Mayotte, Seismo-volcanic Crisis - SEISMIC TEAM

In 2019, a new underwater volcano was discovered at 3500m below sea level (b.s.l.), 50 km east of Mayotte Island in the northern part of the Mozambique Channel. In January 2021, the submarine eruption was still ongoing and the volcanic activity, along with the intense seismicity that accompanies this crisis, is monitored by the recently-created REVOSIMA (MAyotte VOlcano and Seismic Monitoring) network. In this framework, 4 hydrophones were moored in the SOFAR channel in October 2020. Surrounding the volcano, they monitor sounds generated by the volcanic activity and the lava flows. The first year of hydroacoustic data evidenced many earthquakes, underwater landslides, large marine mammal calls, along with anthropogenic noise. Of particular interest are impulsive signals that we relate to steam bursts during lava flow emplacement. A preliminary analysis of these impulsive signals (10 days out of a year, and only 1 day in full detail) reveals that lava emplacement was active when our monitoring started, but faded out during the first year of the experiment. A systematic and robust detection of these specific signals would hence contribute to monitor active submarine eruptions in the absence of seafloor deep-tow imaging or swath-bathymetry surveys of the active area.

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.

A new submarine volcano has been discovered offshore Mayotte, a part of the Comoros volcanic archipelago located between Africa and Madagascar. Regarding this seismo-volcanic activity, a 3-D passive tomography was conducted, in order to determine the structure of the volcanic plumbing system (Foix et al., 2021). Using > 3,000 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 . We are sharing here the 3-D P and S velocity models and 3-D output earthquake locations.

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.

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.

Volcanic eruptions shape Earth’s surface and provide a window into deep Earth processes. How the primary asthenospheric melts form, pond and ascend through the lithosphere is, however, still poorly understood. Since 10 May 2018, magmatic activity has occurred offshore eastern Mayotte (North Mozambique channel), associated with large surface displacements, very-low-frequency earthquakes and exceptionally deep earthquake swarms. Here we present geophysical and marine data from the MAYOBS1 cruise, which reveal that by May 2019, this activity formed an 820-m-tall, ~5 km³ volcanic edifice on the seafloor. This is the largest active submarine eruption ever documented. Seismic and deformation data indicate that deep (>55 km depth) magma reservoirs were rapidly drained through dykes that intruded the entire lithosphere and that pre-existing subvertical faults in the mantle were reactivated beneath an ancient caldera structure. We locate the new volcanic edifice at the tip of a 50-km-long ridge composed of many other recent edifices and lava flows. This volcanic ridge is an extensional feature inside a wide transtensional boundary that transfers strain between the East African and Madagascar rifts. We propose that the massive eruption originated from hot asthenosphere at the base of a thick, old, damaged lithosphere. An ~5 km³ volcanic edifice offshore Mayotte formed between May 2018 and May 2019 by rapid magma intrusion through the entire lithosphere, according to an analysis of marine observations and geophysical data.

The volcano-seismic crisis afflicting Mayotte since May 2018 has motivated France-based seismologists to consider the installment of a permanent seafloor observatory with one or more seismometers for monitoring surfacing magma and the associated seismicity. In general, deploying a seismometer offshore is known to improve earthquake location – particular in depth – and lower magnitude detection. However, how true are these claims for Mayotte when a land-based seismic network already exists? To address this, we investigate location and detection performance when deploying permanent seismometers offshore Mayotte. We modeled location and detection performance using both real and synthetic data in different network configurations. We found that, in the case of Mayotte, only longitude error is significantly reduced by adding seismometers offshore, perhaps due to the North-South configuration of the land network. Moreover, the size of the Mayotte volcano monitoring area, which spans depths and distances up to 50 km for both, prevents accurate location and detection performance with less than 2 permanent seismometers offshore. Therefore, we would need at least 2 cabled seismometers to monitor this volcanic system, i.e. locate and detect events in real-time. Overall, our modeling suggests that a one-side land network can perform relatively well by itself in location (errors <5 km) and detection (magnitude >1.3) so long as the seismicity occurs at epicentral distances and depths <20 km. However, beyond this distance, one or more seafloor seismometers would be needed to improve location and detection performance.

Since May 2018, a major volcano-seismic crisis has been happening offshore Mayotte Island, in the Comoros archipelago. With 5.8 km 3 of magma emitted from a deep reservoir, intense seismic activity and deformation that hasn't stopped after more than 2 years, this crisis is unprecedented and represent the largest basaltic eruption in erupted volume and duration since the Laki eruption (Iceland) in the 18th century. Since May 2019, several oceanographic campaigns deployed a dense network of Ocean Bottom Seismometers (OBS), associated with previous and new land stations, around the newly active region (https://doi.org/10.18142/291). The OBS were installed in 4 months periods, with in total 71 OBS and land stations between February and November 2019 and up to 25 for each period. This network was able to detect two clusters of earthquakes: a main group located 5 to 15 km offshore Petite-Terre, at depths between 20 and 55 km, and a secondary cluster between 25 and 50 km offshore, at depths ranging from 40 km to 15 km, the shallowest events closer to the new volcanic edifice (NVE). The main cluster is focused in a circular « donut » shape, potentially related to the roof collapse of a deep reservoir caused by the withdrawal of magma, while the secondary cluster is focused in a N130-elongated seismic structure, probably related to movements of fluids from the deep reservoir to the NVE and other active vents. These results show that Mayotte ongoing eruption provides exceptional observations on the development and evolution of a volcanic seamount and its associated seismicity. To place even better constraints on the magmatic, hydrothermal and tectonic processes occurring around the seismically active area, we use the best located events from a dataset of more than 4000 manually-picked earthquakes to develop a new 1D velocity model for the area and to relocate all events. The new velocity model greatly improve earthquakes' locations, with better RMS and ellipsoidal errors, particularly for the secondary cluster. The relocation of both clusters with this new model show more detailed seismic structures. The new model also provides better locations of the events without the OBS data, as used for the continuous daily monitoring, greatly improving the day-today hazard assessment for Mayotte exposed population.

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.

Volcanic eruptions are foundational events shaping the Earth's surface and providing a window into deep Earth processes and composition. Most eruptions occur on established volcanoes, exploiting longstanding magma reservoirs and pathways. Those creating new volcanoes are rare and usually too small or too remote to be well monitored, particularly in the offshore domain. We document here a lithosphere-scale magmatic event giving birth to a 5km3 submarine volcano offshore Mayotte island (Western Indian Ocean). This event is associated with large surface displacements and unusually long-lasting and intense seismic activity. Starting in May 2018, hundreds of earthquakes were a felt by the population, whose anxiety increased by the day. As part of a response program for crisis management, high-resolution geophysical, seismological and geodetic data were acquired onshore and offshore. The newborn volcano was discovered by comparing multibeam bathymetry acquired in 2019 to older data. The volcano has only been growing for less than a year and is already 820m tall. Popping rocks dredged on its flank suggest that the melt transferred rapidly from the upper mantle, without storage in intermediate reservoirs. Four main fluid plumes more than 2000 m-high emanate from the volcano summit, attesting to its intense activity. The volcano lies at the eastern tip of a N110° E striking volcanic ridge, made of dozens of older volcanic edifices. An ocean bottom seismometer deployment improved earthquake and tremor locations, revealing two unusually deep (25 to 50 km) seismic swarms along the ridge, and, at shallower depths, the source of many very long period (15s) seismic signals showing an exceptionally long duration (1000s). The new volcano may have erupted as a result of lithosphere-scale rifting process involving the drainage of a large asthenospheric reservoir, possibly confined at the base of the lithosphere.

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.