Florian Schmid, Gesa Petersen, Emilie Hooft, Michele Paulatto, Kajetan Chrapkiewicz , Martin Hensch, Torsten Dahm

The Kolumbo submarine volcano in the southern Aegean (Greece) is associated with repeated seismic unrest since at least two decades and the causes of this unrest are still hardly understood. We present a ten-month long microseismicity dataset for the period 2006-2007. The majority of earthquakes clusters in a cone-shaped portion of the crust below Kolumbo. The tip of this cone coincides with a low Vp-anomaly at 2-4 km depth, that is interpreted as a crustal melt reservoir. Our dataset includes several earthquake swarms, of which we analyse the four strongest in detail. The swarms occupy near-vertical volumes of the crust and together they form a zone of fracturing elongated in SW-NE direction, parallel to major regional faults. All four swarms show a general upward migration of hypocentres and the cracking front propagates unusually fast, compared to swarms in other volcanic areas. We conclude that the swarm seismicity is most likely triggered by a combination of pore-pressure perturbations and the re-distribution of elastic stresses. Fluid pressure perturbations are induced either by obstructions in the melt conduits in a rheologically strong layer between 6-9 km depth or by dynamic triggering from purely tectonic earthquakes. We conclude that the zone of fractures below Kolumbo is exploited by melts ascending from the mantle and filling the crustal melt reservoir. Together with the recurring seismic unrest, our study suggests that a future eruption is feasible and monitoring of the Kolumbo volcanic system is highly advisable.