Recent Earthquake Activities and Seismic Sources in Northwestern Türkiye ^†

Abstract

Northwestern Türkiye (Biga Peninsula, Edremit Gulf, Saros Bay) is a highly seismically active region at the convergence of the Anatolian, Eurasian, and Aegean tectonic plates. It features numerous active faults, including the Yenice–Gönen and Edremit fault zones, in addition to offshore segments of the North Anatolian Fault Zone (NAFZ) in Saros Bay. Earthquakes here exhibit various mechanisms: the 2017 Ayvacik earthquake sequence (Mw 5.4) near the Tuzla Fault featured NE–SW normal faulting, reflecting regional extension. Other moderate events display strike–slips with minor normal components, indicating transtensional forces. These findings enhance our understanding of the area’s complex seismotectonic activity and stress the critical need for continuous seismic monitoring and hazard assessment in this geologically complex and densely populated part of Türkiye.

1. Introduction

Türkiye is characterized by a tectonic window, i.e., an area of intense and current earthquake activity, generated by active faults exhibiting various dynamic characteristics. This tectonic environment is further evidenced by Türkiye’s categorization as one of the most active earthquake belts worldwide. The Anatolian Block is clearly subject to sustained earthquake forces resulting from tectonic interactions between the African Plate to the southwest, the Arabian Plate to the southeast, and the Eurasian Plate to the north. Rupture zones, exemplified by the North Anatolian Fault Zone, the East Anatolian Fault Zone, and Aegean tectonic rupture regime, have been the causative agents of a substantial number of significant destructive earthquakes in both the historical and instrumental periods. The recent Sindirgi–Balıkesir earthquake (Mw = 6.1) of 10 August 2025 occurred in Western Anatolia, with significant regional impacts. This event once again emphasized the urgent need to improve seismic hazard assessments in Northwestern Türkiye; moreover, it demonstrated that the seismotectonic setting of the Biga Peninsula and its surrounding regions continues to pose a considerable risk to densely populated areas, underlining the importance of updated seismic monitoring and evaluation efforts.

Given the theme of the congress, the findings presented here not only provide a scientific basis for understanding active tectonic structures but also have direct implications for disaster preparedness, infrastructure resilience, and regional planning strategies in earthquake-prone areas. As articulated by the authors of [1,2,3,4], the active faults within the Biga Peninsula have been thoroughly delineated. In this area, the faults forming the regional stress regimes are predominantly laterally thrust faults with a normal component. The area under consideration is characterized by the presence of numerous NE–SW strike–slip faults that are oriented parallel to each other. In the Biga Peninsula, the southern tributary reaches the Aegean Sea via Gönen, Yenice, and Edremit Bay, whereas the central tributary passes through Sarikoy, Can, Bayramic, and Ezine. The 1953 Yenice–Gönen earthquake (M = 7.4, KOERI) resulted in a 3.5 m right-lateral movement along the southern branch and an NE–SW-trending lateral strike–slip rupture zone. The Yenice–Gönen earthquake on 3 March 1969 (M = 5.8, KOERI) exhibited a predominant thrust component, suggesting that the Biga Peninsula experienced uplift through this mechanism [5].

2. Seismicity

The 2023 Kahramanmaras earthquake sequence (Mw 7.7 and 7.6) profoundly impacted Southeastern Türkiye, causing widespread destruction and drawing global attention to the seismic hazard threatening the country. The lessons learned from this event, including those surrounding cascading rupture behavior, aftershock distribution, and infrastructure vulnerabilities, are highly relevant in our understanding of the potential risks for Northwestern Türkiye. Similar to the Eastern Anatolian Region, the northwestern part of the country is characterized by complex fault interactions and the potential for multi-segment ruptures, which makes comparative evaluations crucial. Integrating insights from both historical and recent destructive earthquakes into seismic hazard assessments ensures that regional models can remain robust and reflective of present-day realities. The Biga Peninsula is in a region of Türkiye that is of particular significance in the context of historical and instrumental seismicity and tectonic characteristics. The region has been subjected to the influence of various tectonic activities in different periods and has been active in terms of earthquakes in the past and present.

A review of the historical data reveals that a series of significant seismic events transpired in the years 155, 170, 253, 484, 1737, 1855, and 1875, coinciding with specific phases within the instrumental period. This phenomenon is further exemplified by events such as the Saros Bay–Murefte earthquake in 1912, with a magnitude of 7.3 (Mw), the 1935 Erdek Bay earthquake, with a magnitude of 6.4, and the 1935 Can–Biga earthquake, with a magnitude of 6. The 1944 Edremit Bay–Ayvacik earthquake (M = 6.8), the 1953 Yenice–Gönen earthquake (M = 7.2), the 1964 Gönen earthquake (M = 5.8), the 1971 Edremit–Bakircay earthquake (M = 5.5), the 1983 Biga earthquake (M = 5.8), the 2006 Kusgolu–Manyas earthquake (M = 5.2), and the 2006 Bandirma earthquake (M = 5.0) are indicative of significant seismic activity in and around the Biga Peninsula. In Figure 1, historical seismic events with magnitudes greater than or equal to 6.5 on the Richter scale are displayed for the range of IX–XII. In Figure 2, the epicentral distributions of the earthquakes of the instrumental period following 1900 are illustrated.

Edremit Bay is widely regarded as a tectonically active graben that developed during the Neogene–Quaternary period in the northwestern region of Anatolia. This geological formation is bordered by the Sakarya continent. In the evaluation of seismic profiles taken in Edremit Bay, strong reflections belonging to a thick accumulation basin and the presence of faults cutting this basin were discussed in [8]. The present study posits that the low-angle faults oriented in the NW and SSE directions are shaped by the effect of the compressional regime. Furthermore, the dominant synthetic and antithetic faults in the region develop parallel to the basin boundaries. Conversely, the authors of [8] proposed that the Edremit Gulf is not solely influenced by lateral thrust faulting or the conventional characteristics of the North Anatolian Fault. Despite extensive research into tectonic development and faulting patterns in the northwestern Aegean region, including the Ayvacik, Biga, and Gallipoli peninsulas, there is a paucity of similar studies. The authors of [9,10,11,12,13] indicated that the Edremit Gulf is an asymmetric wedge-shaped depression area formed in the gulf region where the North Anatolian Fault enters the Northwest Aegean Sea [12,13]. One of the two primary fault systems is a normal oblique NW–SSE-trending fault between the northern shores of Edremit Bay and the island of Lesbos, which extends toward the Aegean Sea. The other fault system is responsible for the formation of the depression between Ayvalik and Lesbos.

Intense earthquake activity began in Ayvacik and its immediate vicinity at the southwestern tip of the Biga Peninsula on 14 January 2017, with a magnitude of 4.6, continuing on 6 February 2017 with magnitudes of 5.5 (Mw = 5.5, KOERI), 5.3 (AFAD), and 5.4 (Mw = 5.4). To monitor this activity, broadband seismometers were temporarily installed on the Ayvacik Peninsula. Considering the atypical seismic activity observed in the region in recent years, the objective was to enhance the monitoring and evaluation of this activity through the deployment of these temporary stations in conjunction with the earthquake stations comprising the established national networks (see Figure 3).

3. Tectonic Framework and Earthquake Sources

The Biga Peninsula is primarily within the extension of the southern branch of the right-lateral strike–slip North Anatolian Fault in terms of active tectonic elements. The northern branch of the North Anatolian Fault passes through the Saros Gulf north of the peninsula. The Ganos fault is an important strike–slip fault on land here. Its extension in the Saros Gulf especially defines the earthquakes here. South of the peninsula, important normal faults surround the Edremit Fault and the Ayvacik Fault. Far southeast of the peninsula, the eastward extension of the Edremit Fault is the Balıkesir–Havran Fault (Figure 4). Many large and small earthquakes have occurred along these fault lines in the last century and throughout history. These segments are also shown as active faults on Türkiye’s fault map [4,14]. The focal mechanism solutions (Figure 5 and

Table 1. Earthquake Source Mechanism (ESM) parameters resulting from the inverse solutions of the moment tensors of 20 earthquakes.

e/q No.	Origin Times	Latitude N° Longitude E°	Mw	h (km)	Main Fault Plane Strike° Deep° Rake°			ESM
1	12.04.2020 05:05:20.41	39.54 25.93	3.3	12	273	50	−110
2	27.04.2020 17:58:00.05	39.59 26.15	3.5	8	113	41	−88
3	06.05.2020 03:08:38.08	39.59 26.13	3.4	10	122	37	−76
4	04.06.2020 12:14:48.87	40.28 26.12	3.1	9	311	43	−20
5	04.07.2020 03:13:22.41	40.24 26.30	3.1	12	345	49	−28
6	30.08.2020 20:57:13.11	39.47 26.30	3.2	6	340	48	−88
7	11.12.2020 03:03:54.32	39.36 26.95	3.1	14	306	49	−74
8	22.01.2021 18:41:58.02	39.76 26.77	3.3	8	340	77	−25
9	10.02.2021 14:48.00.28	39.56 25.99	3.1	10	291	54	−93
10	15.03.2021 05:55:39.88	39.33 26.14	3.1	12	313	59	−87
11	05.05.2021 06:42:11.05	40.35 27.40	4.1	6	151	50	−31
12	12.07.2021 21:27:34.88	40.02 27.41	3.4	7	290	70	−35
13	07.09.2021 14:15:58.21	40.13 27.35	3.1	5	320	55	−27
14	05.09.2021 11:13:50.87	39.53 26.89	3.4	10	125	60	−53
15	15.09.2021 04:10:20.02	39.32 26.12	3.9	14	295	56	−81
16	20.10.2021 22:09:19.18	39.43 26.32	3.4	8	205	24	−98
17	10.11.2021 05:53:23.89	39.78 26.10	3.1	4	305	55	−35
18	15.01.2022 10:15:32.17	39.38 26.22	3.1	8	195	51	−92
19	04.01.2022 16:14:18.24	39.40 26.25	3.1	8	303	41	−86
20	22.04.2022 06:07:57.22	39.56 26.43	3.4	12	331	66	−24

) of the 20 earthquakes that occurred within the Biga Peninsula are examined by considering the active elements in the region. Examining the focal mechanism solutions (Figure 5) of these 20 earthquakes, and considering the active elements in the region, reveals that earthquakes 1, 2, 3, 6, and 9—the largest of which had a magnitude of Mw = 5.3 on 6 February 2017—occurred in an area that was hit by a series of earthquakes between 14 January and 26 March. The Tuzla, Kocakoy, and Babakale normal faults run through this area from northeast to southwest and northwest to southeast [15]. These five earthquakes occurred on these faults, and their locations are consistent with the extent of the faults. Earthquake number 11 occurred on the Karabiga fault, a right-lateral strike–slip fault extending approximately 15–20 km.

Earthquake number 13 is located on the right-lateral strike–slip Sarıköy–Inova fault, which has a NE–SW strike–slip motion. Earthquake number 12 occurred on the Yenice–Gönen Fault, which is a right-lateral strike–slip fault with a NE–SW strike in the western segment. Earthquake numbers 10, 15, and 19 occurred on normal faults that trend NW–SE and north, bound for the northern part of Lesvos Island. On the other hand, earthquakes 16 and 18 occurred in almost the same region and are faults that intersect the Mytilene Fault at right angles. These are local earthquakes. Earthquake No. 14 is located at the eastern end of the Edremit fault, a south-trending normal fault running in an approximately north–south direction. Earthquake number 7 is located west of the Balıkesir–Havran fault system at the southernmost tip of the Biga Peninsula. It is a normal fault that is distant from active tectonic elements and consistent with the main regional extension direction. Earthquakes No. 4 and No. 5 are located on two right-lateral strike–slip secondary faults that are consistent with the North Anatolian Fault, which bounds the northern part of the Gallipoli Peninsula and extends into the Saros Gulf. Earthquake number 8 is located at the southwest end of the Pazarköy–Hamdibey–Kalkım strike–slip normal fault, which trends approximately northeast–southwest and has a right-lateral strike–slip component. Earthquake number 20 is a south-trending, strike–slip, normal fault with a prominent right-lateral, strike–slip component. It is located just north of the Edremit fault and is consistent with it. Earthquake 17 is located southeast of Bozcaada. Another strike–slip fault compatible with the extension and characteristics of these faults is located just south of the Troia Fault System [16]. In Table 1, the blue-marked earthquakes (1, 4, 5, 7, 8, 11, 12, 13, 14, 17, 20) represent strike–slip faulting, and the red-marked earthquakes (2, 3, 6, 9, 10, 15, 16, 18, 19) represent normal faulting.

Figure 5. Reproduced map for locations and focal mechanism solutions of earthquakes with magnitudes greater than 3.1 that occurred between 2020 and 2022. The events that were analyzed are plotted with lower hemisphere stereographic projection after [17].

Figure 5. Reproduced map for locations and focal mechanism solutions of earthquakes with magnitudes greater than 3.1 that occurred between 2020 and 2022. The events that were analyzed are plotted with lower hemisphere stereographic projection after [17].

4. Results and Conclusions

The Sindirgi–Balıkesir earthquake (Mw = 6.1, KOERI) of 10 August 2025 provides a striking reminder of the region’s seismic hazard. Its occurrence underscores the necessity of integrating new seismic data into ongoing monitoring systems and re-evaluating risk models for Northwestern Türkiye. This study’s findings, when viewed in light of this most recent destructive event, demonstrate that comprehensive and continuous research remains indispensable. The integration of continuous monitoring systems, updated hazard maps, and lessons from recent destructive earthquakes provides essential guidance for reducing future earthquake risks in Northwestern Türkiye. In terms of active tectonic elements, the Biga Peninsula lies mainly within the extension of the southern branch of the right-lateral strike–slip North Anatolian Fault. The northern branch of the North Anatolian Fault passes through the Saros Gulf north of the peninsula. The Ganos fault is an important strike–slip fault on land. Its extension in the Saros Gulf defines the earthquakes here. The Edremit Fault and Ayvacik fault are surrounded by significant normal faults in the southern part of the peninsula. Accordingly, the Biga Peninsula and its northern and central regions are under the influence of a compressional regime ranging from NW–SSE to NE–SW-SSE, which is active today. Earthquakes present a strike–slip faulting mechanism. The active structures in the region generally trend NE–SW or NW–SSE and are right-lateral strike–slip faults.