A seafloor deformation study using A-0-A pressure instruments and ocean models to contribute to the monitoring of the Mayotte volcanic crisis.
Authors: Ballu Valerie , Tranchant Yann-Treden , Dausse Denis , Testut Laurent 
Published: March 2024

The sudden 2018 volcanic eruption offshore Mayotte, in the western Indian Ocean, demonstrated, once again, the crucial need for means to monitor telluric activity occurring on the seafloor and threatening coastal zones. In the Mayotte case, on-land GNSS stations were of primary importance to detect the subsidence induced by the emptying of a deep magma chamber (Peltier et al. 2022), however they are not adequate to properly characterize and monitor the deformation created by further offshore or shallower processes.Ocean bottom pressure (OBP) records can be used to monitor seafloor motion. However, detecting small or slow deformation is challenging due to instrumental drift and oceanic variations at different timescales. New Ambient-Zero-Ambient (A0A) pressure systems allow the estimation of the instrumental drift&#160;in situ by periodic venting from ocean pressures to a reference atmospheric pressure (Wilcock et al., 2021) and therefore allow access to the accurate monitoring of slow deformation. A A0A drift-controlled pressure gauge has been deployed since 2020 (four successive deployments) to monitor the seafloor vertical deformation on the flank of Mayotte island. The deployment site is located within a seismically active circular-shape zone, called the proximal cluster (Lavayssi&#232;re et al., 2022). During the last deployment (2022-2023), an additional reference instrument was installed outside the proximal cluster, to allow for differential deformation analysis.Beside volcanic activity monitoring, the objective of this study is to assess the performance of these new A0A pressure gauges and our ability to reduce the oceanic &#8220;noise&#8221; in corrected OBP records and characterize seafloor deformation in the Mayotte region.&#160;We investigate the use of numerical models, including available global ocean circulation reanalyses (OGCMs) and barotropic simulations, to account for the different oceanic processes contributing to the seafloor pressure variations and therefore limiting our ability to identify crustal deformation in the integrated pressure records.We also use temperature and salinity profiles from repetitive glider transects to validate OGCMs in the region and quantify the contribution of unresolved fine-scale processes to OBP records. Our results provide valuable insights into the feasibility of using numerical modeling for improving the accuracy of OBP-based monitoring at different timescales, in the context of the Mayotte seismic crisis as well as for other seafloor deformation monitoring.&#160;Finally, we present a preliminary work on the combination of sparse regional altimetric data with the glider observations to compute a seafloor pressure series to be compared to the recorded data. Current altimetry spatio-temporal coverage is limited, however, newcoming SWOT observations are likely to provide new perspectives in seafloor geodesy.Our results bring insights for future A0A deployments, especially in the perspective of the planned MARMOR seafloor cabled observatory offshore Mayotte.