Genesis and Propagation of Low-Frequency Abyssal T-Waves

Abstract

Abyssal T-waves are seismo-acoustic waves originating from abyssal oceans. Unlike subduction-zone-generated slope T-waves which are generated through multiple reflections between the sea surface and the gently dipping seafloor, the genesis of abyssal T-waves cannot be explained by the same theory. Several hypotheses, including seafloor scattering, sea surface scattering, and internal-wave-induced volumetric scattering, have been proposed to elucidate their genesis and propagation. The elusive mechanism of abyssal T-waves, particularly at low-frequencies, hinders their use to quantify ocean temperatures through seismic ocean thermometry (SOT) and estimate oceanic earthquake parameters. Here, using realistic geophysical and oceanographic data, we first conduct numerical simulations to compare synthetic low-frequency abyssal T-waves under different hypotheses. Our simulations for the Romanche and Blanco transform faults suggest seafloor scattering as the dominant mechanism, with sea surface and internal waves contributing marginally. Short-scale bathymetry can significantly enhance abyssal T-waves across a broad frequency range. Also, observed T-waves from repeating earthquakes in the Romanche, Chain, and Blanco transform faults exhibit remarkably high repeatability. Given the dynamic nature of sea surface roughness and internal waves, the highly repeatable T-wave arrivals further support the seafloor scattering as the primary mechanism. The dominance of seafloor scattering makes abyssal T-waves useable for constraining ocean temperature changes, thereby greatly expanding the data spectrum of SOT. Our observations of repeating abyssal T-waves in the Romanche and Chain transform faults could provide a valuable data set for understanding Equatorial Atlantic warming. Still, further investigations incorporating high-resolution bathymetry are warranted to better model abyssal T-waves for earthquake parameter estimation.