Seismic Anisotropy Within the Subducting Northern Philippine Sea Plate, SW Japan, Using DONET Seafloor Observation Network

Anisotropy within subducted slabs carries important information about the deformation history of the slabs. Seismic waves propagating a long distance within a subducted slab can be utilized to invert for the slab anisotropy. The Dense Ocean Floor Network System (DONET) deployed near the Nankai trough are located in a suitable position to record seismic waves guided along the subducted northern Philippine Sea plate (nPSP). In this study, we perform the shear wave splitting analysis on local event data recorded by ocean bottom seismometers from DONET to measure the anisotropy within the subducting nPSP. The obtained fast axis is sub-trench-parallel with inferred strength of 1%-3% anisotropy, arising likely from the fossilized fabric formed during the seafloor spreading period and strengthened by trench-parallel serpentinized hydrated faults. This result shows that the fossilized anisotropy of nPSP is preserved during the subduction. Plain Language Summary The geological processes can cause the preferred orientation of anisotropic minerals, resulting in seismic anisotropy. Thus seismic anisotropy beneath a region carries important information about the formation and deformation history of the region. Compared with the anisotropy beneath continents, it remains a challenge to measure the seismic anisotropy beneath oceans, especially within the oceanic lithosphere, due to its relatively thin slab thickness accounting for only a small percentage of the whole seismic ray paths. Here we use seismic data recorded by ocean bottom seismometers (OBSs) deployed near the Nankai trough, where the northern Philippine Sea plate (nPSP) is subducting beneath the Eurasian Plate in the southwest Japan and generating deep earthquakes within nPSP. These OBSs are in a suitable position to detect the seismic waves propagating along the subducting nPSP, which can then be utilized to invert for the anisotropy of nPSP as they avoid the contamination from other structures of the subduction zone. We find that the anisotropy within nPSP may arise from the fossilized fabric during the seafloor spreading and be strengthened by trench-parallel faults associated with plate bending, and is preserved during the subduction.

Automated summary

Anisotropy within subducted slabs provides important information about their deformation history. In this study, we analyze seismic data recorded by ocean bottom seismometers to measure the anisotropy within the subducting northern Philippine Sea plate. The results suggest that the anisotropy is likely formed during the seafloor spreading period and strengthened by trench-parallel faults associated with plate bending.