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% 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%. 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.

中文翻译：

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科摩罗群岛马约特岛以东的 2018-2019 年地震-火山危机：特殊海底喷发的地震活动和地面变形标志

2018 年 5 月 10 日，在科摩罗火山岛最东端的马约特岛以东，一场史无前例的漫长而强烈的地震危机开始了。人们感受到了数百个事件。在一年的时间里，发生了 32 次 5 级以上的地震，其中包括科摩罗有史以来记录的最大地震（2018 年 5 月 15 日 Mw = 5.9）。地震在空间和时间上都是聚集的。还记录了不寻常的强烈持久单色极长周期事件。从 2018 年 7 月上旬开始，全球导航卫星系统站和干涉合成孔径雷达记录到一个大的漂移，证明了大规模的海上通货紧缩。由于对从危机开始到第一年的地震活动进行了分析，我们描述了大型岩浆事件的发生和演变，与地面变形观测（GNSS 和 InSAR）和建模相比。我们区分并表征了初始压裂阶段、岩浆侵入和岩脉从深度到地下传播的阶段，以及从 2018 年 7 月 3 日开始的喷发阶段，大约在第一次地震事件发生后 50 天。火山喷发两年后仍未停止，异常地震活动的持续存在，其模式自 2018 年夏季以来一直相似，包括呈现周期约 16 秒的谐波振荡的偶发性极低频事件。从2018年7月开始，整个马约特岛以小幅增加的速度向东和向下漂移，直到2018年底达到峰值。在顶点处，平均变形速率为向东224 mm yr-1，向下186 mm yr-1。2019年期间，变形平稳下降，2020年1月不到峰值的20%。埋在同质半空间中的岩浆储层的紧缩模型与数据吻合得很好。模拟储层位于马约特岛以东 45 ± 5 公里处，深度为 28 ± 3 公里，推断的顶点岩浆提取量为 ~94 m3 s-1。在马约特岛下方引入一个与主源相同深度的小型次要源，将拟合度提高了 20%。2019 年，主要来源的比率下降了 5 倍，但次要来源的比率保持稳定。这可能是在火山喷发结束后可能会持续一段时间的深度松弛发生的线索。根据我们的模型，2020 年 1 月从深层水库提取的总体积约为 2.65 平方公里。这是有史以来定量记录的最大的海上火山事件。这场地震-火山危机与科摩罗群岛沿线的张力机制一致。