Assessing Potential Geo-Hazards Along the Aegean Volcanic Arc – First Results From MULTI-MAREX-2 Expedition (March–April 2025)

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.