Subduction zones are among the most seismically active and mechanically complex regions of the Earth. Variations in earthquake size distribution with depth provide important constraints on stress conditions and deformation mechanisms within subducting slabs. The Gutenberg–Richter b-value is a widely used statistical parameter that reflects the relative proportion of small to large earthquakes and is sensitive to changes in stress regime, material properties, and faulting behaviour. In this study, depth-dependent seismic b-value variations are examined near subduction slab tips, where slabs reach their maximum penetration depths and experience significant mechanical and rheological changes. A homogeneous earthquake dataset, covering the period 2004–2025, was analyzed for four representative subduction zones: Izu–Bonin, Mariana, Chile, and Indonesia. After declustering the catalogs using the Reasenberg algorithm, b-values were estimated by the maximum likelihood method for successive depth intervals. The results show a systematic decrease in b-value with increasing depth near slab tips, particularly below approximately 300 km. These patterns are interpreted in terms of increasing effective stress, dehydration-related processes, and rheological transitions within the subducting lithosphere. The results indicate that depth-resolved b-value analysis provides useful constraints on seismic behaviour and stress evolution near subduction slab tips.

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