Seismological heterogeneity in subduction zones provides insights into slow earthquakes and potential megathrust earthquakes. Studies at the Kii Peninsula in the Nankai subduction zone suggest that there are high-density and high-velocity plutonic bodies in the accretionary prism over the subducting slab, potentially influencing megathrust earthquakes. The lateral variation of heterogeneity and the spatial extent of plutonic bodies remain to be investigated well. Our passive-source imaging of receiver-side Green's functions, from widely distributed campaign seismic observations, reveals a sharp negative S-wave velocity contrast on the top surface of the subducting Philippine Sea plate common to all along-dip profiles and a positive phase tilted upward in the forearc crust. The low permeability of the forearc crust prevents the infiltration of slab-dehydrated fluid further into the upper crust. In the western area, we also found positive phases tilted upward in the forearc crust. The negative phase extends towards the deeper extent of slow-earthquake sources. Meanwhile, the positive phase likely represents the top surface of plutonic rocks of the Kumano and Ohmine plutons that span all the way down to the plate interface. Together with observations of gravity anomaly, intraslab seismicity, and seismic tomography, our interpretation supports the presence of plutonic bodies which extend deep beneath the forearc crust as well as laterally over the subducting PHS slab, rather than a serpentinized mantle wedge. The upper plate is generally low in permeability, but areas with localized high permeability may exist on the updip side of tremor sources. This condition, wherein fluid can infiltrate upwards locally, may maintain the relatively less active slow earthquakes in the western area. The lateral variation of the upper-plate lithology likely influences fluid processes and slow earthquake activities.

中文翻译：

---

接收器侧格林函数揭示的低频地震上的深层深成体


俯冲带的地震非均质性为了解慢速地震和潜在的大推力地震提供了见解。对南海俯冲带纪伊半岛的研究表明，俯冲板块上方的增生棱柱中存在高密度和高速的深成体，可能会影响大逆冲地震。异质性的横向变化和深成体的空间范围仍有待深入研究。我们从广泛分布的战役地震观测中对接收器侧格林函数的无源成像显示，俯冲菲律宾海板块的顶面上有一个尖锐的负 S 波速度对比，这是所有沿倾剖面所共有的，并且在前弧壳中是一个向上倾斜的正相位。前弧壳的低渗透性可防止板状脱水液进一步渗入上壳。在西部地区，我们还发现弧前地壳中向上倾斜的正相。负相延伸到慢地震源的更深范围。同时，正相可能代表熊野和大峰冥王体的深成岩的顶面，一直延伸到板块界面。结合对重力异常、板内地震和地震断层扫描的观察，我们的解释支持存在深成体，这些深成体深深延伸到弧前地壳下方以及俯冲 PHS 板的横向上，而不是蛇纹岩化的地幔楔。上板的渗透率通常较低，但局部高渗透率的区域可能存在于震颤源的上倾侧。 在这种情况体可以在局部向上渗透，可能会维持西部地区相对不太活跃的慢地震。上板岩性的横向变化可能影响流体过程和缓慢的地震活动。