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.