Mobile Earthquake Recording in Marine Areas by Independent Divers:Now Landing on the Ocean Bottom

Mermaid stands for Mobile Earthquake Recording in Marine Areas by Independent Divers. This autonomous, freely-drifting underwater robot records acoustic conversions of earthquake signals while floating at a water depth of 1.5 km, rising to the surface to transmit such seismograms by satellite. Mermaid is already in use for seismic tomography, which images wave speed anomalies inside the Earth, using earthquake-generated, seismic wave paths. The absence of conventional recording stations in the oceans leads to poor path coverage, which is now being remediated by Mermaid instruments. Under the umbrella of the EarthScope-Oceans network, a collaboration between France, China, the U.S. and Japan, we launched 50 Mermaids in the South Pacific in 2018 as part of the SPPIM (South Pacific Plume Imaging and Modeling) project. Having operated successfully for three years already, the instruments will transmit sufficient data over ~6 years to image the southern Pacific superplume below Tahiti, providing wave paths completely unsampled by land or OBS stations. Mermaid is developed at Geoazur laboratory in collaboration with commercial partner Osean. Prompted by recent seismic swarm activity around a submarine volcano located 50km east of Mayotte, in the Comores archipelago and at 3500 m depth, our team have decided to devise a bottom-landing version of Mermaid. A user command can switch a regular, passively drifting Mermaid into Landing Mode when the instrument is connected through satellite link during a surfacing event. The instrument then proceeds to sink to the seafloor, where it remains stationary during the user-programmed period, operating essentially like an OBS until it resurfaces. A fleet of Mermaid with lander capability could be the technology of choice for rapid aftershock deployments since no dedicated research ship is required for deployment (unlike for OBS arrays). Seismograms can be transmitted at user-defined intervals (e.g., weekly) during the time period of interest (aftershock activity), which can be re-evaluated on the fly. If the Mermaid are ultimately recovered (rather than released into drifting mode), a continuous, one-year data buffer can be analyzed for any signals not originally transmitted by satellite due to bandwidth constraints.