Is a doublet signaling the formation of a new basin in Sındırgı, Western Türkiye?

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A pair of 2025 magnitude 6.1 earthquakes on the Sındırgı segment of Western Türkiye’s strike-slip Simav fault zone appear to have ruptured new normal faults.
 

By Haluk Eyidoğan, Emeritus Professor, İstanbul Technical University, Department of Geophysics, Sarıyer, İstanbul, Türkiye
 

Citation: Eyidoğan, H., 2026, Is a doublet signaling the formation of a new basin in Sındırgı, Western Türkiye?, Temblor, http://doi.org/10.32858/temblor.380
 

On August 10, 2025, a magnitude 6.1 earthquake occurred 11 kilometers south of the Sındırgı district of Balıkesir Province in western Türkiye at a depth of 11 kilometers (Table 1). In the villages of the Sındırgı district, a total of 1,036 residences were severely damaged or destroyed. According to AFAD-DAD (2025a) report, one person perished.

On October 27, 2025, a second earthquake of magnitude 6.1 occurred, 78 days after the first and 15 kilometers to the southeast. The second shock ruptured at a depth of 6 kilometers (Table 1), and damaged 607 residences and 93 workplaces (Hürriyet, 2025; ÇŞİB, 2025).

These two earthquakes present an interesting example of a doublet — two similarly-sized mainshocks that occur near one another both spatially and temporally. Doublet earthquakes can occur as a result of movement along the same fault, on conjugate faults, or on different sections of neighboring faults.

In this article, I explore the fault responsible for this doublet; the seismicity that preceded and succeeded the mainshocks; the significance of the change in location of the earthquake sequence over time; and the style of faulting. In particular, I use this information to argue for the development of a new basin controlled by normal faulting.
 

Table 1. Earthquake parameters of the doublet as determined by different institutions. The first event occurred on August 10, 2025, and the second on October 27, 2025
Table 1. Earthquake parameters of the doublet as determined by different institutions. The first event occurred on August 10, 2025, and the second on October 27, 2025

 

The Simav fault zone

The major fault associated with these events is the Sındırgı Fault, which is the westernmost branch of the Simav fault zone (Figure 1). According to the AFAD-DAD (2025b) earthquake catalog, the only major earthquake that may have been associated with the Sındırgı Fault between January 1, 1900 and August 9, 2025 occurred on October 28, 1942. The event had a surface wave magnitude of 5.4., and ruptured just north of the Sındırgı Fault (Figure 1).

The right-lateral strike-slip Simav fault zone is 205 kilometers long (Emre et al., 2018; MTA, 2025) and is classified as an active fault with significant earthquake potential. The Simav fault zone comprises seven active fault segments with right-lateral strike-slip movement. From west to east, they are: Sındırgı, Çaysimav, Şaphane, Abide, Banaz, Elvanpaşa and Sinanpaşa (Figure 1 and Table 2). According to Emre et al. (2018), basins located along the length of the fault formed as a result of releasing bends caused by changes in the orientation of the fault. In such a scenario, the continued overall right-lateral motion of the fault zone results in normal motion at these bends.
 

Figure 1. The location of the 205-kilometer-long Simav fault zone. The area marked with a dashed black rectangle in the upper left corner demarcates the area examined in this article, the Sındırgı segment. Fault information is taken from Emre et al. (2018) and the MTA (2025). The star shown within the area outlined by the dashed line indicates the location of the earthquake with a surface wave magnitude (Ms) of 5.4 that occurred on the Sındırgı Fault on October 28, 1942 (AFAD-DAD, 2025b). In the inset on the upper right, which is a map of Turkey’s active faults, the blue rectangle shows the location of the Simav fault zone. Credit: Eyidoğan, 2026, CC BY-NC-ND 4.0
Figure 1. The location of the 205-kilometer-long Simav fault zone. The area marked with a dashed black rectangle in the upper left corner demarcates the area examined in this article, the Sındırgı segment. Fault information is taken from Emre et al. (2018) and the MTA (2025). The star shown within the area outlined by the dashed line indicates the location of the earthquake with a surface wave magnitude (Ms) of 5.4 that occurred on the Sındırgı Fault on October 28, 1942 (AFAD-DAD, 2025b). In the inset on the upper right, which is a map of Turkey’s active faults, the blue rectangle shows the location of the Simav fault zone. Credit: Eyidoğan, 2026, CC BY-NC-ND 4.0

 

Table 2. Characteristics of active faults belonging to segments of Simav fault zone (Emre et al., 2018). The locations of fault segments are shown on Figure 1. The Fault Code is the fault number defined on the Turkish Active Fault Map. RL: right-lateral strike-slip fault
Table 2. Characteristics of active faults belonging to segments of Simav fault zone (Emre et al., 2018). The locations of fault segments are shown on Figure 1. The Fault Code is the fault number defined on the Turkish Active Fault Map. RL: right-lateral strike-slip fault

 

The Simav fault zone has attracted the attention of many Earth scientists and has been studied using geological, geophysical and seismological methods (Arpat and Bingöl, 1969; Seyitoğlu, et al., 1992; Doğan and Emre, 2006; Toker, 2014; Kartal and Kadirioğlu, 2015; Toker et al., 2018). It was initially defined as a normal fault by Seyitoğlu (1997). More recently, Gündoğdu et al. (2020) suggested that the Simav fault zone behaved as a strike-slip fault in the Early Miocene, but today acts as a normal fault. What can the doublet and its aftershocks tell us about this apparent discrepancy regarding the sense of motion of this major fault zone?
 

Spatial and temporal variation of seismicity

This research uses the AFAD-DAD (2025b) database of earthquakes with magnitude 1.0 and greater. For the numerous earthquakes with magnitudes of 4.0 and less, the database reports the local magnitude, or ML. To homogenize all magnitudes to a single scale, local magnitudes are converted to moment magnitude (known as Mw) using the following equations from Goertz‐Allmann et al. (2011):

Mw = 0.594*ML+0.985 (ML<2) Mw = 1.327+0.253*ML+0.085*ML2 (2≤ML≤4) Mw = ML-0.3 (ML>4)

Hereafter, all magnitudes are reported as moment magnitude unless otherwise stated.

Between July 1, 2025 and April 24, 2026, a total of 23,385 earthquakes with magnitudes 1.0 or greater occurred in the region around the Sındırgı segment (Figure 2a, b). Prior to August 10, 2025, the date of the initial mainshock, a remarkable burst of microseismic activity occurred here. Because of the location and frequency of seismic activity, I consider these events foreshocks.
 

Figure 2. (a) This map shows the location of epicenters for seismic activity around the Sındırgı Fault between July 1, 2025, and April 24, 2026. The inset shows the spatial distribution of foreshock activity. b) This plot shows the magnitude of shocks over time. The inset shows the temporal variation of foreshock activity. Earthquake data were obtained from AFAD-DAD (2025b). Credit: Eyidoğan, 2026, CC BY-NC-ND 4.0
Figure 2. (a) This map shows the location of epicenters for seismic activity around the Sındırgı Fault between July 1, 2025, and April 24, 2026. The inset shows the spatial distribution of foreshock activity. b) This plot shows the magnitude of shocks over time. The inset shows the temporal variation of foreshock activity. Earthquake data were obtained from AFAD-DAD (2025b). Credit: Eyidoğan, 2026, CC BY-NC-ND 4.0

 

Migration of Sındırgı seismicity

The most striking feature of the Sındırgı earthquake sequence is the migration of seismicity from west to east in a southeast direction from the western end of the Sındırgı Fault (Figure 3).

The aftershocks of the first magnitude 6.1 earthquake migrated southeastward (Figure 3a, seismic activity in blue and yellow), extending from the western end of the Sındırgı Fault. The epicenter of the second magnitude 6.1 earthquake, which occurred on October 27, 2025, was located southeast of the previous mainshock, where aftershocks were concentrated (Figure 3b). On January 23, 2026, a large aftershock of magnitude 5.1 occurred east of the October 27 earthquake (Figure 3c).
 

Figure 3. The spatial distribution of earthquakes and their aftershock activity in the vicinity of the Sındırgı Fault. (a) After the August 10, 2025 magnitude 6.1 earthquake, aftershock activity migrated to the southeast. (b) The October 27, 2025 magnitude 6.1 mainshock occurred in the area in which the initial event's aftershocks were concentrated. (c) On January 23, 2026, a magnitude 5.1 aftershock occurred in the second mainshock's aftershock zone. The time-dependent variation characteristics of this seismic activity are shown in Figure 2. The figure was prepared by using the ZMAP7.1 algorithm (Wiemer, 2001) in MATLAB (2025). Credit: Eyidoğan, 2026, CC BY-NC-ND 4.0
Figure 3. The spatial distribution of earthquakes and their aftershock activity in the vicinity of the Sındırgı Fault. (a) After the August 10, 2025 magnitude 6.1 earthquake, aftershock activity migrated to the southeast. (b) The October 27, 2025 magnitude 6.1 mainshock occurred in the area in which the initial event’s aftershocks were concentrated. (c) On January 23, 2026, a magnitude 5.1 aftershock occurred in the second mainshock’s aftershock zone. The time-dependent variation characteristics of this seismic activity are shown in Figure 2. The figure was prepared by using the ZMAP7.1 algorithm (Wiemer, 2001) in MATLAB (2025). Credit: Eyidoğan, 2026, CC BY-NC-ND 4.0

 

Figure 4. Graphical representation of the longitude-to-time positions of earthquakes with magnitude 4.0 and greater that occurred between July 1, 2025 and April 24, 2026. Focal mechanism solutions are given in Figure 5. Data were obtained from AFAD-DAD (2025b). Credit: Eyidoğan, 2026, CC BY-NC-ND 4.0
Figure 4. Graphical representation of the longitude-to-time positions of earthquakes with magnitude 4.0 and greater that occurred between July 1, 2025 and April 24, 2026. Focal mechanism solutions are given in Figure 5. Data were obtained from AFAD-DAD (2025b). Credit: Eyidoğan, 2026, CC BY-NC-ND 4.0

 

Figure 4 shows the graph of the longitudes of the epicenter of earthquakes with magnitude 4.0 or greater changing over time. In Figure 4, we see the southeastward migration of the doublet earthquakes and their aftershocks.
 

Focal mechanism solutions of the Sındırgı doublet

The locations of 75 focal mechanism solutions (AFAD-DAD, 2025c) — those of the three major earthquakes of magnitude 6.1, 6.1 and 5.1, and 72 aftershocks of magnitude 4.0 and above — are plotted on a map in Figure 5. The focal mechanisms (beachball diagrams) indicate that northwest-southeast trending normal faults ruptured progressively southeast of the Sındırgı Fault during the earthquake sequence. Taken together, the focal mechanism solutions and spatial distributions of aftershocks suggest that normal faults are developing in the shallow parts of Earth’s crust south of the Sındırgı Fault.

Because focal mechanism solutions provide two fault planes according to the double-couple rule, it is difficult to determine which plane hosted the normal fault movement without finding a visible fault trace in the field. In other words, do the fault planes dip to the southwest or the northeast?
 

Figure 5. This map shows focal mechanism solutions for earthquakes of magnitude 4.0 and greater that occurred around the Sındırgı Fault between August 10, 2025 and April 24, 2026. The base map with focal mechanism solutions is taken from AFAD-DAD (2025c). The inset in the lower left corner shows major tectonic movements in Turkey. Western Türkiye is under the influence of northeast-southwest extension. Information regarding the focal mechanism solutions for the 75 earthquakes shown here is provided in the appendix at the end of the article. Credit: Eyidoğan, 2026, CC BY-NC-ND 4.0
Figure 5. This map shows focal mechanism solutions for earthquakes of magnitude 4.0 and greater that occurred around the Sındırgı Fault between August 10, 2025 and April 24, 2026. The base map with focal mechanism solutions is taken from AFAD-DAD (2025c). The inset in the lower left corner shows major tectonic movements in Turkey. Western Türkiye is under the influence of northeast-southwest extension. Information regarding the focal mechanism solutions for the 75 earthquakes shown here is provided in the appendix at the end of the article. Credit: Eyidoğan, 2026, CC BY-NC-ND 4.0

 

Additional observations

Several studies have been published recently on the topic of these earthquakes. Their results sometimes conflict.

By integrating field-based geology, seismology and InSAR data, Sözbilir et al. (2026) determined that the initial earthquake and aftershocks were related to newly mapped secondary faults of the Emendere Fault Zone. Coulomb stress transfer modeling in that work showed that the stress from the August 10, 2025 earthquake was transferred southeast, triggering the October 27, 2025 earthquake and potentially loading neighboring faults. The authors suggested that the characteristics of Sındırgı earthquake activity differ from typical dip-slip normal faulting events in Western Anatolia.

Using InSAR and stress inversion methods, Kutoğlu et al. (2026) found that the Sındırgı seismic activity that began on August 10, 2025 does not coincide with a single fault plane, but rather belongs to a tectonic structure composed of previously unmapped fault segments. The authors suggested that the switch from normal faulting during the first mainshock to oblique faulting with strike-slip motion during the second main shock may be related to stress transfer occurring within a complex fault system. The researchers noted that their field investigations in the earthquake zone revealed new surface cracks and fault traces. These observations provide evidence of shallow deformation, but the authors were unable to determine the dip direction.

In other work, Seyitoğlu (2025) argued that the initial mainshock behaved as a low-angle normal (listric) fault, similar to examples found elsewhere in western Anatolia (Eyidoğan and Jackson, 1985), and that this fault extended northward at an angle of 34°. However, using Sentinel-1 InSAR observations and Coulomb stress modeling, Eski and Tepe (2026) determined that a normal fault dipping 41° to the northeast hosted the initial rupture. This finding does not confirm a low-angle listric faulting model. Eski and Tepe (2026) noted that their observations could be explained by a transtensional environment rather than a simple normal fault.

On the other hand, based on the combination of geological, geodetic (GNSS) and seismological data, Tiryakioğlu et al. (2026), stated that the initial mainshock occurred along a northwest-southeast trending normal fault dipping to the southwest — opposite the dip direction reported by Seyitoğlu (2025) and Eski and Tepe (2026).

Outstanding questions

The Simav fault zone classified as a right-lateral strike-slip fault according to Türkiye’s officially published Active Fault Map (MTA, 2025). The oblique normal fault plane solution for the second mainshock, the October 27, 2025 magnitude 6.1 event, indicates a different stress field than the initial mainshock, which had a pure normal fault solution.

The fault plane solutions of 75 aftershocks with a magnitude of 4.0 and above between August 10, 2025, and April 24, 2026, predominantly exhibit normal fault movement. This complex tectonic scenario, described as a “hybrid mechanism” by Sözbilir et al. (2026) necessitates an investigation into whether a northeast-southwest extensional regime is developing in the northwestern region of the strike-slip Simav Fault Zone.

These many normal faults raise the question: Is a new basin being formed and controlled by developing normal faults? If a new basin is developing, what seismotectonic “hybrid mechanism,” or process, determines its formation?

According to the studies conducted after the earthquake, it is clear that the earthquakes occurred on a northwest-southeast trending zone with normal motion, but the direction of fault dip is still debated. Of the studies conducted to date, only Eski and Tepe (2026) indicate that the initial mainshock dipped northeast.
 

Conclusion

In examining the temporal and spatial distribution of earthquakes near the Sındırgı Fault, starting with the magnitude 6.1 mainshock on August 10, 2025, the magnitude 6.1 mainshock 78 days later, I’ve shown how seismic activity migrated southeastward. Other studies show this same behavior via InSAR (Sözbilir et al, 2026; Kutoğlu et al, 2026; Qiao et al, 2026; Eski and Tepe, 2026).

These earthquakes occurred at the northwestern end of the Simav Fault Zone, officially classified as a right-lateral strike-slip fault. The earthquakes indicate an extensional tectonic regime developing along the characteristic northwest-southeast trend of Western Anatolia. However, the dip direction of the faults that hosted the 2025-2026 events remains debated. Additional geological, seismological, and geodetic investigations would help definitively determine dip direction. Furthermore, additional examination of stress transfer (Das and Scholz, 1981; Stein, 1999; King and Cocco, 2001; Riga and Balocchi, 2018) would contribute to a better understanding of the seismotectonics of the region.
 

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