Archaeos-Age Amorgos Fault Prolongation Guiding 2025 Diking into Anhydros Ridge

In the aftermath of the 2025 seismic crisis involving Santorini, the submarine volcano Kolumbo, and the Anhydros Ridge, several studies published earthquake hypocentre distribution maps interpreted as evidence for dike intrusion. Notably, the relocations by Isken et al. (2025) and Lomax et al. (2025) show that hypocentres cluster along the southern Anhydros Ridge. However, the two studies differ in their estimates of hypocentre depths and in their interpretations of how seismicity relates to the south-westward continuation of the Amorgos Fault along the ridge. The Amorgos Fault is well expressed in the bathymetry of northern Anhydros and was responsible for the devastating Mw 7.7 earthquake in 1956. Despite this, neither relocation directly correlates the 2025 seismicity with mapped tectonic faults in the southern Anhydros Ridge. Here we present a joint interpretation of multichannel reflection seismic data acquired during the 2025 MULTI-MAREX-research-cruise-2 (MSM135) aboard RV MARIA S. MERIAN together with reprocessed legacy seismic data from the University of Hamburg. These data reveal that the Amorgos Fault is connected south-westward along the Anhydros Ridge as a sediment filled crestal graben that is not expressed in bathymetry. The graben can be traced along the ridge and is defined by two oppositely dipping normal faults that dissect the ridge and are aligned with the regional extensional stress field. The crestal graben is parallel to the hypocentre alignment proposed by Lomax et al. (2025) and is most clearly developed where Isken et al. (2025) locate the shallowest seismicity close to the seafloor. Core-seismic integration with stratigraphic information from IODP 398 Site U1600 (Preine et al., 2025) indicates that graben opening occurred around 700-800 ka, a time period, in which the Archaeos eruption occurred. No subsequent fault activity is detectable in the seismic data, which have a vertical resolution of ~15 m. These observations suggest that the 2025 dike intrusion exploited a pre-existing zone of structural weakness, highlighting the importance of inherited volcano-tectonic structures in governing magma transport and seismicity in the Santorini–Kolumbo volcanic system. Isken, M.P. et al. Volcanic crisis reveals coupled magma system at Santorini and Kolumbo. Nature 645, 939–945 (2025).Lomax A. et al. The 2025 Santorini unrest unveiled: Rebounding magmatic dike intrusion with triggered seismicity. Science 390, eadz8538 (2025).Preine, J. et al (2025). Data report: core-seismic integration and time-depth relationships at IODP Expedition 398 Hellenic Arc Volcanic Field sites. Texas A & M University.