The 2018–2019 seismo-volcanic crisis east of Mayotte, Comoros islands: seismicity and ground deformation markers of an exceptional submarine eruption

Type Article
Date 2020-10
Language English
Author(s) Lemoine Anne1, Briole Pierre2, Bertil Didier1, Roullé Agathe1, Foumelis Michael1, Thinon Isabelle3, Raucoules Daniel1, De Michele Marcello1, Valty Pierre4, Hoste Colomer Roser1
Affiliation(s) 1 : BRGM, Direction Risque et Prévention, Orléans, France
2 : UMR 8538 CNRS - Ecole Normale Supérieure - PSL Research University, France
3 : BRGM, Direction Géoressources, Orléans, France
4 : IGN- Institut National de l'Information Géographique et Forestière / Service de Géodésie et Métrologie- St Mandé, France
Source Geophysical Journal International (0956-540X) (Oxford University Press (OUP)), 2020-10 , Vol. 223 , N. 1 , P. 22-44
DOI 10.1093/gji/ggaa273
Keyword(s) Indian Ocean, Mayotte, seismic swarm, volcano seismology, ground deformation, magma migration
Abstract

On May 10th, 2018, an unprecedented long and intense seismic crisis started offshore, east of Mayotte, the easternmost of the Comoros volcanic islands. The population felt hundreds of events. Over the course of one year, 32 earthquakes with magnitude greater than 5 occurred, including the largest event ever recorded in the Comoros (Mw = 5.9 on May 15th, 2018). Earthquakes are clustered in space and time. Unusual intense long lasting monochromatic very long period events were also registered. From early July 2018, Global Navigation Satellite System stations and Interferometric Synthetic Aperture Radar registered a large drift, testimony of a large offshore deflation. We describe the onset and the evolution of a large magmatic event thanks to the analysis of the seismicity from the initiation of the crisis through its first year, compared to the ground deformation observation (GNSS and InSAR) and modelling. We discriminate and characterise the initial fracturing phase, the phase of magma intrusion and dike propagation from depth to the sub-surface, and the eruptive phase that starts on July 3rd, 2018, around fifty days after the first seismic events. The eruption is not terminated two years after its initiation, with the persistence of an unusual seismicity, whose pattern has been similar since summer 2018, including episodic very low frequency events presenting a harmonic oscillation with a period of ∼16 s. From July 2018, the whole Mayotte Island drifted eastward and downward at a slightly increasing rate until reaching a peak in late 2018. At the apex, the mean deformation rate was 224 mm yr−1 eastward and 186 mm yr−1 downward. During 2019, the deformation smoothly decreased and in January 2020, it was less than 20 per cent of its peak value. A deflation model of a magma reservoir buried in a homogenous half space fits well the data. The modelled reservoir is located 45 ± 5 km east of Mayotte, at a depth of 28 ± 3 km and the inferred magma extraction at the apex was ∼94 m3 s−1. The introduction of a small secondary source located beneath Mayotte Island at the same depth as the main one improves the fit by 20 per cent. While the rate of the main source drops by a factor of 5 during 2019, the rate of the secondary source remains stable. This might be a clue of the occurrence of relaxation at depth that may continue for some time after the end of the eruption. According to our model, the total volume extracted from the deep reservoir was ∼2.65 km3 in January 2020. This is the largest offshore volcanic event ever quantitatively documented. This seismo-volcanic crisis is consistent with the trans-tensional regime along Comoros archipelago.

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Lemoine Anne, Briole Pierre, Bertil Didier, Roullé Agathe, Foumelis Michael, Thinon Isabelle, Raucoules Daniel, De Michele Marcello, Valty Pierre, Hoste Colomer Roser (2020). The 2018–2019 seismo-volcanic crisis east of Mayotte, Comoros islands: seismicity and ground deformation markers of an exceptional submarine eruption. Geophysical Journal International, 223(1), 22-44. Publisher's official version : https://doi.org/10.1093/gji/ggaa273 , Open Access version : https://archimer.ifremer.fr/doc/00632/74401/