S波スプリッティングから見た下北半島下のリソスフェアの不均質構造

概要

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We measured shear-wave splitting parameters from local earthquakes at three stations in the Shimokita Peninsula, northeast Japan, and found the station-dependent feature of splitting. Polarization of fast S-waves at station OGM in the central part of the Peninsula is restricted to NNW-SSE, irrespective of source location. Because the polarization direction is consistent with the trend of P-axes of shallow earthquakes that occurred near the station, we interpret the origin of anisotropy as the preferred orientation of cracks aligned parallel to the direction of the maximum compressional stress in the crust. Conversely, the splitting observed at stations KMN and STK is characterized by N-S polarization for eastern ray paths and E-W polarization for western ray paths, which suggests the existence of two anisotropic bodies in the mantle wedge. The boundary of anisotropic bodies parallels the strong, positive magnetic anomaly that runs N-S from Hokkaido to the east coast of central Iwate prefecture. The N-S polarized rays pass through a region beneath the Kitakami pluton that causes the magnetic anomaly. The origin of N-S polarized anisotropy is possibly the alignment of solidified melt that had formed during the intrusion of magma in the Cretaceous age, and/or the mineral re-orientation enhanced by the magmatism.