[1] We retrieve three-dimensional structures of isotropic and anisotropic velocities of P-waves of the Tohoku district from first P-arrival time data, assuming azimuthal anisotropy to be caused by hexagonal symmetry axes distributed horizontally in the Earth. The results show that the high-velocity Pacific slab is clearly imaged in the isotropic velocity structure, even though the azimuthal anisotropy is taken into account. In addition, small-scale low-velocity regions and prominent low-velocity anomalies are found just below the active volcanoes and in the mantle wedge above the high-velocity Pacific slab, respectively. The fast propagation axis of P-waves is in mostly E-W direction in the upper crust, nearly N-S and E-W directions in the lower crust, E-W direction in the mantle wedge, and N-S direction in the descending Pacific slab. These features of the P-wave anisotropy structure are consistent with those of lateral variations of the fast polarization directions measured previously by shear-wave splitting observations. The plausible factor that causes the crust anisotropy is interpreted as being alignment or preferred orientation of microcracks and crust minerals. The mantle wedge anisotropy is attributed to lattice preferred orientation of the mantle minerals arising from present-day mantle process such as the mantle wedge convection and the plate motion. However, the fast propagation axis of P-waves in the slab is almost perpendicular to the magnetic lineation of the oceanic plate under the northwest Pacific, and thus the slab preserves the original anisotropic property that the Pacific plate gained when it formed.Citation: Ishise, M., and H. Oda (2005), Three-dimensional structure of P-wave anisotropy beneath the Tohoku district, northeast Japan,

We investigate the effect of seismic anisotropy on P-wave receiver functions, calculating synthetic seismograms for P-wave incidence on multilayered anisotropic structure with hexagonal symmetry. The main characteristics of the receiver functions affected by the anisotropy are summarized as (1) appearance of seismic energy on radial and transverse receiver functions, (2) systematic change of P-to-S (Ps) converted waveforms on receiver functions as ray back-azimuth increases, and (3) reversal of the Ps-phase polarity on the radial receiver function in a range of the back azimuth. Another important influence is shear-wave splitting of the Ps-converted waves and other later phases reverberated as S wave. By numerical experiments using synthetic receiver functions, we demonstrate that the waveform cross-correlation analysis is applicable to splitting Ps phases on receiver functions to estimate the seismic anisotropy of layer structure. Advantages to utilizing the Ps phases are (1) they appear more clearly on receiver functions than on seismograms and (2) they inform us about what place along the seismic ray path is anisotropic. Real analysis of shear-wave splitting is executed to the Moho-generated Ps phases that are identified on receiver functions at six seismic stations in the Chugoku district, southwest Japan. The time lags between the two arrivals of the split Ps phases are estimated at 0.2-0.7 sec, and the polarization directions of the fast arrival components are from north-south to northeast-southwest. This result is consistent with recent results of shear-wave splitting measurements and the trend of linear epicenter distributions of crustal earthquakes and active fault strikes in the Chugoku district.

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The Kanto region of Japan, including the Tokyo metropolitan area, is located on the thick sediment of the Kanto Basin in the Okhotsk (OKH) plate. The tectonics of the area are dominated by the dual subduction of the Philippine Sea (PHS) plate and the Pacific (PAC) plate (Figure 1). As a result, the Kanto region has been severely damaged by M8-class earthquakes, which occur along the plate interfaces, as well as many M7class earthquakes within the subducting slabs (in Japanese, 2004, available at http://www.bousai.go.jp/jishin/syuto/taisaku_wg/pdf/syuto_wg_siryo04.pdf). Many studies have investigated seismic activity, seismic

We investigate slab and mantle structure of the NE Japan subduction zone from P wave azimuthal and radial anisotropy using travel time tomography. Trench normal E-W-trending azimuthal anisotropy (AA) and radial anisotropy (RA) with VP V > VP H are found in the mantle wedge, which supports the existence of small-scale convection in the mantle wedge with flow-induced LPO of mantle minerals. In the subducting Pacific slab, trench parallel N-S-trending AA and RA with VP H > VP V are obtained. Considering the effect of dip of the subducting slab on apparent anisotropy, we suggest that both characteristics can be explained by the presence of laminar structure, in addition to AA frozen-in in the subducting plate prior to subduction.Plain Language Summary There is increasing importance and interest in seismic anisotropy because it can provide crucial constraints on the lithospheric structure as well as the nature of dynamics of mantle flow. In this study, we performed two types of anisotropic tomography analyses using the same data set and estimated three-dimensional P wave azimuthal and radial anisotropy structures beneath NE Japan. These tomography analyses show that mantle wedge of the subduction zone is characterized by E-W-trending azimuthal anisotropy and radial anisotropy with VP V > VP H . On the other hand, N-S-trending azimuthal anisotropy and radial anisotropy with VP H > VP V are shown in the Pacific slab. Assuming flow-induced lattice preferred orientation of mantle minerals, the observed mantle wedge anisotropy can be explained by 3-D mantle flow with small-scale convection. Also, we evaluated the effect of dip of the slab that is anisotropic and carefully considered resulting apparent radial and azimuthal anisotropies in our tomography. Then, the anisotropy in the Pacific slab can be explained by a combination of positive radial anisotropy due to laminar scatterers in the oceanic plate and azimuthal anisotropy frozen-in during the formation of an oceanic plate whose fast direction is parallel to the ancient spreading direction.