The Izu-Bonin subduction zone, located in the northwest Pacific Ocean and southeast of Japan, is a typical subduction system formed by the westward subduction of the Pacific slab beneath the Philippine Sea Plate, and the strike direction is nearly N-S. This region is characterized by frequent seismic and volcanic activities, complex tectonic processes, and has long served as a key area for studying geodynamics and plate interactions. Seismicity within the subduction zone provides valuable insights into the movement and deformation of Earth’s interior materials. The accompanying shear-waves record anisotropic characteristics along the propagation paths, offering opportunities to understand plate dynamics, mantle flow mechanisms, and the interaction between subducting slabs and surrounding mantle structures. Compared to previous studies using land-based stations, this study aims to investigate the anisotropic characteristics within the subducting Pacific Plate and the sub-slab mantle using ocean bottom seismometers.

Based on the seismic data from 17 Japan’s S-net seafloor seismometers deployed near the trench at the northern end of the Izu-Bonin subduction zone, eight shallow and deep seismic events are chosen from the central and southern parts of the subducted plate. Such station-event geometry excludes the influence of the overriding plate and mantle wedge in the subduction zone, allowing for an analysis of anisotropy within the subducting Pacific slab and the sub-slab mantle. Then two shear-wave splitting analysis methods, the minimum eigenvalue minimization method and waveform rotation cross-correlation method, are employed to investigate the anisotropic characteristics of the Pacific subducting slab and the sub-slab mantle in the Izu-Bonin subduction zone. By conducting shear-wave splitting analysis on the direct S phases and the seismic upward-propagating s phases, a total of 36 reliable shear-wave splitting measurements were obtained and the results were categorized into three types based on the ray paths reflecting anisotropic characteristics across different regions.

Type Ⅰ events, with focal depths of 61−67 km, primarily samples the shallow part of the subducting slab. The fast axis orientations are NNE−SSW, with an average orientation of 67.8°, intersecting the trench at a small angle. These results reflect the anisotropy associated with trench-parallel faults formed during subduction, later modified by local stress field, causing the fast-axis to deviate from a trench-parallel orientation. Variations in the local stress field may enhance or reduce the regional anisotropy, resulting in significant variations in splitting time （0.14−0.82 s） along different seismic ray paths.

Types Ⅱ and Ⅲ events are deep earthquakes （399−464 km） with distinct ray path characteristics. Type Ⅱ ray paths mainly propagate within the slab, with limited propagation in the sub-slab mantle, while Type Ⅲ paths sample both the subducting slab and the sub-slab mantle, incorporating more sub-slab mantle anisotropy. The fast-axis orientations of Type Ⅱ events are NNW−SSE, with an average orientation of −80.7°, and splitting times ranging from 0.08−0.6 s. Similarly, Type Ⅲ events have fast-axis orientations of NNW−SSE, with an average orientation of −74°, and splitting times ranging from 0.12−0.86 s.

Integrated analysis of Type Ⅱ and Type Ⅲ results reveal that the fast axis orientations in the deep Pacific slab aligns with the seafloor spreading direction, indicating the “fossil” anisotropy formed during plate solidification. In contrast, the anisotropy in the sub-slab mantle is primarily driven by the dynamics of the subducting slab. As the slab dip angle steepens from north to south, the slab undergoes back-rotation, inducing significant trench-parallel mantle flow in the sub-slab mantle. This flow results in lattice-preferred orientation （LPO） aligning along the trench direction, producing pronounced trench-parallel anisotropy. This dynamic interaction highlights the critical role of slab geometry and motion in shaping subduction zone anisotropic patterns.

A comparison of splitting times indicates that the average splitting time for Type Ⅲ events is 0.34 s, significantly greater than the 0.226 s average for Type Ⅱ events. This suggests that anisotropy intensity in the sub-slab mantle is markedly higher than the “fossil” anisotropy within the subducting slab. The sub-slab mantle anisotropy is driven by the dynamic processes of the subducting slab, with its intensity reflecting the impact of these processes on mantle flow. It is noteworthy that mantle anisotropy intensity generally varies with depth: shallow regions exhibit enhanced anisotropy due to dislocation creep under high stress, while deeper regions show reduced anisotropy due to diffusion creep. Furthermore, the splitting times range from 0.08−0.86 s, indicating the heterogeneous nature of observed anisotropy, which may reflect the intensity variations of mantle anisotropy along the trench direction, influenced by slab geometry and associated mantle flow pathways.

These results provide new insights into the spatial distribution and genesis of anisotropy within the subducting Pacific slab and sub-slab mantle in the Izu-Bonin subduction zone. They highlight the combined effects of plate formation history, local stress field variations, and subduction dynamics on anisotropy. Future research will integrate more seismic network data, particularly data with longer time spans, to further validate and refine the anisotropic characteristics of the study area. Additionally, by incorporating more geological, geochemical, and tomographic imaging results, a more comprehensive model can be built for the Izu-Bonin subduction zone to show its flow field and deformation history.