Coseismic rupture, aftershock migration and stress triggering in the 2025 Balıkesir, Turkey earthquake sequence

The 2025 Balıkesir earthquake sequence in western Turkey, consisting of an M_W6.1 event on 10 August and an M_W6.0 event on 27 October, occurred within an extensional transition zone between the southern Marmara region and western Anatolia. To investigate the rupture characteristics, seismogenic structure, and earthquake interaction of this sequence, we integrate Sentinel-1 InSAR coseismic deformation, geodetic fault-slip inversion, aftershock relocation, shallow slip deficit (SSD) analysis, Coulomb stress modeling, and regional strain-rate constraints. The August earthquake was characterized by a compact rupture patch with a peak slip of ~1.5 m, concentrated mainly at 8-10 km depth, whereas the October event showed a broader, more distributed rupture pattern with a peak slip of ~1.4 m and principal slip concentrated at 10-12 km depth. Inversions based on the preferred fault planes also fit the observed deformation better than alternative nodal-plane solutions. Relocated aftershocks and slip models together indicate that both coseismic rupture and subsequent seismicity were mainly confined to a vertically limited seismogenic layer in the middle upper crust. SSD estimates show strongly reduced shallow slip for both events, reaching 93% and 87% within the upper 3 km, implying inefficient upward rupture propagation and possible rheological weakening in the shallow crust. Coulomb stress modeling further suggests that stress interaction within the sequence was depth-dependent, and that static stress loading likely contributed to failure in the shallow-to-intermediate seismogenic layer. Regional strain-rate results indicate that the source area is situated in an actively deforming extensional to transtensional setting. Overall, the Balıkesir sequence highlights the importance of depth-dependent rupture behavior, shallow rheological effects, and nonuniform stress interaction in controlling moderate earthquake sequences in continental extensional transition zones, with implications for seismic hazard assessment in western Anatolia.