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The 2018-2019 Mayotte Seismic Crisis: Evidence of an upper Mantle Rifting Event ?
Abstract
The Mayotte Island (Indian Ocean, Comoros archipelago) is facing an exceptional, offshore, volcano-tectonic crisis. It started on May 2018, with a seismic activity eastward of Mayotte and peaked in May - June 2018 with 29 5<M<5.9+ shocks. The shocks migrated both ESE and WNW, along a N115°E trending volcanic ridge (Lemoine et al., 2019). At the ESE tip of the migration, a newly born volcano was discovered by the MAYOBS1 cruise. The crisis has involved ~50 km length of the ridge.
Using OBS deployed since February 2019, we have evidence that earthquakes are occurring in the upper mantle. According to the literature, the lithosphere of the Comoros region is likely of oceanic origin, of Mesozoic age and its Moho lies at a depth of ~15km. Below the Mayotte Island, receiver functions obtained by Dofal et al. (2019) suggest that the Moho interface is 18 km deep. All the ~2500 1.0<M<5.4 relocated earthquakes (February 2018-July 2019) took place at 30- 50km depth. Using improved velocity models, the relocations of the earlier earthquakes of the crisis suggest most of them also occurred at the same depth range.
At present, there are very few regions where earthquakes are documented in the upper mantle. Among them, below the Hawaii Island, many earthquakes seem related to the elastic bending of the lithosphere under the load of the island. Below the continental East African and Baikal Rifts, less than a few tens of earthquakes have been located in the upper mantle, but their mechanical meaning remains unclear. Near Sumatra, in the Wharton basin (WB), in 2012 a sequence of major shocks broke the entire lithosphere. The M8+ events initiated at depth of ~50 km. Seismic experiments in the ~60 Myrs old WB indicate it is cut by lithospheric faults down to 45km. It demonstrates that the upper mantle is capable to generate major brittle ruptures. The Mayotte crisis involves smaller ruptures concentrated along a narrow, 15 km wide and ~50 km long zone, below a volcanic ridge. Furthermore, the principal swarm seems to be well organized in a cylinder shape under a caldera like structure. It is the first time that a rifting event has been documented in the mantle with such a level of detail. Surprisingly, no accurately located shocks were shallower than 20-30 km. The Mayotte crisis is a unique opportunity to study the rheology of the lithospheric upper mantle.
... That experiment was located 100 km southeast of Mayotte (Coffin et al. 1986, instrument 449;Fig. 1b) and extended beyond 10 km depth with a Moho interface 15 km deep (Jacques et al. 2019). The second profile, named 'ADofal', is based on a S-wave velocity (V S ) profile determined from receiver functions (Dofal et al. 2018) using the MAYO temporary station deployed on Mayotte island between 2011 and 2014 (RHUM-RUM project, doi:10.15778/RESIF.YV2011; Fig. 1b). ...
... With this new hybrid 1-D velocity model, the earthquake lie between 25 and 55 km depth (Figs 4a-c). They are in the mantle, below the Moho discontinuity, which has been estimated either at 17 km depth under Mayotte island by Dofal et al. (2018) or at 15 km depth offshore by Jacques et al. (2019). This is very unusual compared to other volcanoes where deep seismicity is usually sparse and of low energy (White & McCausland 2019). ...
... For a reasonable half-space P-wave velocity between 5 and 9 km s -1 , all events are located between 20 and 60 km depth, with an average RMS lower than 1 s and best results obtained with V P = 6 km s -1 . Except in the extreme cases, earthquakes were always located deeper than 15 km (Jacques et al. 2019). ...
The brutal onset of seismicity offshore Mayotte island North of the Mozambique Channel, Indian Ocean, that occurred in May 2018 caught the population, authorities, and scientific community off guard. Around 20 potentially felt earthquakes were recorded in the first 5 days, up to magnitude Mw 5.9. The scientific community had little pre-existing knowledge of the seismic activity in the region due to poor seismic network coverage. During 2018 and 2019, the MAYOBS/REVOSIMA seismology group was progressively built between four French research institutions to improve instrumentation and data sets to monitor what we know now as an on-going exceptional sub-marine basaltic eruption. After the addition of 3 medium-band stations on Mayotte island and 1 on Grande Glorieuse island in early 2019, the data recovered from the Ocean Bottom Seismometers were regularly processed by the group to improve the location of the earthquakes detected daily by the land network. We first built a new local 1D velocity model and established specific data processing procedures. The local 1.66 low VP/VS ratio we estimated is compatible with a volcanic island context. We manually picked about 125,000 P and S phases on land and sea bottom stations to locate more than 5,000 events between February 2019 and May 2020. The earthquakes outline two separate seismic clusters offshore that we named Proximal and Distal. The Proximal cluster, located 10km offshore Mayotte eastern coastlines, is 20 to 50 km deep and has a cylindrical shape. The Distal cluster start 5 km to the east of the Proximal cluster and extends below Mayotte's new volcanic edifice, from 50 km up to 25 km depth. The two clusters appear seismically separated, however our dataset is insufficient to firmly demonstrate this.
... That experiment was located 100 km southeast of Mayotte (Coffin et al. 1986, instrument 449;Fig. 1b) and extended beyond 10 km depth with a Moho interface 15 km deep (Jacques et al. 2019). The second profile, named 'ADofal', is based on a S-wave velocity (V S ) profile determined from receiver functions (Dofal et al. 2018) using the MAYO temporary station deployed on Mayotte island between 2011 and 2014 (RHUM-RUM project, doi:10.15778/RESIF.YV2011; Fig. 1b). ...
... With this new hybrid 1-D velocity model, the earthquake lie between 25 and 55 km depth (Figs 4a-c). They are in the mantle, below the Moho discontinuity, which has been estimated either at 17 km depth under Mayotte island by Dofal et al. (2018) or at 15 km depth offshore by Jacques et al. (2019). This is very unusual compared to other volcanoes where deep seismicity is usually sparse and of low energy (White & McCausland 2019). ...
... For a reasonable half-space P-wave velocity between 5 and 9 km s -1 , all events are located between 20 and 60 km depth, with an average RMS lower than 1 s and best results obtained with V P = 6 km s -1 . Except in the extreme cases, earthquakes were always located deeper than 15 km (Jacques et al. 2019). ...
The Mayotte island (Indian Ocean, Comoros archipelago) is undergoing an exceptional offshore volcano-tectonic seismic crisis which started May 10th, 2018, peaked with a magnitude Mw5.9 on the 15th of the same month, and has produced more than two thousand M4+ events to date (Bertil 2019). To monitor this crisis, since February 2019, several French research institutes combined their efforts to deploy and operate for the first time a unique mixed sea/land seismic network and to quickly process the acquired data. On land, 4 broadband stations from INSU-SISMOB were collocated by IPGS with accelerometric stations from BRGM. An initial 6 OBS deployment from INSU-IPGP facility (short period seismometer, broadband hydrophone and absolute pressure gauge) was recovered in April 2019 and redeployed until October 2019. Height IFREMER microOBS (short-period sensors) were then deployed and recovered monthly.
After each OBS recovery, onboard MAYOBS oceanographic missions, a 24/7 team of operators manually reviewed more than 3000 earthquakes starting from the land stations based catalog maintained by BRGM and BCSF-RENASS, between magnitude 1.0 and 5.4. We used a virtual observatory setup with a central SeisComP3 server and WebObs (Beauducel 2010). Locations were done with NonLinLoc using an hybrid velocity model defined during the first mission, from the first set of 100 manually picked earthquakes (apparent Vp/Vs ratio) and two initial models from the literature (Dofal 2018 and Coffin 1986). With azimuthal gaps less than 100 degrees and location uncertainties below 5km, this combined sea/land network and onboard work has proved essential for accurately monitoring the still evolving crisis. The earthquakes are today clustered in two main areas: a primary annular shaped swarm centered 10 km east of Mayotte with additional diffuse seismicity beneath Mayotte upper slope and a secondary swarm 30 km east of Mayotte. All the events cluster between 25 and 50km depth. Present-day activity beneath the newly discovered volcano, 50 km east of Mayotte, is extremely sparse.
... The majority of the seismic energy is released during the first six weeks of activity [Cesca et al., 2020, Lemoine et al., 2020a. This seismicity starts as a swarm 40 km east of Mayotte and around 30-40 km deep, hence below the Moho, estimated to be around 17 km [Jacques et al., 2019, Dofal et al., 2021. From there, the local and regional networks record a migration of earthquakes southeastwards and upwards, interpreted as magma migration from a deep large reservoir (>10 km 3 ) to the surface [Cesca et al., 2020, Lemoine et al., 2020a, Berthod et al., 2021a. ...
... Following Cesca et al. [2020] and Feuillet et al. [2021], we suggest that the western part of the active area corresponds to the injection point of magma into the surrounding lithosphere, magma that was previously stored in the deep magma reservoir. This segment of the distal cluster remains active (In October 2022), forming the western, nearly E-W oriented part of the distal cluster [Jacques et al., 2019, REVOSIMA, 2022. ...
... The OBSs deployed in February were retrieved, and new ones were released. The data allowed relocating the earthquakes and specifying the location of the seismic swarms Feuillet et al., 2019Feuillet et al., , 2021aJacques et al., 2019;Saurel et al., 2019). Scientists also acquired high-resolution marine geophysical data, studied the water column, and carried out rock dredging operations on the seafloor. ...
... In December 2019, the American Geophysical Union fall meeting hosted a special session dedicated to Mayotte's new volcano discovery in which the scientific results from the first MAYOBS campaigns were presented (e.g., Feuillet et al., 2019;Jacques et al., 2019;Saurel et al., 2019). From our interviews, we understood that some tensions emerged between the authorities and the scientists about one of the poster communications (Poulain et al., 2019), which mentioned a delay of a few minutes between a triggering event due to the volcanic activity and the arrival of a tsunami on land. ...
Population information is a fundamental issue for effective disaster risk reduction. As demonstrated by numerous past and present crises, implementing an effective communication strategy is, however, not a trivial matter. This paper draws lessons from the seismo-volcanic “crisis” that began in the French overseas department of Mayotte in May 2018 and is still ongoing today. Mayotte's case study is interesting for several reasons: (i) although the seismo-volcanic phenomenon itself is associated with moderate impacts, it triggered a social crisis that risk managers themselves qualified as “a communication crisis”, (ii) risks are perceived mostly indirectly by the population, which poses specific challenges, in particular to scientists who are placed at the heart of the risk communication process, and (iii) no emergency planning or monitoring had ever been done in the department of Mayotte with respect to volcanic issues before May 2018, which means that the framing of monitoring and risk management, as well as the strategies adopted to share information with the public, has evolved significantly over time. Our first contribution here is to document the gradual organization of the official response. Our second contribution is an attempt to understand what may have led to the reported “communication crisis”. To that end, we collect and analyze the written information delivered by the main actors of monitoring and risk management to the public over the last 3 years. Finally, we compare its volume, timing, and content with what is known of at-risk populations' information needs. Our results outline the importance of ensuring that communication is not overly technical, that it aims to inform rather than reassure, that it focuses on risk and not only on hazard, and that it provides clues to possible risk scenarios. We issue recommendations for improvement of public information about risks, in the future, in Mayotte but also elsewhere in contexts where comparable geo-crises may happen.
... Because the region around Mayotte was, historically, seismically quiet (Bertil and Regnoult, 1998), local subsurface velocity information was sparse before the 2018 Mayotte activity. Previous models include: 1) a regional P-wave velocity profile, hereafter named the "Coffin" model, from a 1980 active-seismic sonobuoy experiment 100 km southeast of Mayotte (instrument 449, Coffin et al., 1986) (Fig. 1), completed for depths >10 km by a bibliographic study concerning the crustal structure of the Somali basin and the Mozambique channel (Jacques et al., 2019;Saurel et al., 2021a;Leinweber et al., 2013;Phethean et al., 2016); and 2) an S-wave velocity profile (Dofal et al., 2018(Dofal et al., , 2019(Dofal et al., , 2021, hereafter named the "ADofal" model, obtained using receiver functions at a temporary station (MAYO) on Mayotte, deployed as part of the 2012-2013 RHUM-RUM experiment (Barruol et al., 2012) ( Fig. 1). The Coffin model places the Moho discontinuity at 13-15 km depth and infers a constant Vp/Vs ratio of 1.72, whereas the ADofal model places the Moho at~17 km depth and yields a Vp/Vs ratio of 1.66. ...
... The focal mechanism of the May 14th, 2019 M W 4.9 event (GCMT solution), mentioned by Feuillet et al. (2021) and relocated in the deep part of the proximal cluster with our velocity model, is consistent with the reverse outward-dipping motion on the ring faults defined by Geyer and Martí (2014) (Figs. 7, 9). From our relocations, the deeper deflating reservoir responsible for these faults would be located at depths greater than 45 km and would be consistent with the deflating reservoir inferred in previous works (Feuillet et al., 2021;Jacques et al., 2019;Berthod et al., 2021aBerthod et al., , 2021b (Fig. 9). This kind of subvertical ring faults have been observed during caldera collapse events (Acocella, 2007;Filson et al., 1973;Geshi and Oikawa, 2008;Gudmundsson et al., 2016) but these examples concern much shallower structures. ...
In May 2018, a seismically quiet region of the Indian Ocean awoke. More than 130 magnitude 4+ earthquakes were recorded in the first month, including a MW 5.9 event on May 15th, 2018. This seismic activity was later identified as being related to an exceptional underwater volcanic eruption offshore Mayotte Island, which had emitted more than 6.5 km³ of lava by the time of writing. To better constrain the geodynamic processes responsible for the seismic and volcanic activity, a new network of ocean-bottom seismometers and land stations has been deployed around the seismically active region since February 2019. We present here an improved 1D velocity model for the active area and relocations of manually-picked earthquakes using this new model. The best-constrained events image detailed structures within two clusters of seismic activity east of Mayotte. The westernmost, proximal cluster, close to Mayotte's Petite-Terre island, has a “donut” shape horizontally and an “hourglass” shape in depth. The events distribution suggests the presence of a magma reservoir at around 27 km depth, with earthquakes focused along its sides, and a collapsing system underneath, related to the drainage of another, deeper magma storage zone. The distal cluster, focused 30–50 km offshore of Petite-Terre island, highlights the propagation of a dike between 45 and 25 km depth, aligned towards the new volcanic activity on the seafloor. We interpret this cluster as the fluid pathway towards the new volcano and nearby active seafloor lava fields. The improved velocity model also permits more robust daily monitoring of the seismicity using land stations, allowing local authorities to better assess seismic and volcanic hazards and to communicate them to the island's population.
... However, depth distribution depends on the velocity model which is far from being constrained around Mayotte and surrounding areas (e.g. Jacques et al. 2019). Finally, we could state that the locations including more than 10 manually picked seismic phases, are stable, especially horizontally whereas depth distribution should be considered with caution. ...
... Large dashed line: proposition for Moho position (e.g. Jacques et al. 2019from Dofal et al. 2018and Coffin et al. 1986). ...
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.
Geophysical and geological data from the North Mozambique Channel acquired during the 2020–2021 SISMAORE oceanographic cruise reveal a corridor of recent volcanic and tectonic features 200 km wide and 600 km long within and north of Comoros Archipelago. Here we identify and describe two major submarine tectono-volcanic fields: the N’Droundé province oriented N160°E north of Grande-Comore Island, and the Mwezi province oriented N130°E north of Anjouan and Mayotte Islands. The presence of popping basaltic rocks sampled in the Mwezi province suggests post-Pleistocene volcanic activity. The geometry and distribution of recent structures observed on the seafloor are consistent with a current regional dextral transtensional context. Their orientations change progressively from west to east (${\sim }$N160°E, ${\sim }$N130°E, ${\sim }$EW). The volcanism in the western part appears to be influenced by the pre-existing structural fabric of the Mesozoic crust. The 200 km-wide and 600 km-long tectono-volcanic corridor underlines the incipient Somalia–Lwandle dextral lithospheric plate boundary between the East-African Rift System and Madagascar.
A new submarine volcano has been discovered offshore Mayotte, a part of the Comoros volcanic archipelago located between Africa and Madagascar. The edifice arose from the sea-floor following a seismo-volcanic crisis that started in May 2018. This seismo-volcanic activity highlights very deep magma reservoirs and dykes in the East Mayotte volcanic system. Since the crisis, the region has experienced >2000 earthquakes with magnitude ≥3.5 and activity continues today (August 17, 2021). The earthquakes are unusually deep and distributed into two swarms: one 5–15 km east of Petite-Terre at 25–55 km depth and a second 25 km away at 30–50 km depth. Significant subsidence of Mayotte to the East has been assigned to the drainage of a deep magma chamber, inferred to be located 30 km from the coast. However, at present, the earthquake locations and geodetic observations have not been sufficient to image entirely the structure of the volcanic plumbing system. In this study, we construct Vp, Vs, dVp, dVs and Vp/Vs 3D velocity models to assess the deeper structure of the young volcano plumbing system, offshore and East of Mayotte. Using >3000 earthquakes from an ongoing monitoring effort, and a 1D velocity model determined onboard, we jointly inverted for velocity structures, earthquake locations, origin times, and station corrections using LOTOS software. The calculated 3D velocity models highlight a complex volcanic system down to 40 km depth. Specifically, we image 3 interpreted reservoirs, more or less consolidated/old. The main reservoir is located at about 30 km depth and deeper, making it one of the deepest magmatic chamber imaged. The reservoirs are connected by several old crystallized conduits, whose existence could have been influenced by the presence of an old fracture zone, globally oriented N130°, due to a regional strike-slip motion of the lithosphere. Moreover, gas-saturated rock may be present below the currently degassing Horse Shoe structure. We were unable to image connections between the new volcanic edifice and reservoirs or conduits due to a lack of resolution in that part of the study area.
The Mayotte seismic sequence that started on 10 May 2018, with a main shock of magnitude Mw 5.9 on May 15, followed by a major offshore volcanic activity, raises several questions of seismo-volcanic hazards in the Comoros region. The unexpected size and duration of the crisis is an opportunity to reassess the distribution and magnitude of the seismicity near Mayotte Island, but also regionally. We present a comprehensive seismicity catalogue of the region including the Mozambique Channel, the Mozambique coast and Madagascar, based partly on previously published data in the region and partly on unpublished data from local catalogues for the Comoros and Madagascar. Our catalogue extends from 1900 onward with a completeness of magnitude 5.5 until 1980s and decreasing only recently to 4.5 after 2010. It comprises the events of magnitude Mlv ≥3.5 for the seismic sequence of Mayotte from May 2018 to October 2020 as the crisis is still ongoing. Present knowledge of the seismicity, largely partial in distribution and magnitude before 1980, makes the seismic sequence of 2018-2020 an exceptional and unprecedented seismo-volcanic event in the region. We discuss the distribution of seismicity in time and space within the context of the south eastward propagation of the East African rift system towards Madagascar.
On 10 May 2018, an active seismic crisis began on French island of Mayotte, which a year later will be shown to be related to offshore volcanic activity. It affects a vulnerable territory exposed to risks of many kinds (poverty, violence, lack of basic resources). In the absence of known events in human memory, the population is naive with regard to seismic and volcanic hazards. The concern is therefore very strong. In spite of a large number of publications, the communication set up by the main actors of the risk chain does not answer the population's concern. To understand why, we analyse a large corpus of the textual communications (press releases, web pages, scientific bulletins, reports, etc.) issued by the authorities and scientists from May 2018 to April 2021. We draw lessons on the communication strategy put in place in the first three years of the crisis; and we issue recommendations for improvement in the future, in Mayotte, but also elsewhere in contexts where comparable geo-crises may happen. We notably stress the importance of ensuring that communication is not overly technical, that it aims to inform rather than reassure, that it focuses on risk and not only on hazard and that it provides clues to possible risk scenarios.
On 2018 May 10, a seismic crisis started ~50 km east of Mayotte, the easternmost of the Comoros volcanic islands. Here we analyze its first six months, from 2018 May 10 to November 14. In that period, 29 earthquakes with magnitude greater than 5 occurred, the main one reaching Mw = 5.9 on 2018 May 15. In mid November, the crisis was continuing with the persistence of an unusual seismicity including episodic tremors and steady anomalous velocities measured by GNSS. The seismicity shows three successive clusters, overlapping in time and space. The coordinates of the GNSS stations also evolve in three phases, with moderate deformation during two weeks at the beginning of the crisis, quiescence from early June to early July, and the main deformation phase starting around July 10. While models of seismic dislocation cannot fit the GNSS motions, a model of deflation of magma reservoir buried in a homogenous half space fits them very well. From mid of July the deformation is steady and strong with the whole Mayotte drifting at 16 ± 1 mm/month towards east and subsiding globally at 9 ± 1 mm/month. The deflating magma chamber inferred by the model is located 45 ± 5 km east of Mayotte, at a depth of 28 ± 3 km. The rate of magma release is 82 m3 s-1 which means more than 1 km3 emitted during the first six months of the crisis. The magma might have reached the seafloor at a single point, producing a new seamount (there are others visible in the bathymetry offshore Mayotte) or along of volcanic fissure. Yet, due to the sediments thickness around Mayotte, the magma might not have reached the seafloor but instead have flown at the base of the sediments. Because of the pressure of the water (2 to 3 km depth) in the eruption area, the volcanic gasses remain in the magma. More observations at sea and seafloor in the near field are needed to constrain the characteristics of this event which could be the major submarine eruption ever documented quantitatively.
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