The distribution of split shear-wave parameters in the backarc region of the South Kuril subduction zone

Abstract

This paper reports a study in the distribution of parameters of split shear waves excited by deep earthquakes in the Sakhalin and Hokkaido area, with the orientation of the axes of symmetry in the earth being estimated assuming a viscoelastic anisotropic model for the mantle. A mantle flow along NW 310° ± 20° has been identified beneath the Japan Sea. The flow is inclined at an angle of 20–30° relative to the horizontal plane and is consistent with the motion of the Pacific plate. Beneath the southern Sea of Okhotsk the fast a-axis [100] of olivine is oriented NE 30 ± 15°, nearly parallel to the trend of the Kuril arc, while the c-axis [001] is inclined at ∼35° relative to the horizontal plane. Bearing in mind the increased heat flow in the region, we assume the development of a mantle flow along the olivine c-axis at NW 300° ± 20° and with the shear plane [010] in the conditions of partial mantle melting (the B-type LPO). The lowest anisotropy (1–2.5%) was identified beneath Sakhalin and the greatest (3–5%) beneath the Japan Sea. An increasing degree of anisotropy is also noted to occur with an increasing depth of focus (down to 350 km).