The dynamics of magma deep in the Earth’s crust are difficult to capture by geophysical monitoring. Since May 2018, a seismically quiet area offshore of Mayotte in the western Indian Ocean has been affected by complex seismic activity, including long-duration, very-long-period signals detected globally. Global Navigation Satellite System stations on Mayotte have also recorded a large surface deflation offshore. Here we analyse regional and global seismic and deformation data to provide a one-year-long detailed picture of a deep, rare magmatic process. We identify about 7,000 volcano-tectonic earthquakes and 407 very-long-period seismic signals. Early earthquakes migrated upward in response to a magmatic dyke propagating from Moho depth to the surface, whereas later events marked the progressive failure of the roof of a magma reservoir, triggering its resonance. An analysis of the very-long-period seismicity and deformation suggests that at least 1.3 km3 of magma drained from a reservoir of 10 to 15 km diameter at 25 to 35 km depth. We demonstrate that such deep offshore magmatic activity can be captured without any on-site monitoring.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: The discovery of the underwater volcano is a result of the MAYOBS 1 campaign that took place from 2 to 19 May, 2019 aboard the Marion Dufresne oceanographic vessel. This campaign was conducted by several French research institutions and laboratories (IPGP/CNRS/BRGM/IFREMER/IPGS) as part of a CNRS-INSU programme20. We thank CNRS-INSU for making public the location and size of the discovered volcano. M.I. thanks H. Sudhaus for her valuable contribution and acknowledges funding by the German Research Foundation DFG through an Emmy Noether Young Researcher Grant (no. 276464525). G.P. is funded by the German Research Foundation DFG project (no. 362440331), a subproject of “SPP 2017: Mountain Building Processes in 4D” (project number no. 313806092). We thank T. James and T. Davis for revising the English text.