Stress field and megathrust strength in the Western Hellenic subduction system: insights from the 2024 Mw 5.9 Strofades earthquake sequence, Greece.

An offshore M5.9 earthquake occurred on 29 March 2024 in the western Hellenic subduction system near the Strofades Islands. The mainshock and the related sequence occurred during a period of unusually dense onshore broadband seismic station coverage across the Peloponnese, including a temporary station deployment operated by the Ruhr University Bochum, Adria Array temporary stations, and permanent stations from the Hellenic Unified Seismological Network. Here we present a study of the seismotectonic context of the M5.9 sequence that capitalizes on the dense coverage and its fortuitous location to investigate subduction dynamics in the region, including interactions between the upper and lower plates and the strength of the megathrust. We compute high-resolution hypocentral locations and focal mechanism solutions that point to an association of the earthquake sequence with the lower plate. The absence of triggered upper-plate seismicity, together with contrasting stress orientations between the overriding and subducting plates, are consistent with a decoupled stress field between the two plates and suggest a weak megathrust interface.
Our analysis of the distribution of high-precision hypocenter locations and focal mechanism solutions is coupled with an interpretation in the context of local stress field and previously mapped intraslab faults. High-precision hypocenter locations and focal mechanisms indicate rupture on a NNE–SSW striking, left-lateral strike-slip fault within the slab. P- and T-axis focal mechanism orientations differ from those of nearby interplate and upper-plate earthquakes, consistent with the intraslab nature of the sequence and indicative of a distinct stress regime. The stress pattern of the M5.9 earthquake sequence lies approximately orthogonal to the NE–SW shortening direction of the upper plate and reflects arc-parallel shortening within the lower plate, similar to that observed for intermediate-depth earthquakes in the Aegean. The orientation of the intraslab stress field relative to the plate margin suggests that slab rollback controls the intraslab stress regime by reducing horizontal compressional stress normal to the margin. Our results suggest that previously mapped intraslab faults, if present, play a limited role in controlling the intraslab stress field, and that a weak megathrust limits interaction and stress transfer between the lower and upper plates in the shallow portion of the subduction zone.