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

俯冲带板块边界薄弱部位的侧向空间变化主要是通过主要发生在发震带周围的慢地震的分布来估计的。但是，微弱断层的详细深度分布仍然难以捉摸。在这里，我们部署了6个海底地震仪在南海俯冲带，日本，观察思考源于钻井船地球号船噪声（水声P这是坚持从固定的位置在海面辐射波），和检索的P -到- s（Ps）来自板边界附近的多个倾角断层的反射。该诗根据潮汐应力的程度对振幅进行叠加和比较，并在涨潮（压缩）时观察到高振幅。一项迁移技术表明，在兆张断层和兆张断层与洋壳顶部之间的另一个断层，都估计了速度对比波动的位置。这表明潮汐应力改变了这些断层的物理性质。物理属性的变化归因于潮汐应力波动引起的断层内的流体连接和隔离，从而引起了地震各向异性的变化。这种变化通过各向异性介质的波传播的三维数值模拟得到了证实。我们的观察表明，板块边界附近存在多个弱断层，