2019 Seismicity clusters structure of Mayotte volcano-tectonic crisis

Abstract:

Off Mayotte Island (Comoros Archipelago, Northern Mozambique Channel), an unprecedented volcano-tectonic seismic event is ongoing since May 2018, including the largest earthquake ever recorded in the Comoros archipelago and hundreds of felt earthquakes on Mayotte. This volcano-tectonic crisis is associated with a long-duration and intense seismic activity and testimonies of volcanic activities as revealed for example by geodesy and offshore observations (Feuillet et al., in review). In this poorly instrumented area, the combined effort of the French ministries and different research institutes allowed (1) increasing the number of seismological stations onland since early 2019, but also (2) deployments of OBS stations and the discovery of a new offshore volcano edifice. The data recorded by these different networks are processed to create a collaborative seismicity catalogue involving different institutes within the framework the MAYOBS/REVOSIMA Seismology group (fundings CNRS Tellus and Revosima).

In this study, we use the double difference relocation method to process the events of the catalogue of 2019 whose absolute location and ad hoc 1D velocity model was made with land and OBS stations (Lavayssière et al., abstract AGU 2020). The main results are the morphology of the deep seismicity organized in two clusters and the spatial distribution of the seismicity within the clusters. The spatial analysis reveals structures and patterns linked to this unprecedented volcanic episode. A first cluster is aligned between the centre of deflation and the new volcanic edifice, in coherency with the seismicity of the beginning of the crisis that lead to the eruption, early July 2018 (Lemoine et al., 2020; Mercury et al., abstract AGU 2020). Epicentral distribution of the second main cluster, which appeared during summer 2018, is organized as a donut shape, forming a hourglass shape in cross section. This configuration persists for two years. The evolution of the seismicity is confronted to focal mechanisms (Jacques et al., abstract AGU 2020).

The confrontation of seismicity patterns to volcanic and/or tectonic structures makes a step forward imaging the active volcano-tectonic system of Mayotte and its interpretation in the regional context of the Northern Lwandle plate and in the ongoing large offshore eruption.