Mayotte Seismic Crisis: Advancing Knowledge in Near Real-Time Through Integrated Land and Ocean-Bottom Seismic Monitoring – First Results

The sudden onset of intense seismic activity off the coast of Mayotte Island, located north of the Mozambique Channel (Indian Ocean), in May 2018 took residents, authorities, and scientists by surprise. Within the first five days, about 20 felt earthquakes were recorded, reaching magnitudes of up to Mw 5.9. At the time, scientific understanding of the region’s seismic behavior was limited, largely due to poor network coverage.

In response, the MAYOBS/REVOSIMA seismology group was formed between four French research institutions throughout 2018 and 2019. Their goal was to enhance monitoring capacity and data quality to study what is now recognized as an exceptional ongoing submarine basaltic eruption.

By early 2019, three medium-band stations were installed on Mayotte and one on Grande Glorieuse Island, complementing the data collected from Ocean Bottom Seismometers (OBS). The seismic data were regularly processed to refine earthquake locations detected by the land-based network.

A new local 1-D velocity model was developed, along with specialized data-processing procedures. The team estimated a local VP/VS ratio of 1.66, consistent with a volcanic island environment. Approximately 125,000 P and S wave phases were manually picked from both land and seafloor stations, enabling the precise location of over 5,000 earthquakes recorded between February 2019 and May 2020.

The analysis revealed two distinct offshore seismic clusters, termed Proximal and Distal:

• The Proximal cluster, about 10 km east of Mayotte, lies between 20 and 50 km depth and exhibits a cylindrical structure.
• The Distal cluster begins roughly 5 km east of the Proximal one and extends beneath Mayotte’s new volcanic edifice, from 50 to 25 km depth.

Although the clusters appear seismically distinct, the current dataset does not yet allow a definitive conclusion about their separation.