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Underwater observatory south of Terre-de-Bas, Guadeloupe

This Tuesday, June 1st, 2021, teams working on the PREST project (led by OVSM-IPGP) will be setting up an underwater observatory, equipped with innovative seismic, inclinometric and pressiometric sensors with optical measurements, to improve monitoring of the seismicity zone in Les Saintes. A team from Géoazur, specialists in this type of operation, will lead the FIBROSAINTES oceanographic campaign in charge of deploying this new monitoring system.

Underwater observatory south of Terre-de-Bas, Guadeloupe

Maps showing the location of the Saintes underwater observatory.

Most of the major seismic sources (and therefore also tsunamis) threatening the Caribbean islands are located at sea. It is therefore essential to develop techniques for installing networks of underwater geophysical sensors in order to study these hazards as closely as possible (earthquakes, volcanoes, landslides, tsunamis, etc.).

As part of the PREST project (Interreg V Caribbean) led by the Observatoire volcanologique et sismologique de Martinique of the Institut de physique du globe de Paris (OVSM-IPGP), an exceptional oceanographic campaign is starting on Tuesday June 1st, 2021. It is the culmination of several years of research and development into fibre-tip optical sensors, carried out by the IPGP, ESEO and ENS, with a view to monitoring as closely as possible the seismicity of the Les Saintes area, which was hit by the 2004 earthquake and is still affected by earthquakes.

Designed and developed under the direction of Pascal Bernard, a physicist at the IPGP and head of this axis in PREST, and Guy Plantier, director of research at the ESEO and a member of the University of Le Mans, this innovative instrumentation overcomes many of the problems associated with the usual deployment of electronics and energy on the seabed (such as conventional underwater seismometers known as OBS). Here, a purely mechanical seismometer, inclinometer and pressure meter, installed at a depth of 40 m underwater and located 4-5 km south of Les Saintes, are linked by an optical cable to Terre-de-Bas (tourist office) where a laser interrogator enables its movements to be measured remotely with great precision. The prototype seismometer was tested and validated in 2019-2020 off the coast of Brest and this is the first time it will be installed in real monitoring conditions, since the data will be transmitted in real time to the volcanological and seismological observatories of Guadeloupe and Martinique (OVSG-OVSM-IPGP).

In addition, the fibre itself will be used to track the propagation of seismic waves at any point along its route, thanks to another optical interrogator placed on land by the Géoazur team. This innovative approach is made possible by monitoring disturbances in the light signal caused by the passage of seismic waves. With a measurement point every 10 metres along the length of the cable, the optical fibre will represent the equivalent of 500 seismometers. The interrogator was developed by the SME Febus Optics.

This new underwater monitoring system will provide a better understanding of this zone of persistent activity in Les Saintes, in the form of small sequences of earthquakes that are sometimes felt, the origin of which is still debated, since Les Saintes is still one of the most active zones in the Caribbean arc, even though the magnitude 6.3 earthquake occurred 17 years ago (November 21st, 2004).

The qualification of the FIBROSAINTES campaign as part of the PREST project will make it possible to launch more ambitious projects with longer cables, 50 to 100 km long, capable of reaching the source zones of the major earthquakes in the Caribbean subduction zone off Guadeloupe and Martinique, and thus to finally identify the ongoing deformation activity that is preparing the next major earthquakes.

PREST (Plateforme Régionale de Surveillance Tellurique) is a project under the Interreg Caribbean V Programme 2014-2020 (European ERDF funds).

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