Abstract
SUMMARYOn 10 May 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 1 yr, 32 earthquakes with magnitude greater than 5 occurred, including the largest event ever recorded in the Comoros (Mw = 5.9 on 15 May 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 (GNSS) stations and Interferometric Synthetic Aperture Radar (InSAR) 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 characterize the initial fracturing phase, the phase of magma intrusion and dyke propagation from depth to the subsurface, and the eruptive phase that starts on 3 July 2018, around 50 d after the first seismic events. The eruption is not terminated 2 yr 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.
Highlights
The volcanic Comoros archipelago (Fig. 1) is composed of four main volcanic islands, from west to east: Grande Comore, Mohéli, Anjouan, and Mayotte, and further east, the Geiser and Leven volcanic banks (Daniel et al, 1972)
More than one year after the onset of the crisis, and after ~2 months of intense seismic unrest at the beginning of the sequence, the seismicity was persisting with a large number of small earthquakes (Bertil et al, 2019) and still ongoing ground deformations are observed by Global Navigation Satellite System (GNSS) and Interferometric Synthetic Aperture Radar (InSAR)
As the Comoros axis is associated with volcanism since 20 Ma and seismically active, while there is no identified seismicity within the Somalia basin and between Seychelles and the Comoros, a significant part of this NE-SW extension between Seychelles and Mozambique could well be accommodated across the volcanic line of the Comoros, through oblique extension along the volcanic axis
Summary
The volcanic Comoros archipelago (Fig. 1) is composed of four main volcanic islands, from west to east: Grande Comore, Mohéli, Anjouan, and Mayotte, and further east, the Geiser and Leven volcanic banks (Daniel et al, 1972). As for example in Afar (Ethiopia) where a phenomenon that began in 2005 involved seismicity organised in clusters, including M > 5 earthquakes: between 2005 and 2010, 14 dike intrusions were identified along a 60 km long rift segment and earthquakes are organised in clusters, i.e. distributed at spatially reduced areas close to the dikes (e.g., Ayele et al, 2007; Grandin et al, 2011) Another example illustrating both seismicity and ground deformation patterns as markers of magmatic phenomenon is the rifting event near the Bárdarbunga volcanic system (Iceland) that lasted from August 16th to September 6th, 2014. We present an analysis of the first year of the Mayotte 2018-2019 seismo-volcanic episode combining both seismological analysis and deformation observations (GNSS and InSAR) before the significant improvement of the initial seismic monitoring network in May 2019 operated through the installation of ocean bottom seismometers (OBS). We propose a model of scenario that explains, with different phases, the spatio-temporal evolution of the seismicity and the geodetic observations
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