Efficient T-wave generation by small-magnitude earthquakes along the NE Brazil continental margin

Abstract In 2021 and 2022, two offshore small-magnitude earthquakes (3.5 and 3.7) occurred in the northeastern Brazilian continental margin. Both occurrences produced hydroacoustic T-waves, which were recorded by H10N and H10S hydrophone arrays located near Ascension Island and owned by the Comprehensive Nuclear-Test-Ban Treaty Organization. We analyzed hydroacoustic T-waves recorded by the two arrays using cross-correlation of three-hydrophone pairs with a 2D plane-wave fitting approach. Back-azimuth and apparent velocity were derived from slowness vector inversion to locate T-wave excitation points along the northeastern Brazilian continental margin. Despite the powerful acoustic signals, no seismic T-waves were recorded at neighboring coastal land seismic stations. This pattern shows that seismic energy is efficiently coupled into the SOFAR channel from the continental slope. The high acoustic levels for such small magnitudes suggest favorable source conditions, most likely involving thick sediment cover, shallow hypocenters, and fault geometries dominated by normal or reverse motion, which are consistent with regional stress regimes and known offshore faulting systems in other continental margin areas of Brazil. Comparisons with empirical relationships provide support for the theory of greater coupling at the margin-water contact. These results suggest that active faulting likely occurs along the northeastern Brazilian margin and highlight the critical role of margin morphology and sedimentary structures in T-wave generation and coupling efficiency.