The 2025 Balıkesir Sındırgı (Türkiye) Mw 6.1 Doublet: Insights from InSAR and Seismology

The recent 2025 sequence that occurred near Sındırgı, a town in Balıkesir (western Türkiye) started with a Mw 6.1 earthquake on August 10^th (Sunday, 19:53 local time) on the westernmost part of the Simav graben. The normal faulting event was initially reported to be on the Sındırgı fault since the epicentre was located along its surface trace. As per the Turkish Active Faults Database, the fault is considered as one of the seven active segments of the Simav Fault Zone. Initial coseismic models calculated using the Sentinel-1 radar images acquired 24 hours after the earthquake revealed that the event could not have occurred on the proposed Sındırgı fault but on an unknown fault either to its south or its north. However, it also became evident that fault plane ambiguity could not be resolved using InSAR alone.

To aid in resolving this ambiguity and to monitor the distribution of the aftershocks TÜBİTAK Marmara Research Center’s Earth Sciences Research Group installed a temporary 16-station seismic network in the area. Using artificial intelligence techniques the spatiotemporal evolution of the seismic activity was determined using >30.000 relocated aftershocks. The seismic data favors the north dipping fault plane which intersects the surface about 7 km south of the Sındırgı fault.

A second Mw 6.1 event took place about two months later on October 27^th (Monday, 22:48 local time).  Both InSAR and the aftershocks distribution clearly exhibit that the event had occurred this time on a portion of a known fault to the east of the first mainshock. The coseismic models validate the strike slip dominant nature of the faulting that took place again within a depth range of 5 to 12 km on a ~60° south dipping fault.

The two earthquakes are the biggest to occur along the Simav Fault Zone since the 1970 M7.1 Gediz earthquake. In this study, the spatiotemporal evolution of the sequence will be discussed using both InSAR time series and seismic data as well as the elevated seismic hazard in the region where the activity was still continuing as of January 2026.