Managing the progressive awakening of Karthala volcano (Comoros Archipelago) after 14 years of quiescence: lessons learned from a long phase (2021-2023) of unrest.

Karthala shield volcano (Grande Comore) is the most frequently active volcano of the Comoros Archipelago. On a centennial scale, Karthala alternates phases of dominantly eccentric activity, when eruptive fissures open along the rift zones cutting the northern and southern volcano flanks, with phases of dominantly summit activity. As a whole, the volcano alternates periods of intense activity (4 eruptions in the 2005-2007 period) with decade-long periods of total rest. Since 1926, the activity of the volcano has occurred inside or close to the summit caldera, with the notable exception of the 1977 eruption located at a low altitude on the SW flank, close to the village of Tsingani. The last eruption occurred in 2007 in the northern part of the summit caldera.

The Centre National de Documentation et de Recherche Scientifique (CNDRS) of Comoros is in charge of volcanic and seismic monitoring for the three islands of Grande Comore, Anjouan and Moheli. The headquarters of the Karthala Volcano Observatory (OVK) are located on Grande Comore. Geophysical and geochemical volcano monitoring and educational programs are performed in collaboration with international partners, while communication during seismo-volcanic events is performed in collaboration with the Civil Defence and a panel of stakeholders.

Since November 2021, the OVK seismic network has detected the beginning of a new phase of unrest, after 14 years of quiescence. Swarms of deep eccentric seismicity below the western flank and minor shallow seismicity below the summit during June-October 2022 have been associated with significant movement of the western flank of the volcano towards the satellite as detected by satellite radar interferometry. A continuous trend of subsidence is measured inside the summit crater, possibly related to cooling of the 2007 lava lake. Geochemical monitoring has confirmed the absence of major changes in composition or temperature of the summit intra-caldera fumaroles and the occurrence of CO₂ soil emissions mostly focused on the volcano flanks.

On one side, the early detection of a possible dyke injection below the edifice, the progressive awakening of the volcano and the existence of major international cooperation programs (Interreg “Hatari”) have facilitated the rapid consolidation of the scientific and operational framework and have permitted to inform the national authorities and the population regularly. Conversely, the long duration of the alert represents a major challenge in maintaining a permanent and efficient scientific-operational interface able to face the several possible scenarios associated with the reawakening of Karthala volcano.