Regional variation in shear-wave polarization anisotropy of the crust in southwest Japan as estimated by splitting analysis of Ps-converted waves on receiver functions

Abstract

Regional variation in shear-wave polarization anisotropy of the crust is investigated in the Kyushu, Chugoku, Shikoku and Kinki districts, southwest Japan, using Ps-converted phases generated at the Moho discontinuity. The Ps phases on radial and transverse receiver functions are analyzed to determine shear-wave splitting parameters of fast polarization direction and time lag by waveform cross-correction analysis. As a result, the fast polarization directions are shown to be obviously different between the Chugoku district and the Kyushu and Shikoku districts, which are separated from each other by the Seto Inland Sea. In the Kyushu, Shikoku and south Kinki districts, the Ps phases exhibit fast polarization directions ranging from E–W to NW–SE. The polarization directions are not only consistent with those of S waves from shallow crustal earthquakes but also parallel to the direction of maximum horizontal compression acting on southwest Japan and the strike directions of metamorphic belts, accretionary zones and rift zones. Thus the polarization anisotropy is understood to reflect mainly the upper crustal anisotropy that is attributable to stress-induced vertical cracks and geological lineament structure. On the other hand, in the Chugoku and north Kinki districts, fast polarization directions are in a range from N–S to NE–SW. They are not consistent with the tectonic stress field and the S-wave splitting observed for shallow crustal earthquakes. The disagreement between the fast polarization directions estimated from the S waves and the Ps-converted phases is interpreted as arising because the lower crustal anisotropy attributed to mineral preferred orientation has a significant influence on the Ps-phase splitting. The regional variation in Ps-phase polarization anisotropy of the crust in southwest Japan is dependent on which anisotropy of the upper and lower crust is prominently reflected in the Moho Ps-phase splitting.