Extraction of P Wave From Ambient Seafloor Noise Observed by Distributed Acoustic Sensing.

Seismic wave extractions have been performed using ambient noise records observed by distributed acoustic sensing (DAS) technology. Extractions of microseisms can be investigated at a local scale using such DAS records observed in the ocean. Here, we show P and Scholte wave extractions from ambient seafloor noise observed by DAS along a submarine cable deployed off Cape Muroto in the Nankai subduction zone, Japan. The P waves can be observed at a frequency band of 0.1–0.3 Hz and up to a distance of 8 km. The distance at which the P waves can be observed is controlled by the P incident angle and the DAS sensitivity to the observable apparent velocity. Effective extractions of P and Scholte waves, corresponding to large intensity of wave‐wave interaction at the sea surface, are performed, indicating that the waves are originated from pressure field at the sea surface. Plain Language Summary: The density of seismic observations on land is typically higher than that in the ocean. However, higher‐density observations in both land and ocean regions can be made using the recently developed distributed acoustic sensing (DAS). In this study, we focus on the waves persistently propagating through the Earth, which are excited by ocean swell and are called microseisms. Ocean swell at the sea surface primarily excites surface waves: however, P and S waves excited by ocean swell, especially during tropical storms, were observed on a global scale. Because this indicates that body waves may be locally excited by sea surface disturbances, we investigate the problem using DAS records. Our results show that large amplitudes of P waves occur when an atmospheric low pressure passes through Japan, and those amplitudes become weak when the wave‐wave interaction intensity at the ocean surface is weak. Although many previous studies at global or regional scales have indicated that microseisms are originated from fluid disturbances at the sea surface, the current study observed the excitation of P and surface waves right beneath the sea surface disturbances. Key Points: P waves were extracted from ambient seafloor noise records observed by distributed acoustic sensing (DAS) at a frequency band of 0.1–0.3 Hz and up to a distance of 8 kmExtraction of P waves depends on weather conditions, particularly for wave‐wave interaction intensity at the sea surfaceThe distance at which the P waves emerge is controlled by the P incident angle and the DAS sensitivity to apparent velocities [ABSTRACT FROM AUTHOR]