Optical fiber available to researchers to better understand Mayotte earthquakes

For the IPGP, this is an unprecedented observation mission on a large-scale submarine cable. This full-scale experiment will enable us to test new types of measuring instruments. Like terrestrial fibres, submarine cables not only carry communications, but also detect vibrations in the ground on which the fibre is laid. The equipment placed at the end of the fibre pair will enable it to be used as an antenna to better locate seismic signals in the region.

18 months ago, a first long series of seismic swarms were felt by the population of Mauritania and recorded by the regional seismic stations. In order to shed light on the mechanisms behind these movements, observation and research operations involving numerous French research bodies (CNRS, IPGP. BRGM, Ifremer…) were launched in autumn 2018 by the authorities. At the end of May 2019, these operations led to the discovery of a new active submarine volcanic edifice off Mayotte. Following this discovery, and to organise the scientific and operational response to this large-scale geological phenomenon close to Mayotte, the “Réseau de surveillance volcanologique et sismologique de Mayotte” (REVOSIMA) was set up in summer 2019. However, analysis of the crisis and real-time monitoring are limited by the low density of measurements on land (Mayotte and Glorieuses). Data at sea is currently recorded for several weeks, but is only processed afterwards when the instruments are changed. It is therefore of the utmost importance to implement at-sea measurements that enable real-time data acquisition and whose accuracy and resolution are adapted to current challenges.

While the use of optical fibres for monitoring civil engineering structures has become widespread over the last twenty years or so, with sensors located on portions of the fibre, scientists are seeking to exploit the various intrinsic backscattering properties of fibres as sensors to obtain information about their environment (temperature, pressure, stress, vibration, etc.). Optical fibres themselves can be used as sensors, in this case we talk about “Fibre Optic Sensors” (FOS).

For vibration measurements, the interrogation systems now available (DAS system, for Distributed Acoustic Sensor) make it possible to have the equivalent of thousands of seismometers distributed all along the fibre, over several tens of kilometres. This equipment has been acquired as part of an ERDF project involving the ESEO Group in Angers, the Laboratoire d’Acoustique de l’Université du Mans (LAUM UMR CNRS 6613) and IPGP. The DAS will be connected to the end of one of the available fibres of the FLY-LION3 cable in Mayotte to test its ability to measure seismic signals from the active zone.

This agreement between the FLY-LION3 consortium and the IPGP opens up the possibility of diversifying the use of submarine cables beyond supporting connectivity needs.