The seafloor of the southern Aegean Sea is shaped by potentially hazardous Earth processes, including submarine volcanism, active plate tectonics, and mass wasting. The MULTI-MAREX research project of the German Marine Research Alliance (DAM) aims to improve the assessment of geomarine extreme events in the region and to develop mitigation strategies through a living-lab approach. During MULTI-MAREX cruise 2 (RV MARIA S. MERIAN expedition MSM135), nearly 5,000 km of 2D multichannel seismic reflection profiles were acquired, complemented by hydroacoustic and magnetic data as well as geological sampling. Although data analysis is ongoing, several key findings already emerge.

Submarine volcanism: Seismic data calibrated with results from IODP Expedition 398 allow, for the first time, a systematic discrimination between effusive and explosive submarine volcanic products. This approach is applied to the Pausanias volcanic field (Saronic Gulf), where some volcanic edifices initially formed during likely phreatomagmatic eruptions before transitioning to weak explosive or effusive activity. A comparable evolutionary pattern is observed for cones of the Kolumbo volcanic chain, where an initial explosive phase is revealed exclusively by the new seismic data. A dense seismic grid in the eastern Christiana Basin, which hosts 10 volcanic cones beside the Christiana volcano itself, enables a partially dated reconstruction of volcano-tectonic evolution and its links to Santorini and Kolumbo (Hartge et al., this session). Integrated seismic and magnetic interpretation further identifies a previously undocumented submarine caldera south of Milos. The associated phreatomagmatic eruption may have generated the Green Lahar deposits on Milos (T. Cavailhes, pers. comm.). Hydrothermal alteration of volcanic cones is suggested as a potential trigger for flank instability and collapse. A previously unknown submarine crater exceeding 2 km in diameter with collapsed flanks was discovered near Kos. All these observations indicate that explosive submarine volcanism represents a previously underestimated geohazard along the South Aegean Volcanic Arc.

Tectonics: Reflection seismic profiles from the Epidavros Basin provide a revised interpretation of two previously identified NW-SE-striking fault systems. The complex geometry, characterized by alternating dip directions, resembles fault patterns associated with lateral spreading (cf. Friedrich et al., this session). We propose that tephra layers from the early volcanic phase of Methana act as mechanically weak detachment horizons. Ongoing analyses focus on active fault systems surrounding Milos, Kos, Nisyros, and Yali. The investigation of active fault systems around Crete concentrated on the Ierapetra and Messara fault zones where recent tectonics are particularly pronounced. It has been shown that marine seismic and hydroacoustic methods are particularly effective for analyzing tectonic processes due to the high sedimentation rate in marine environments.

Submarine landslides: Submarine mass-wasting processes were systematically investigated offshore Crete (cf. Theden et al., this session). Acoustic mapping enabled the compilation of an integrated geomorphological map, revealing pronounced spatial variability in landslide occurrence. Landslides cluster along parts of the southern Cretan slope and the northern to northwestern flanks of Gavdos, whereas other sectors show a near absence of slope-failure features. These differences likely reflect variations in slope gradient, sediment supply, tectonic activity, and hydrodynamic conditions.

How to cite: Hübscher, C., Dittmers, C., Egelhof, C., Eisermann, J. O., Ford, J., Friedrich, A., Gross, F., Haimerl, B., Hartge, M., Kreh, J., Kutterolf, S., Papazoi, A.-G., Theden, C., Krastel, S., and Party, S.: Assessing Potential Geo-Hazards Along the Aegean Volcanic Arc – First Results From MULTI-MAREX-2 Expedition (March–April 2025), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5822, https://doi.org/10.5194/egusphere-egu26-5822, 2026.

The western Saronic Gulf is part of the active South Aegean Volcanic Arc and hosts the dormant Methana volcanic system and the adjacent submarine Pausanias Volcanic Field. Although Methana last erupted around 230 BCE, ongoing hydrothermal activity and the proximity to densely populated regions, including the greater Athens metropolitan area, motivate detailed seismic investigations. A key prerequisite for the precise location of microseismicity and potentially magmatic seismicity in this region is the availability of accurate regional P- and S-wave velocity models.

Within the framework of the Methana Magmatic Observational Experiment (MeMaX), we densified the regional seismic network to improve event detection, ray coverage and hypocentral resolution. Since 2019, six permanent seismic stations operated by the National Observatory of Athens and the University of Patras have been recording seismicity on Methana and the nearby Peloponnese mainland. In March 2024, this network was expanded by 15 temporary short-period seismic stations deployed across Methana, the islands of Aegina, Agistri, Kyra, and Poros, and the Peloponnese mainland, resulting in a dense network geometry. MeMaX is well suited for local earthquake detection, location and the inversion for seismic velocity models to outline active faults and possible magmatic activity.

Noise analyses indicate low background noise levels at most temporary stations, allowing the detection of low magnitude earthquakes. Using the recorded waveform data, we compile a high-quality dataset of local earthquakes for an enhanced event catalog. We apply machine learning for phase picking (PhaseNet) and robust event association (PyOcto). Hypocenter parameters are determined with NonLinLoc and quality is controlled by sorting out events with too large location uncertainties. The seismic arrival times provide the basis for the inversion of new minimum 1-D P- and S-wave velocity models and corresponding station delay times using VELEST. Numerous starting models are tested to sample the model space and assess uncertainties together with the best-fit models.

The resulting velocity models are used to relocate the seismicity with improved accuracy and to refine the spatial distribution of earthquakes beneath Methana and the western Saronic Gulf. MeMaX thus establishes a robust seismological framework for future high resolution relative relocations, fault imaging, and the investigation of potential deep low frequency seismicity in this part of the South Aegean Volcanic Arc.

This study was supported by grant no. FKZ: 03F0952C of the German Federal Ministry of Research, Technology and Space (BMFTR) as part of the DAM mission “mareXtreme”, project MULTI-MAREX.

How to cite: Föst, J.-P., Ritter, J. R. R., Evangelidis, C. P., Sokos, E., Richter, N., and Reicherter, K. R.: Precise Earthquake Distribution and Seismic Velocity Models in the Western Saronic Gulf, Greece, based on the MeMaX Experiment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11338, https://doi.org/10.5194/egusphere-egu26-11338, 2026.

Tsunamis are among the most impactful natural hazards, yet their rarity results in incomplete historical and instrumental records. Tsunami hazard assessment is therefore strongly affected by uncertainties, mainly related to the source representation. For earthquake-generated tsunamis, the location of future ruptures and their rupture characteristics (geometry, kinematics, slip distribution) is poorly constrained, leading to some subjective choices regarding the source representation. A probabilistic approach allows us to formally incorporate these uncertainties and to calculate the probability that a given tsunami intensity measure will be exceeded at a target location within a specified time window.

The MULTI-MAREX project established two living-labs in Greece, with the purpose of strengthening preparedness and awareness of natural hazards from marine environments. For these two sites, we estimate the offshore hazard from earthquake-generated tsunamis from different source representations. We adopt the regional probabilistic NEAMTHM18 model to select most representative sources based on de-aggregation analysis. These include interface subduction earthquakes, mainly associated with the Hellenic Arc, and both strike- and dip-slip crustal earthquakes distributed over the region. Source geometries are derived from the mean values of established scaling relationships between fault parameters and earthquake magnitude, and alternative scaling relationships. To explore the sensitivity of tsunami hazard estimates to earthquake source variability, we perturb the selected source geometries by considering further alternative scaling relationships and their associated uncertainties, rather than only the mean values.

In addition, we provide a preliminary assessment of the impact of constraining scenarios to a mapped offshore fault in the EFSM20. This provides the basis to verify the effect of including more mapped faults in NEAMTHM18, which is an improvement in principle, provided that faults are well-mapped. This work complements ongoing research in Sicily (within a Transnational Access provided by the Geo-INQUIRE project), where the influence of source scaling laws on both offshore and onshore probabilistic tsunami hazard is explored using nested high-resolution grids. At the MULTI-MAREX sites, offshore-only analyses are performed, yet using much higher resolutions to simulate the offshore propagation.

This work is contributing to enhancing  project tsunami scenario databank by better accounting for source-related uncertaintiest, that finds applications in high-resolution inundation modelling for onshore tsunami hazard and virtual reality modelling.

How to cite: Abbate, A., Babeyko, A., Bayraktar, H. B., Scala, A., Lorito, S., and Kalligeris, N.: Exploring earthquake source uncertainty in probabilistic tsunami hazard assessment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21388, https://doi.org/10.5194/egusphere-egu26-21388, 2026.

Tsunami wave amplification in semi-enclosed coastal basins is fundamentally influenced by resonance effects, as the local geometry and bathymetry determine the hydrodynamic response to an incoming tsunami wave. Previous research studies emphasize that the shape of the basin and its bathymetric features often exert a more decisive influence on the resulting coastal impact than the characteristics of the seismic source itself.  This study investigates the natural oscillation modes of the Messiniakos Gulf, a deep semi-enclosed basin on the peninsula Peloponnese, Greece, to characterise the spatial distribution of tsunami amplitudes from near-by tectonic sources and its implications for coastal hazard assessment.

We employed a Delft3D Flexible Mesh model of the Messiniakos Gulf to determine the natural oscillation modes of the gulf. For this purpose, we analysed a set of tsunami events using the Okada approach, with different source locations and fault parameterisations within the subduction zone of the Western Hellenic Arc. The numerical outputs were validated against background spectra derived from long-term tidal gauge records at Kalamata harbour, located at the north coast of the gulf.

Our results show a high correlation between the observed and simulated spectral peaks, indicating that the resonance periods in the Messiniakos Gulf remain stable across all tested scenarios. This suggests that the local bathymetry and the resulting natural modes have a greater influence on the propagation patterns and spectral distribution of the tsunami energy at the coast than the source mechanism itself.
The results further demonstrate that the impact of a tsunami shows significant spatial variability across the gulf. While the oscillation period remains consistent throughout the basin, energy concentrates at specific coastal areas, and can lead to extreme local wave heights that may even persist for longer time spans than the original wave itself. In contrast, other areas remain relatively unaffected. Identifying these high-amplification zones is essential for hazard assessment, as it provides a basis for local evacuation planning and effective early warning strategies.

How to cite: Gieseking, M., Welzel, M., Schlurmann, T., and Jordan, C.: Tsunami Resonance and Wave Amplification in Semi-enclosed Basins: A case study of the Messiniakos Gulf, Greece, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17921, https://doi.org/10.5194/egusphere-egu26-17921, 2026.

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.

How to cite: Dittmers, C., Hübscher, C., Preine, J., Berndt, C., and Karstens, J.: Archaeos-Age Amorgos Fault Prolongation Guiding 2025 Diking into Anhydros Ridge, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5428, https://doi.org/10.5194/egusphere-egu26-5428, 2026.

The Epidavros Basin in the Saronic Gulf is located in close proximity to active volcanic centers, including the Pausanias volcanic field. The Saronic Gulf is affected by extensional back-arc tectonism predominantly oriented N–S, while evidence for older E–W-directed rifting is also preserved. The Epidavros Basin is bounded to the north and south by NW–SE-striking fault systems. Previous studies have suggested the presence of additional NW–SE-striking fault patterns within the basin interior, which have been mapped and interpreted in differing ways. Within the framework of the MULTI-MAREX project, the MSM135 expedition aboard RV MARIA S. MERIAN in spring 2025 acquired the first high-resolution multichannel seismic reflection data covering the entire basin, enabling a reassessment of the intrabasinal deformation mechanisms and their relevance for submarine geohazards.

The time-migrated seismic data reveal two deformation zones comprising complex extensional fault systems, including listric normal faults, rotational fault blocks, and synthetic and antithetic connecting faults. Prolonged or recurrent growth faulting and recent activity are indicated by an increase in vertical fault displacement with depth, and by faults reaching the seafloor.

Such fault patterns are commonly associated with transtensional deformation and the development of negative flower structures. However, this interpretation is inconsistent with both the regional tectonic framework and the absence of seismological evidence within the Epidavros Basin. The observed fault architecture is consistent with lateral spreading above a mechanically weak detachment layer. We propose Early Pleistocene tephra deposits from explosive Methana volcanism as the primary detachment horizon. Chaotic seismic reflection patterns beneath the faulted sedimentary cover, comparable to tephra facies documented during IODP Expedition 398, support this interpretation. Lateral spreading is likely facilitated by regional NE–SW extension and could promote submarine slope instability, fault-controlled seafloor deformation, and localized mass wasting.

Amplitude anomalies associated with near-vertical pipe structures and laterally confined chaotic zones in the overlying sediments are interpreted as tephra injections, some of which likely extruded at the paleo-seafloor. These features indicate fluid- and sediment-mobilization processes that may further weaken the basin fill.

Due to the presence of a mechanically weak décollement, lateral spreading can be initiated not only by large-scale basement extension but also by earthquake activity, volcanic eruptions, or fluid migration into the weak zone. Our results suggest that lateral spreading above volcanic tephra may represent a previously unrecognized geohazard in the Saronic Gulf, particularly in settings where mechanically competent lava flows overlie mechanically weak tephra deposits. This may be particularly relevant for populated coastal regions located in close proximity to volcanic flanks.

How to cite: Friedrich, A., Hübscher, C., Reicherter, K., Eisermann, J. O., and Gross, F.: Lateral Spreading Above Volcanic Tephra as a Potential Geohazard in the Epidavros Basin (Saronic Gulf, Greece), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5382, https://doi.org/10.5194/egusphere-egu26-5382, 2026.

The South Aegean Volcanic Arc (Greece) is among the most active volcanic systems in Europe and poses an ideal natural laboratory to study the interplay of volcanism and tectonics as drivers of explosive eruptions, earthquakes, submarine landslides and tsunamis. This study focuses on a structurally independent sub-basin in the eastern Christiana Basin, located between the Christiana and Santorini island groups and southeast of the regionally significant Christiana Fault. Although the Christiana-Santorini-Kolumbo volcanic field has been extensively investigated, this basin has not yet been specifically targeted in a comprehensive study.

During the MULTI-MAREX research cruise 2 (MSM135), nearly 640 km of hydroacoustic and 2D multi-channel seismic reflection data were acquired across the eastern Christiana Basin. The MSM135 seismic grid provides increased profile density and signal penetration and establishes a connection with the IODP 398 sites U1591 and U1598. Using the prominent Archaeos Tuff (765 ka) as a marker unit, we updated and harmonised the regional seismostratigraphic model. We refine the estimated volume of the Archaeos Tuff, and map deposits of the Poseidon eruption, providing an initial minimum bulk-volume estimate of 9 km³.

We discovered a syncline, measuring around 8 km in diameter, beneath the almost flat seafloor. The Archaeos Tuff drapes a pre-existing central cone in a W-shaped geometry and reaches a maximum thickness of almost 200 m near the central cone. The syncline accommodates an additional 500 m of post-Archaeos deposits, primarily the Thera Pyroclastic Formation. The infill transitions quickly from an undulating W-shape to a horizontal stratification, indicating short-lived sag-style subsidence. To the northwest, the syncline is bounded by a major fault system, dubbed Thera Fault System, that strikes parallel to the Christiana Fault exhibiting vertical offsets of up to 160 m. Like the Christiana Fault, the Thera Fault System is likely a continuation of the normal faults northeast of Santorini.

The seismostratigraphic model constrains the timing of eruptive and tectonic events, assembled in a comprehensive timeline. We date the activity of at least 10 previously little-considered volcanic cones near the margin of the basin to the Late Pleistocene, based on their relative position between known stratigraphic units. Our findings imply a slow, continuous down-faulting at the Christiana Fault, likely related to the rift extension in the region, whereas the Thera Fault System faulted in two stages of shorter duration. The timing of the subsidence coincides approximately with the first explosive eruption cycle on Santorini.

How to cite: Hartge, M., Hübscher, C., Preine, J., Dittmers, C., Eisermann, J. O., Gross, F., and Kutterolf, S.: Volcano-Tectonic Evolution of the Eastern Christiana Basin (South Aegean Volcanic Arc): Insights from the MULTI-MAREX cruise 2, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5820, https://doi.org/10.5194/egusphere-egu26-5820, 2026.

The Santorini–Amorgos Tectonic Zone in the South Aegean Sea is a major hotspot for marine geohazards, where strong earthquakes, pronounced deformation, and tsunamis interact within an actively extending back-arc setting. The 1956 tsunamigenic Mw 7.5 Amorgos earthquake stands out as the largest instrumented earthquake in the region during the 20th century. While previous focal mechanism analyses have provided a good characterization of the seismogenic source as a NE-striking extensional rupture, little is known about the shallow deformation occurring within the upper kilometer below the seafloor. This shallow deformation associated with this large normal-fault earthquake is of fundamental importance for investigating tsunami triggers.

Previous interpretations of 2D seismic, bathymetric, and ROV data provided first-order insight into the regional tectonic framework, but the geometry and segmentation of the fault system could not be fully characterised due to the sparsely spaced profiles. Here, we present newly acquired high-resolution 3D seismic data, integrated with detailed seafloor mapping to unravel the shallow structural configuration and deformation of the southwestern part of the Amorgos Fault Zone, close to the epicentral area of the 1956 earthquake.

Detailed seismic interpretation and seismic attribute analysis reveal distinct segmentation of the shallow part of the fault system and spatially heterogeneous shallow deformation. Our analyses are aimed at shedding light on the specific shallow rupture patterns that triggered the tsunami and, in particular, determining why there was strong regional variability in tsunami run-up heights reported along the surrounding coasts. Our work will help to improve the understanding of how large normal fault ruptures can generate hazardous tsunamis. 

How to cite: Varotsou, E., Karstens, J., Crutchley, G., Urlaub, M., Berndt, C., Nomikou, P., Pandolpho, B., and Kopp, H.: Shallow structural deformation associated with the 1956 Amorgos Earthquake, Aegean Sea - an investigation from 3D seismic reflection data , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14240, https://doi.org/10.5194/egusphere-egu26-14240, 2026.