The unexpected Mayotte 2018–2020 seismic sequence: a reappraisal of the regional seismicity of the Comoros

Abstract

The Mayotte seismic sequence that started on 10 May 2018, with a main shock of magnitude Mw 5.9 on May 15, followed by a major offshore volcanic activity, raises several questions of seismovolcanic hazards in the Comoros region. The unexpected size and duration of the crisis is an opportunity to reassess the distribution and magnitude of the seismicity near Mayotte Island, but also regionally. We present a comprehensive seismicity catalogue of the region including the Mozambique Channel, the Mozambique coast and Madagascar, based partly on previously published data in the region and partly on unpublished data from local catalogues for the Comoros and Madagascar. Our catalogue extends from 1900 onward with a completeness of magnitude 5.5 until 1980s and decreasing only recently to 4.5 after 2010. It comprises the events of magnitude Mlv ≥3.5 for the seismic sequence of Mayotte from May 2018 to October 2020 as the crisis is still ongoing. Present knowledge of the seismicity, largely partial in distribution and magnitude before 1980, makes the seismic sequence of 2018-2020 an exceptional and unprecedented seismo-volcanic event in the region. We discuss the distribution of seismicity in time and space within the context of the south eastward propagation of the East African rift system towards Madagascar.

Risk communication during seismo-volcanic crises: the example of Mayotte, France

“…About 140 of these earthquakes were reported as having been felt by the population in the LastQuake crowdsource-based information app of the Euro-Mediterranean Seismological Center (EMSC-CSEM, 2021). There was a sharp decrease in the number of felt earthquakes after June 2018, in line with the decrease in the number of instrumentally recorded earthquakes and of their average magnitude (e.g., Lemoine et al, 2020;Bertil et al, 2021). The EMSC-CSEM catalog reports only about four felt events per month until the end of 2018 and then a moderate recovery in the number of felt events between February and June 2019 (nine felt events per month on average) (the red curve in Fig.…”

“…In May 2018, Mayotte's area was poorly instrumented. The ability to identify and precisely locate the earthquakes improved gradually with the development of the network of seismic stations (Bertil et al, 2021;Saurel et al 2021). The inclusion of underwater stations (OBS for ocean bottom seismometer) from February 2019 (Feuillet et al, 2021a;Saurel et al, 2021) and the use of refined seismic velocity models (Lavayssière et al, 2021;Saurel et al, 2021) were determinant in this respect.…”

“…Inhabitants have mainly experienced the ongoing activity through felt earthquakes. More than 20 earthquakes of magnitude 5+ were recorded during the first month of the crisis, from 10 May to mid-June 2018 (Bertil et al, 2021), while ∼ 1900 events with magnitudes > 3.5 happened during the first year (Cesca et al, 2020;Lemoine et al, 2020). About 140 of these earthquakes were reported as having been felt by the population in the LastQuake crowdsource-based information app of the Euro-Mediterranean Seismological Center (EMSC-CSEM, 2021).…”

Storytelling, language, and the earthquake swarm of May 2018: Insights into Shimaore and Kibushi from narrative analysis

“…Mayotte island was an exception to this before May 2018, when inhabitants experienced hundreds of earthquakes within a period of two months, including the largest recorded earthquake on May 15, 2018, which was a 5.9 on the Richter scale. These earthquakes, whose epicenters are located about 50 km east of Mayotte [Bertil et al, 2021], were felt throughout the island. While structural damage to buildings did occur [see Sira et al, 2018], fortunately no major catastrophic destruction happened.…”

Evolution of the East African Rift System from trap-scale to plate-scale rifting

“…QSA: Quatlamba seismic axis. b) Zoom of a), with seismicity added (≥M3.5 from Bertil et al (2021) for the Mozambique channel and Madagascar and the USGS database for the rest of the EARS). We define the East African Rift System (EARS) as these connected segments of volcanism, faulting and seismicity over a ~ 5000 km-wide zone, including not only the classical East and West branches but also a South-East branch corresponding to the previously defined seaward continuity of the rift system along the Davie ridge (i.e., the Southeastern branch; Chorowicz, 2005) and the Quatlamba seismic axis, and an Indian Ocean branch separating the Lwandle and Somali plates along the Comoros archipelago and Madagascar.…”