Three‐dimensional structure of <i>P</i>‐wave anisotropy beneath the Tohoku district, northeast Japan

Abstract

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.