The RSBR-Mar Project: Monitoring Offshore Small-Magnitude Earthquakes in Southeastern Brazil with Marine and Land-Based Instrumentation

Developed after the breakup of Gondwana in the Early Cretaceous, the southeastern margin of Brazil comprises a large, economically important hydrocarbon province that includes two of the world’s most prolific offshore sedimentary basins: Santos and Campos. At the same time, this passive margin hosts the country's main offshore seismic zone, characterised by frequent small-magnitude earthquakes and occasional larger events (M ≥ 4.8), occurring on average every 20-25 years, mostly within the extended and submerged continental crust. The larger events include the 1955 Vitória earthquake (mb 6.1), the 1939 Tubarão earthquake (mb 6.0), the 1972 Campos earthquake (M 4.8), the 1990 Porto Alegre earthquake (mR 5.2), and the 2008 São Vicente earthquake (mR 5.2). Due to insufficient instrumentation, limited to a few distant and unevenly distributed coastal stations of the Brazilian Seismographic Network (RSBR), and the low frequency of larger events, both the origin of this seismic activity and the risk it poses to key offshore infrastructure remain poorly understood. To better understand the processes related to the seismic activity in southeastern Brazil, the Brazilian Seismographic Network at the Sea Project (RSBR-Mar) intends to improve coverage by deploying: i - six temporary broadband coastal stations; ii - five Ocean Bottom Seismometers (OBSs); and iii - eight Mobile Earthquake Recorder in Marine Areas by Independent Divers (MERMAIDs) between São Paulo and Espírito Santo states. To date, two temporary land stations have been installed in Espírito Santo in June 2025. During a cruise mission between late September and early October 2025, we deployed all five OBSs and eight MERMAIDs. The OBSs will be recovered by September 2026, after one year of data acquisition, at which time the batteries will be replaced and the OBSs redeployed for an additional year. MERMAIDs periodically surface and establish satellite communication, allowing them to send important data back to us, collected during each operational cycle. To date, we have collected 50 waveforms detected by the MERMAIDs. Although MERMAIDs were originally designed to record high-quality P-waveforms for teleseismic tomography, rather than to detect small local events, we will explore this possibility by requesting data stored in their one-year internal storage. If successful, MERMAIDs could improve the localisation of earthquakes recorded in continuous records from land stations and OBSs. First, we plan to automatically detect and locate earthquakes in continuous seismograms from land stations and OBSs using machine-learning-based algorithms (e.g., LOC-FLOW, MALMI). These preliminary locations allow us to identify which data windows to request for each MERMAID. Then, we can relocate the small earthquakes with the acoustic data. Finally, relocation methods may help us to delineate possible seismogenic offshore structures (e.g., HypoDD).