Heralds of a Future Eruption? Swarms of Microseismicity beneath the submarine Kolumbo Volcano indicate Opening of near-vertical Fractures exploited by ascending Melts

Abstract

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.

Magma chamber detected beneath an arc volcano with high-resolution velocity images

“…The earthquakes were clustered in several swarms starting with the strongest events at 6-9 depth and migrating upwards, terminating at the base of the LVZ. The swarm propagation was faster than expected for hydrothermal gases diffusing from a point source or for active diking (Schmid et al, 2022). However, the speed and focal mechanisms associated with the swarms remain consistent with melt ascent along pre-existing, subvertical pathways through a rheologically strong layer at 4-9 km b.s.l.…”

“…A recent seismicity study by Schmid et al (2022) shows that the observed anomaly coincides with the tip of a cone-shaped region hosting enhanced seismic activity in the 2006-2007 period. The earthquakes were clustered in several swarms starting with the strongest events at 6-9 depth and migrating upwards, terminating at the base of the LVZ.…”

“…1a). Kolumbo is the most active centre of seismic (Bohnhoff et al, 2006;Dimitriadis et al, 2009;Andinisari et al, 2021;Schmid et al, 2022) and high-temperature hydrothermal activity within the SVF (Sigurdsson et al, 2006;Hübscher et al, 2006;Kilias et al, 2013;Carey et al, 2013;Rizzo et al, 2016). It last erupted in 1650 AD in a highly explosive, tsunamigenic event (Ulvrova et al, 2016) which produced a few km 3 of hydrous, biotite-rich rhyolitic lava (Cantner et al, 2014;Konstantinou, 2020).…”

“…Although a multi-level hydrothermal system extending to the base of the observed anomaly is conceivable, the persistent seismic unrest down to 16 km b.s.l. (Schmid et al, 2022) makes this possibility less likely, as explained below. (Hashin and Shtrikman, 1963).…”

“…However, the speed and focal mechanisms associated with the swarms remain consistent with melt ascent along pre-existing, subvertical pathways through a rheologically strong layer at 4-9 km b.s.l. Schmid et al (2022) interpret this seismic activity as a result of either transport of melt from larger depths to the observed reservoir, or formation of fractures enabling such transport which occurred after the recording period. These findings support our interpretation of a melt-filled reservoir below Kolumbo.…”