Seismic Evidence for Large-Scale Intraslab Heterogeneity Beneath Northeast Japan

Understanding the heterogeneity within a subducting slab is essential for elucidating its rheological properties, which can significantly affect subduction dynamics. Despite the importance, the fine structure of the slab has remained largely enigmatic due to the limited resolution of seismic tomography. Here, we utilize a deep learning approach, PickNet, to collect a comprehensive data set of arrival-times of the first P and S waves from local earthquakes in Northeast Japan. This enables the determination of a high-resolution ($0.2°\times 0.2°\times 30\text{km}$) model of P-wave velocity (Vp), S-wave velocity (Vs), and Vp/Vs ratio within the subducting Pacific slab. Our model reveals a distinct intraslab structure characterized by relatively high Vp (>+3%), slightly high Vs (<+1%), and a high Vp/Vs ratio (>+1%), extending deep into the lithospheric mantle of the slab, approximately $0.4°\times 0.4°\times 80\text{km}$ in size between longitudes 140.6°E and 142.0°E, and latitudes 39.8°N and 40.2°N beneath Northeast Japan. This anomalous structure is associated with a decrease or absence in lower-plane intermediate-depth seismicity of the double seismic zone within the slab. This result suggests that the presence of intrusive minerals, potentially affected by increased iron (Fe) content due to hot mantle upwelling, may reduce the viscosity of the slab at lithospheric depths and subsequently diminish the lower-plane seismicity. Our results highlight the presence of rheological heterogeneity within the subducting slab, providing new insights into its role in impacting the deep structure and geodynamics of the Earth, potentially facilitating slab detachment or tearing.