Weak faults at megathrust plate boundary respond to tidal stress

Abstract Lateral spatial variations of weak portions at the plate boundary in subduction zones have been estimated primarily by distribution slow earthquakes mainly occurring around seismogenic zones. However, detailed depth profile faults remains elusive. Here, we deployed six ocean bottom seismometers Nankai zone, Japan, to observe reflections originated from drilling vessel Chikyu ship noise (hydroacoustic P wave) that was persistently radiated a fixed position sea surface, and retrieved -to- s ( Ps ) multiple dipping near boundary. The amplitudes were stacked compared according degrees tidal stresses, high observed tide (compression). A migration technique shows locations where velocity contrasts fluctuate both megasplay fault another between top oceanic crust. This indicates physical properties these are altered stress. physical-property changes attributed fluid connections isolations within due stress fluctuations, inducing variation seismic anisotropy. Such confirmed three-dimensional numerical simulation for wave propagation with anisotropic medium. Our observation suggests present boundary, obtained may implications in-depth understanding triggering earthquakes.