Revealing the source mechanism of the August 10, 2025 Sındırgı Earthquake (Mw 6.3) with geological and geodetic evidence

Abstract

On August 10, 2025, a moderate normal faulting earthquake (Mw 6.3) struck near Sındırgı (Balıkesir, Western Anatolia), attracting significant scientific interest due to its epicentral location where two major strike-slip dominated active fault zones in western Anatolia, the NNE-SSW trending İzmir-Balıkesir Transfer Zone and WNW-ESE trending Simav Fault Zone intersect. Moreover, the southward propagation of aftershocks, further south than known faults, suggested complex fault kinematics and interactions between the Sındırgı–Simav and Gelenbe Fault Zones. The focal mechanism solutions of the main shock indicate mainly dip-slip motion with NE-SW extension. The ∼25 km-long Çaygören normal fault, which strikes NW–SE and dips to the southwest, is one of them. Maximum predicted magnitudes are calculated as 6.7 based on fault length. Structural field data in the region also reveal that the youngest stage shows an NE-SW-trending extension with a NE–SW trending σ₃ and a vertical σ₁. GNSS campaign was initiated to quantify the coseismic surface deformation using both continuous and campaign-style observations. The inversion of the GNSS-derived displacements yielded a geodetic moment of 2.78 × 10¹⁸ dyne·cm, corresponding to a moment magnitude of Mw 6.3. Uniform slip model indicated that a buried patch approximately 10.5 km long and 4 km wide ruptured, exhibiting 2.20 m of normal and 0.20 m of right-lateral slip. The analysis of a combination of geologic, geodetic and seismological data confirm that the earthquake originated along the SW dipping and NW-SE trending normal fault, most probably occurred in the 7.4 km deep part of the ÇF and ruptures do not reach the surface.