Characteristics of the Oceanic Ambient Seismic Noise Around Tristan da Cunha in the South Atlantic From OBS Data

It has long been recognized that specific ambient seismic noise is generated within the ocean. Therefore, ocean bottom seismometers have advantages to reveal the noise characteristics directly in the deep sea. We use the seismic recordings around Tristan da Cunha collected by 20 OBSs and two island stations to investigate the ambient noise sources in the middle of the South Atlantic. We analyzed the power spectral density, ambient noise cross-correlations, and correlations with wave heights model. While primary microseisms (similar to 10-20 s) in this region are deficient, the secondary microseisms are strong and dominate the ambient noise at periods of similar to 1-10 s, with the peak broadening to shorter periods (1-3 s). The secondary microseisms are strongly correlated with the local wave heights, especially at shorter periods (similar to 1-3 s), suggesting that the microseisms are affected mostly by local wave interactions. We observed clear Empirical Green's functions at periods of similar to 3-10 s, indicating the existence of distant sources of secondary microseisms. Results of asymmetry analysis of cross-correlations suggest the prominent direction of the distant source should be located to the Southwest and show barely seasonal variations. The correlations with wave heights indicate a possible source region in the southeast Pacific Ocean. Therefore, besides the dominating local wave-wave interactions, the secondary microseisms around Tristan region are affected simultaneously by persistent distant sources from the deep basin in the Southeast Pacific Ocean, which might be related to the intense wave-wave interactions caused by the blocking effect of the Drake Passage on ocean currents and waves. Plain Language Summary Ambient seismic noises are the ubiquitous and low-amplitude background signals recorded by the seismic stations worldwide. Microseisms are the strongest parts of ambient noise, which are generated within the oceans by wave interactions. Previous studies of ambient noise or microseisms had been carried out in many regions, but no study had focused yet on the South Atlantic Ocean due to the lack of seismological data. In this study, we computed the probabilistic power spectral densities to analyze the spectral characteristics of oceanic ambient noise around Tristan region in the middle of the South Atlantic Ocean. We observe that the microseisms have strong correlations with the local wave heights, indicating that local wave interactions are dominant sources here. The analysis of ambient noise cross-correlations shows there are distant sources for this region obscured by dominant local sources. Based on asymmetry analysis of the ambient noise cross-correlations, we estimate the direction of the major distant sources. Combined with the correlation analysis with global wave heights model, we identify the predominant distant sources, which are located in the Southeast Pacific Ocean and related to the blocking effect of the Drake Passage on ocean currents and wave systems.

Automated summary

Through analyzing seismic recordings collected in the Tristan da Cunha region, the sources of ambient noise in the middle of the South Atlantic were investigated. It was found that secondary microseisms were strong and correlated with local wave heights, indicating that local wave interactions were the dominant source of noise. Additionally, distant sources of secondary microseisms were identified, located in the Southeast Pacific Ocean and influenced by the blocking effect of the Drake Passage on ocean currents and wave systems.