Seismic anisotropy within the subducting Philippine Sea slab beneath the central Japan

Subduction of the Philippine Sea slab (PHS) causes recurrent megathrust earthquakes every 100 to 150 years. Knowledge of slab geometry has been increased by using the recently established dense seismograph networks in Japan, but anisotropic feature, which is related to the tectonic stress field and/or rock properties, within the slab is still unclear. In order to reveal depth-dependent anisotropic feature within the PHS by using teleseismic receiver functions (RFs), we select 33 stations located in the Kii Peninsula, central Japan. We choose teleseismic events (M>6.0) from October 2000 to April 2013 for RF analysis, and use seismograms with good signal-to-noise ratio. Low-pass filters with fc = 1.0 and 1.5 Hz are applied to estimate RFs. To estimate the orientation of anisotropy symmetry axis, we apply the harmonic expansion method to the RFs at each station. In the depth range just below the slab Moho (slab mantle), the symmetry axes correspond to the dip direction of the PHS well, and 2-lobe component is dominant. Within the oceanic crust, this trend is the almost same in the eastern Kii Peninsula, though the 4-lobe component is larger than those in deeper part. On the other hand, the axes show EW direction in the southeastern part of the peninsula, and NS direction in the southwestern part. These directions are not consistent with the slab geometry or the regional stress field. This disturbance of seismic anisotropy may reflect complex local stress field as the PHS slab is bending like a valley at this region.