17:15 〜 18:45
[SIT14-P11] 方位依存レシーバ関数とマルチモード表面波を用いたベイズ推定による豪州大陸の上部マントル不連続面マッピング
キーワード：上部マントル、リソスフェア、アセノスフェア、異方性、レシーバ関数、表面波
Major seismic discontinuities such as the Lithosphere-Asthenosphere Boundary (LAB) are key to understanding the evolutional history of the Earth and mantle dynamics. The Australian continent is the fastest-moving continental plate (6-7cm/yr), comprising the Archean and Proterozoic cratons in the west and the Phanerozoic orogenic zone in the east. Mapping discontinuities of this continental plate will lead us to advance our understanding of the formation and deformation process of the continental lithosphere drifting over a long geological time.
Recent studies investigating the upper mantle structure have employed joint inversions with surface-wave dispersions (SWDs) and body-wave receiver functions (RFs) (e.g., Calo et al., 2016, EPSL; Taira & Yoshizawa, 2020, GJI). In such studies, the azimuthal dependence of RFs is generally ignored under the assumption of a 1-D stratified structure beneath a seismic station. However, if there is a rapid lateral variation of seismic interfaces under the station, RFs would vary with the incoming direction of teleseismic body waves (e.g., direct P-waves for P-RF).
In this study, at first, we estimate the orientation of horizontal components of the seismometer at all the employed stations in Australia based on the time-domain polarization analysis (Vidale, 1986, BSSA) using the direct P-waves for the better estimation of azimuth-dependent P-RFs. Then, we performed the trans-dimensional Bayesian inversion with multimode SWDs and azimuth-dependent P-RFs to estimate the radially anisotropic S-wave models (SV and SH wave speeds) and the conversion points from P to S for various incoming directions of P waves.
Our retrieved models indicate the azimuth-dependent LAB and Lehmann Discontinuity (L-D). In eastern Phanerozoic Australia, significant SV-wave speed drops, representing LAB, are generally observed at shallow depths (70-80km). However, LAB becomes deeper (130-140 km) in the north and west of a station near the margin with the western cratonic region, where the lithosphere thickens. Below the LAB, we could also detect seismic discontinuities accompanied by the S-wave velocity jump, which can be interpreted as L-D. Compared with LAB, the L-Ds are generally observed as flat and multiple interfaces in eastern Australia. On the other hand, in the western cratonic region, although the S-velocity drop seems unclearer, the LAB-like signature can be found at deeper depths (110-180 km) than in eastern Australia. In addition, L-D is seen as a single interface at around 300 km depth. As expected from earlier studies, radial anisotropy (xi = (VSH/VSV)2) increases below the LAB, reflecting the horizontal shear flow in the asthenosphere. On the contrary, below the L-Ds, radial anisotropy becomes weak and closer to isotropy, suggesting the transition from the dislocation to diffusion creep as expected from mineral physics.
Recent studies investigating the upper mantle structure have employed joint inversions with surface-wave dispersions (SWDs) and body-wave receiver functions (RFs) (e.g., Calo et al., 2016, EPSL; Taira & Yoshizawa, 2020, GJI). In such studies, the azimuthal dependence of RFs is generally ignored under the assumption of a 1-D stratified structure beneath a seismic station. However, if there is a rapid lateral variation of seismic interfaces under the station, RFs would vary with the incoming direction of teleseismic body waves (e.g., direct P-waves for P-RF).
In this study, at first, we estimate the orientation of horizontal components of the seismometer at all the employed stations in Australia based on the time-domain polarization analysis (Vidale, 1986, BSSA) using the direct P-waves for the better estimation of azimuth-dependent P-RFs. Then, we performed the trans-dimensional Bayesian inversion with multimode SWDs and azimuth-dependent P-RFs to estimate the radially anisotropic S-wave models (SV and SH wave speeds) and the conversion points from P to S for various incoming directions of P waves.
Our retrieved models indicate the azimuth-dependent LAB and Lehmann Discontinuity (L-D). In eastern Phanerozoic Australia, significant SV-wave speed drops, representing LAB, are generally observed at shallow depths (70-80km). However, LAB becomes deeper (130-140 km) in the north and west of a station near the margin with the western cratonic region, where the lithosphere thickens. Below the LAB, we could also detect seismic discontinuities accompanied by the S-wave velocity jump, which can be interpreted as L-D. Compared with LAB, the L-Ds are generally observed as flat and multiple interfaces in eastern Australia. On the other hand, in the western cratonic region, although the S-velocity drop seems unclearer, the LAB-like signature can be found at deeper depths (110-180 km) than in eastern Australia. In addition, L-D is seen as a single interface at around 300 km depth. As expected from earlier studies, radial anisotropy (xi = (VSH/VSV)2) increases below the LAB, reflecting the horizontal shear flow in the asthenosphere. On the contrary, below the L-Ds, radial anisotropy becomes weak and closer to isotropy, suggesting the transition from the dislocation to diffusion creep as expected from mineral physics.