Insights into stress-loading mechanisms in the western Peloponnese, Greece from a high-resolution earthquake catalog

Abstract. The western Peloponnese exhibits a continuum of earthquake faulting styles that reside in a complex stress field near the western termination of the Hellenic Subduction System. Here we present a detailed study of recent seismicity in western Peloponnese, including the spatiotemporal distribution of earthquake sequences and their clustering and migration properties to infer possible driving mechanisms. We build a detailed earthquake catalog from December 2023 – September 2024 with a magnitude-of-completeness M_c ~1 and a location precision on the order of 100 s of meters by combining new data from temporary seismic station deployments with publicly available data from permanent stations. Catalog statistical and clustering analysis shows increased background seismicity rates and seismic moment release in the northern part of the study area that is consistent with larger strain rates and a higher stressing rate reported from geodetic data. The seismicity distribution and focal mechanism solutions suggest that the predominantly north-south extensional regime in the north changes to east-west extension in the central-western Peloponnese near the town of Zaharo. Nearest-neighbor cluster analysis reveals mainshock-aftershock-type sequences in the northwest near the town of Vartholomio that are consistent with tectonic loading. Conversely, clustering properties in the central-western Peloponnese near Zaharo are consistent with swarm-like sequences driven by external forcing, such as pore-fluid pressure changes, and potential aseismic slip. Independent studies of slab dewatering suggest fluid-driven pore-pressure gradients that might be responsible for migrating seismicity and swarm-like behavior in the upper plate.