Integrated Seismological and Geodetic Analysis of the 2025 Balıkesir–Sındırgı Earthquakes

Western Türkiye, situated on the westward-extruding Anatolian Plate, is one of the most actively deforming regions of the Eurasian-Arabian-African tectonic system. The shear regime of the North Anatolian Fault (NAF) Zone to the north and subduction of the Hellenic Trench to the south together drive significant N–S extension across western Türkiye. This extension is accommodated by major E-W-trending graben systems, including Gediz, Simav, and Menderes, making the region an excellent natural laboratory for studying stress transfer and seismic hazard. This tectonic setting, together with elevated heat flow and locally high crustal permeability, gives rise to a highly complex seismotectonic environment with multiple active fault systems in the Balıkesir–Sındırgı region. In 2025, two earthquakes (Mw 6.1 and Mw 6.0) ruptured the Balıkesir–Sındırgı segment of the Simav Fault Zone (SFZ) in this region, initiating an intense aftershock sequence characterized by Mw 3–4 events and several Mw ≥ 5 shocks. This short, spatially clustered sequence offers an opportunity to investigate the stress transfer, seismic productivity, and coseismic deformation in this complex extensional domain. In this study, to understand these processes better, both historical and instrumental period events are compiled and analyzed to describe the spatio-temporal distribution of earthquakes before and after the 2025 events. Coseismic Coulomb stress changes (ΔCFS) are computed for each mainshock, and the results are compared with the aftershock distribution. A regional ΔCFS analysis is also performed to assess cumulative loading on neighboring fault segments. To evaluate seismic productivity and magnitude–frequency characteristics, a- and b-values are estimated using the Gutenberg–Richter relationship, and spatial variations in b-values are compared with the ΔCFS models. Furthermore, Sentinel-1 SAR images are analyzed with the Interferometric Synthetic Aperture Radar (InSAR) technique to map coseismic deformation and to define the source geometry and slip behaviour. Finally, these results are discussed in conjunction with published seismic velocity, magnetotelluric, and geothermal studies, which together indicate a relatively thin and thermally elevated crust that may facilitate shallow normal/oblique faulting and efficient stress transfer.