Deciphering the relationship between fluids and seismicity in the Lesser Antilles – LAVAS

The LAVAS (Lesser Antilles subduction : Volatiles And Seismicity) project proposes a multi-disciplinary study of the relationship between the migration of deep-seated fluids to the seafloor/surface and seismicity in the Lesser Antilles region. The project has three components: (a) evaluation of data from the Manta-Ray cruises, (b) analysis of satellite images to detect natural hydrocarbon seepage, and (c) acquisition of additional data using seismometers and a methane detecting glider at active seeping sites. This 5-year project will use a versatile set of geophysical, geochemical, and satellite radar data to quantify and characterize fluids entering and exiting the subduction zone, to elucidate their migration pathways, and to monitor their discharges at the seafloor and surface to link them to locally occurring seismicity. LAVAS consists of 4 scientific Work Packages (WP) and an educational project.
The first WP will quantify the amount of fluid entering the subduction system in oceanic rocks and sediments and image fluid pathways at depth as well as expulsion sites at the seafloor. Wide-angle and multi-channel seismic data from the Manta-Ray 1 cruise (2022) will be used to calculate fluid content in the incoming slab and to image flow pathways in the fore-arc. A 3D model of p- and s-wave velocities will be made to constrain the degree of hydration of the oceanic crust and fluid pathways. The second WP will characterize the relationship between variation of fluid expulsion rates and seismicity. Seismicity will be investigated using newly acquired seismological data at actively fluid seeping structures as well as data from previous deployments. In addition, a previously deployed piezometer will be used to estimate flow rates and their variation. Fluids from piston cores taken at fluid escape sites during the Manta-Ray 1 cruise will be analyzed for the physicochemical processes they undergo during migration. The identified physicochemical processes fluids undergo will be compared to the spatio-temporal behavior of seismicity to ascertain any correlations. The third WP will investigate satellite radar images for natural hydrocarbon seeps at the ocean’s surface. This approach, as shown in a preliminary study, can be applied to areas substantially larger than seismic profiles, enhancing our capabilities to characterize seep distribution and recurrences and the extent that deep-seated fluid discharges at the seafloor reach the ocean surface. As this approach requires large quantities of data to be analyzed, deep learning techniques will be developed in this WP. A comparison study using the newly developed tools will test the applicability on other seismically active zones, such as the Sea of Marmara. The fourth WP will synthesize results from the other WPs as a basis to decide on the exact locations of the future data acquisition proposed in this project. Using a sailboat a glider will be used to record methane in the water column at active fluid seeping sites and ocean-bottom seismometers to acquire signals from hydrocarbons leaving the seafloor. In addition, we will make a comparison to other subduction zones, such as the Alaska and Ecuador subduction zones where analysis of fluid circulation is ongoing. Finally, we have planned an ambitious educational project which will install 5 seismometers in schools on the islands of Saint-Martin and Saint-Barthélemy. The instruments will serve both educational and scientific purposes. It will enable local populations to learn about the seismicity of the region and also extend the seismological network in a region otherwise scarcely covered.
The results of the project will answer several questions including (a) are fluid seeps triggered by earthquakes, or do they act as precursors (b) does a link exist between the spatial distribution of seeps and the hydration state of the subducted slab and (c) what is the contribution of fluids migrating in subduction zones in the fluid cycle.