsequent studies have shown that the a-value is directly related to the size of the region under investigation, the time period considered, and the catalog used. It has also been emphasized that the a-value may reflect different seismic activity levels for each region. In general, a value between three and four indicates low seismic activity in the region, a value between four and six suggests moderate seismic activity, and a value greater than six indicates high seismic activity. In various studies conducted for different regions of the world, a-values have been estimated using different methods. On a global scale, the a-value has been calculated as 5.65 to 7.23 in oceanic subduction zones, 6.83 to 8.37 in mid-ocean ridges, 6.11 for the San Andreas Fault, and 6.37 for the Bitlis–Zagros collision zone [89]. In another study focusing on the EAFZ, another significant tectonic feature of the Anatolian Plate, the a-values for the Palu-Hazar Lake segment were estimated to range between 6.5 and 7.0. This suggests that the region is seismically more active than other segments of the EAFZ [94]. In a separate study examining the East African Rift System, a different continental plate, the study area was divided into seven regions, and the a-values were found to vary between 3.78 and 8.4. An a-value of 5.75 was calculated for the Afar Triple Junction, which the authors identified as a highly seismically active region [121]. The a-value obtained in this study indicates that the SFZ and its surrounding area have an active seismic structure and indicate intense earthquake activity. Additionally, a significant number of small-to-moderate magnitude earthquakes that occurred in the region during the studied time period further validate the high a-value obtained (Figure 2).

According to Wiemer and Wyss [122], regions with low crustal heterogeneity and stress are associated with b-values greater than 1.0, while values less than 1.0 are related to high differential stresses. A high b-value indicates that small earthquakes dominate in the studied region, while a low b-value suggests that larger earthquakes suppress smaller events in that area. In addition, it has been stated by many researchers that the b-value is related to the Earth’s crustal stress. Regions or faults with high stress rates and large deformation velocities tend to have lower b-values, while areas with low stress accumulation exhibit higher b-values [94,104,120,123,124,125]. The results obtained from this study, which present findings related to the region’s strain rates, have been compared with conducted studies in the literature. In studies where Western Anatolia is generally considered as a whole, with no specific focus on the SFZ, Alptekin [126], Erdik et al. [127], and Kayabali [128] found the a-values to range between 4.4 and 5.4, and the b-values to range between 0.52 and 0.88, within a source zone that includes some part of the SFZ. Papazachos and Kiratzi [129] calculated the a parameter as 3.82 and b parameter as 0.8, Orhan et al. [7] estimated an a-value of 6.82 and b-value of 1.0 in a study that focused on mostly Eskişehir, while Kalyoncuoglu [114] found a-values varying between 5.5 and 6.3 and b-values between 0.8 and 0.9. Also, Polat et al. [130] found the same b-values around SFZ. Bayrak et al. [131] and Öztürk [132] estimated the b-value around 1.02 for the region including the Kütahya, Simav, and Zeytindağ–Bergama faults (KSZBF) together; for the same fault zones, Öztürk et al. [133] found a b-value of 0.65. Similarly, Sayıl and Osmansahin [134] calculated b-values between 0.52 and 0.66 and a-values between 3.85 and 4.78 for two separate sub-region containing different sections of the SFZ. In another study, where Western Anatolia was divided into 15 sub-regions, Bayrak and Bayrak [135] and Bayrak et al. [136] calculated the a-value as 5.7 and the b-value as 0.94 for the region containing the Simav and Gediz-Dumlupınar Faults. In addition to b-values, some studies suggest that modal values (a/b) are compatible with tectonic environments and also reflect seismic activity in the region of interest better than a– and b-values alone. In this study, the modal value was calculated as 6.9, and this value points out that it is both in agreement with b-values and the general seismicity of the study area [90,95,108,137,138]. As a result of this study, when compared with previous studies conducted in the region, most of the studies are in concordance with our seismic parameters, except for those where the b-value was estimated to be considerably low. Most of the studies where the b-value was estimated to be low are those that examine Western Anatolia partially or completely, particularly without focusing on a specific fault zone. Therefore, it is believed that the low b-values obtained do not reflect the general tectonic regime of Western Anatolia, which releases energy through a large number of low-magnitude earthquakes. When considering the high a-value and low-average b-value obtained specifically for the SFZ in this study, these values suggest that the SFZ generally produces numerous low-to-moderate magnitude earthquakes. However, due to the associated subduction tectonics, it is capable of accumulating stress that could potentially generate large earthquakes, such as the Gediz earthquake (Mw = 7.2), despite being less frequent. A similar calculation was made by Öztürk [132], who interpreted the low b-values obtained for the SFZ and its surroundings as an indication of relatively uniform structural characteristics and significant stress accumulation due to subduction tectonics. This, in turn, could lead to stress buildup over time, which is released by earthquakes that, although less frequent, are great in magnitude [139]. Also, the main location of the İzmit earthquake in 1999 (Mw = 7.4) is situated in a region characterized by a higher a-value and an average b-value [130,140,141].