Abstract
SUMMARYThe 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 d, 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 submarine 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 1-D 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 5000 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 10 km offshore Mayotte eastern coastlines, is 20–50 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 to 25 km depth. The two clusters appear seismically separated, however our data set is insufficient to firmly demonstrate this.
Highlights
Before May 10th 2018, Mayotte island, part of the volcanic Comoros archipelago in the NorthMozambique Channel of the Indian Ocean (Figure 1), was not considered as a significantlyT seismically active area (Bertil et al, 1998)
N Together with an unfelt M 5.1 event on March 23rd 1993, 80 km south-west of Mayotte, A these were the only M5+ earthquakes recorded within 100 km of the island since the advent M of the global seismological networks in 1964 (Storchak et al, 2017; ISC, 2020)
In 2019, we developed a new protocol to efficiently process Ocean Bottom Seismometers (OBS) data during IT pickathons, when, at the same place, several analysts dedicate a few days to work together ED on the newly recovered data
Summary
Before May 10th 2018, Mayotte island, part of the volcanic Comoros archipelago in the North. Only one real-time seismic station (RA.YTMZ; Résif, 1995) was installed on the ITED island at the onset of the 2018 seismic crisis. This station was deployed by BRGM (Bureau de recherches géologiques et minières, the French geological survey) for the French. ED funding, 4 seismic stations were installed onshore (3 on Mayotte island and 1 on Grande IT Glorieuse island) and 6 Ocean Bottom Seismometers (OBS) were deployed offshore, within a ED radius of 40 km around the seismically active area.
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