A MERMAID Lander seismo-acoustic float records numerous seismic events in the Ligurian Sea

A MERMAID float was deployed in the Mediterranean Sea offshore Nice between July and September 2024. It dove to a depth of 2,200 meters to record earthquake time series using its hydrophone. MERMAID floats have been developed as part of a collaboration between GEOAZUR research lab and their manufacturing partner OSEAN since 2014.

An anchoring system, called guiderope, allows the float to rest on the seabed at depth up to 4000 meters, preventing its drift in deep sea currents. In two and a half months, the float surfaced three times and changed position by only 10 km, mainly due to suface drift for transmitting data. Without an anchoring system, the drift would have been in excess of 100 km, as for standard MERMAID floats, which have been operating for example in the southern Pacific since 2018.

The onboard signal acquisition and processing system automatically detected 11 teleseismic P-waves from earthquakes of magnitudes between 6.0 and 7.4, as well as one T wave from a nearby earthquake in the Mediterranean of magnitude 4.1. These recordings were transmitted by Iridium satellite each time the float surfaced.

Manual analysis of the continuous times series recording after recovery of the float, too voluminous to transmit by satellite during the mission, revealed numerous additional events. Namely 17 teleseismic P-waves from earthquakes of magnitudes 4.5 to 7.9 and 57 T-waves from the Mediterranean basin from earthquakes of magnitudes 1.6 to 4.5, as well as 24 T waves not identifiable in the catalogs. Numerous other signals related to maritime navigation and weather conditions were also recorded.