Abstract
Excitations of seismic background noises are mostly related to fluid disturbances in the atmosphere, ocean and the solid Earth. Earthquakes have not been considered as a stationary excitation source because they occur intermittently. Here we report that acoustic-coupled Rayleigh waves (at 0.7–2.0 Hz) travelling in the ocean and marine sediments, retrieved by correlating ambient noise on a hydrophone array deployed through a shallow to deep seafloor (100–4,800 m) across the Nankai Trough, Japan, are incessantly excited by nearby small earthquakes. The observed cross-correlation functions and 2D numerical simulations for wave propagation through a laterally heterogeneous ocean–crust system show that, in a subduction zone, energetic wave sources are located primarily under the seafloor in directions consistent with nearby seismicity, and secondarily in the ocean. Short-period background noise in the ocean–crust system in the Nankai subduction zone is mainly attributed to ocean-acoustic Rayleigh waves of earthquake origin.
Highlights
We demonstrate that obtained cross-correlation functions (CCFs) represent the retrieval of persistently excited Acoustic-coupled Rayleigh (ACR) waves, and the direction in which strong ACR waves propagate coincides with seismicity around the Nankai subduction zone
This phase can be identified in the observed CCFs with small amplitudes, indicating that sources at the sea surface may slightly contribute to the production of ambient noise observed at the seafloor at WD2
Our results indicated that the observed features in the CCFs in the Nankai subduction zone would be mostly explained by ACR waves due to seismic sources
Summary
Detection of persistently excited ACR waves in the CCFs. Figure 2 and Supplementary Fig. 1a–c show CCFs using two adjacent stations along Lines A–E (Methods). In case (2) (Supplementary Fig. 5e), the CCFs show several groups of signals with a common apparent velocity of B4 km s À 1, which are interpreted as multiple reflections of P waves between the sea surface and seafloor This phase can be identified in the observed CCFs with small amplitudes (arrows in Supplementary Fig. 1d), indicating that sources at the sea surface may slightly contribute to the production of ambient noise observed at the seafloor at WD2. Seismic sources would be appropriate for the persistent generation of ACR waves in the Nankai Trough region
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