Deep plutonic bodies over low-frequency earthquakes revealed from receiver-side Green's functions

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.