Seismic anisotropy of the Pacific slab and mantle wedge beneath the Japanese islands

Abstract

[1] The mantle beneath the Japanese islands is complex because of subduction of the Pacific and Philippine Sea plates and the deformation associated with it. Detection of seismic anisotropy should be useful for understanding the processes occurring in such mantle. Here we resolve seismic anisotropy of the mantle wedge from that of the underlying subducted Pacific slab using a large number of measurements of shear wave splitting for a family of core-reflected shear phases: ScS, sScS, ScS2, and sScS2. The anisotropy of the mantle wedge changes sharply across the volcanic front. On the Pacific side of the volcanic front a vertically propagating shear wave is polarized with the fast direction approximately parallel to the trench, whereas on the back-arc side it is polarized with the fast direction approximately parallel to the plate convergence direction. The Pacific slab is uniformly anisotropic with the NNW fast direction, consistent with the paleospreading direction of the northwestern Pacific seafloor. The preferential alignment of anisotropic crystals frozen in the plate at the time of its formation appears to be preserved down to depths beyond 400 km within the slab.

Elasticity of single-crystal olivine at high pressures and temperatures

“…In addition to the low-velocity wedge in the slabs, seismic anisotropies have also been identified below the 410-km depth (Liu et al, 2008;Sandvol and Ni, 1997;Tono et al, 2009). The observed seismic anisotropy in the slabs in the mantle transition zone could be caused by the preferred alignment of the metastable olivine crystals (Liu et al, 2008;Mainprice, 2007;Sandvol and Ni, 1997;Tono et al, 2009).…”

“…The observed seismic anisotropy in the slabs in the mantle transition zone could be caused by the preferred alignment of the metastable olivine crystals (Liu et al, 2008;Mainprice, 2007;Sandvol and Ni, 1997;Tono et al, 2009). Using the obtained singlecrystal elasticity, we have computed the maximum azimuthal compressional-wave anisotropy, A P [(V P ,max − V P ,min )/V P ,ave ], and shear-wave splitting, A PO S [(V S2 − V S1 )/V S,ave ], of olivine at various slab temperatures (Fig.…”

“…Recently, a number of seismic studies have reported the existence of a low-velocity and anisotropic wedge with 3-5% lower velocities and shear-wave splitting of ∼1 s in the slabs below the 410-km depth, which was viewed as evidence for the presence of the metastable olivine in the subducted slabs (e.g. Jiang et al, 2008;Kaneshima et al, 2007;Liu et al, 2008;Pankow et al, 2002;Tono et al, 2009). It is thus of great interest to model the velocity of metastable olivine in the transition zone depth.…”

“…Knowledge of the elasticity of single-crystal olivine is thus the key to understanding the anisotropic seismic structures of the upper mantle (Mainprice, 2007;Mainprice et al, 2000;Nunez-Valdez et al, 2010;Zha et al, 1996). Furthermore, seismic anisotropies have also been observed in the subduction slabs in the transition zone and used as potential evidence for the presence of the metastable olivine wedge in the region (Chen and Brudzinski, 2003;Liu et al, 2008;Sandvol and Ni, 1997;Tono et al, 2009). We note that the phase transition of olivine to ringwoodite has been proposed to be a potential mechanism for occurrence of the deep earthquakes (e.g.…”

Seismic Anisotropy Analysis Beneath Sumatra Revealed by Shear-Wave Splitting

“…The knowledge of mantle movement can be obtained through a study of anisotropic structure of the earth mantle using a measurement such as share-wave splitting. A lot of attention had been given to investigate the other subduction zones, for example a subduction zone in Japan (Ando et al, 1983;Fouch and Fischer, 1998;Long and Hilst, 2005;Nakajima and Hasegawa, 2004;Tono et al, 2009) and in Cascadia (Eakin et al, 2010).…”