POSEIDON Project: Seismic Hazards in the western Peloponnese - Ionian Islands Domain

The cruise POSEIDON from 10-22 June 2023, mapped the tectonic structure of the region extending from the western Peloponnese across the Ionian Islands. This is one of the most complex and comparatively little evaluated regions, with demonstrated seismic hazard, in the Mediterranean. The region contains a complex fault system with numerous strands controlling much of the submarine and subaerial relief and with dramatic lateral changes in deformation rates. The fault system has produced large earthquakes, mostly offshore, recorded during the past few decades in the Greek national seismological network. However, the largest recent Mw~6.8-7.0 Kephalonia 1953 event was recorded in few stations available then. This earthquake - possibly the most destructive in recent Greek history - cause the collapse of ~85% of all buildings on Kephalonia Island, ~1k deaths, and ~145k people homeless. However, the 1953 earthquake is poorly understood compared with more recent, albeit less destructive events. The epicentre of the 1953 event is not well constrained, and the location and dimensions of the causative fault are unknown. Likewise, the hypocenter depth of the 1953 event thrust-fault focal mechanism, that occurred E or SE of Kefalonia, is defined from <50 km to <20 km, depending on the analysis. Studies in the islands interpret active shallow thrusting to propose that the 1953 event ruptured the upper 5 km of several shallow faults, but such a rupture cannot explain a Mw~6.8-7.0 even. The goal of POSEIDON is to define the regional fault system structure and kinematics and to place it in the proper geodynamic context that helps understand hazards and eventually evaluate associated risks. The DEM displays a rugged terrain from the Ionian Islands to the Peloponnese Peninsula, with numerous features that indicate active deformation across the entire region. Major submarine tectonic structures around the islands trend from NE-SW to NNW-SSE, similar to the basins and ranges on the islands. Elongated shallow troughs offshore laterally project to the relief trends on the islands, supporting widespread active faulting. A further complication in the research area is that the upper-crust fault system, might be located above, or sole into, a mega-thrust plate boundary fault of the Hellenic subduction zone, inferred -in publications- to dip in a NE direction at ~25-40 km depth under the surface. However, the location of a mega-thrust seismogenic zone is yet not well constrained. POSEIDON seismic data image the tectonic features in the crust and integrated with the high-resolution bathymetry allow to define the main faults across the offshore region in unprecedented detail. Acknowledgements We acknowledge the professional and dedicated work of the National Institute of Oceanography and Applied Geophysics (OGS-Trieste) technical party and the technical party of the Marine Technology Unit (UTM) from the Spanish National Research Council (CSIC) to make the experiment a success. We also acknowledge the professional and dedicated work of the master, officers and crew of the R/V Laura Bassi during the POSEIDON experiment.