3:00 PM - 3:15 PM
[SCG49-06] Crust and Mantle Structure of the Tohoku Region from the Viewpoint of Seismic Anisotropy
★Invited Papers
Keywords:seismic anisotropy, Tohoku region, tomography, tectonics
Seismic anisotropy is closely related to stress fields, flow fields, and structural geometries, and thus provides useful information for understanding tectonics and dynamics of subduction zone. In this presentation, we present the results of azimuthal and radial anisotropy tomography analyses of the Tohoku region (Ishise et al., 2018) and show the characteristics of crustal and mantle anisotropy.
The main features of the azimuthal and radial anisotropy of the upper crust, lower crust, and upper mantle (mantle wedge) obtained by tomographic analyses and their interpretations are as follows:
Upper crust (0 and 10 km depths)
At a depth of 10 km, N–S-trending anisotropy (from NNW–SSE to N–S) is dominant in the Kitakami region, and NW–SE and NE–SW anisotropy coexist. At a depth of 0 km, both N–S- and EW-trending anisotropy are observed. The distribution of N–S-trending anisotropy is almost consistent with the distribution of earthquakes with N–S compression seismic mechanism and distribution of major active faults with N–S strike. In general, anisotropy in the upper crust is explained by regional stress fields, but in the Tohoku region, it is thought to be caused by regional tectonics and geological structure.
Beneath the land area, the forearc region shows radial anisotropy of VPH > VPV and the backarc region show radial anisotropy of VPH < VPV, respectively. This distribution pattern suggests the existence of large-scale structure that separates the forearc region from the backarc region.
Lower crust (25 km depth)
E–W-trending azimuthal anisotropy is dominant beneath the inland area. The direction is almost coincides with the direction of motion of the PAC slab relative to the Okhotsk plate, and is presumed to reflect the plastic deformation and flow field of the lower crust caused by the subduction of the PAC plate.
The N–S-trending azimuthal anisotropy is zonally distributed beneath the coastal areas of the Pacific Ocean and Japan Sea. This anisotropy can be attributed to plastic flow in the N–S direction or the developed internal structure (such as laminated structure) orientated in the N–S direction. However, since it is unlikely that orthogonal deformation fields exist next to each other, the N–S anisotropy can be attributed to the geometry of the internal structure.
In addition, a characteristic zonal region with NW–SE azimuthal anisotropy is found beneath the Honjo-Sendai tectonic line. This may reflect the deep structure of the tectonic line.
The coastal regions indicating N–S azimuthal anisotropy shows distinct radial anisotropy of VPH > VPV, which suggests that the laminated structure has gentle slope. On the other hand, strong radial anisotropy of VPH < VPV are observed beneath the inland area, suggesting that the lower mantle in the Tohoku region dose not have a horizontal layering structure.
Mantle wedge (deeper than 40 km)
N–S-trending azimuthal anisotropy is widely distributed beneath the land area. The direction is almost parallel to the strike of the iso-depth contours of the PAC slab, which suggests that the flow field in the mantle wedge would be controlled by the geometry of the PAC slab.
Radial anisotropy of VPH < VPV is found beneath the land area, suggesting the presence of small scale convection (e.g., Honda and Yoshida, 2005). Although it is not clear, the areas around the active volcanoes indicate strong VPH < VPV anisotropy.
In this study, the anisotropic features the upper crust, lower crust, and mantle wedge are presented and interpreted individually. As the next step, we aim to relate them and comprehensively interpret the anisotropic structure of the Tohoku region.
The main features of the azimuthal and radial anisotropy of the upper crust, lower crust, and upper mantle (mantle wedge) obtained by tomographic analyses and their interpretations are as follows:
Upper crust (0 and 10 km depths)
At a depth of 10 km, N–S-trending anisotropy (from NNW–SSE to N–S) is dominant in the Kitakami region, and NW–SE and NE–SW anisotropy coexist. At a depth of 0 km, both N–S- and EW-trending anisotropy are observed. The distribution of N–S-trending anisotropy is almost consistent with the distribution of earthquakes with N–S compression seismic mechanism and distribution of major active faults with N–S strike. In general, anisotropy in the upper crust is explained by regional stress fields, but in the Tohoku region, it is thought to be caused by regional tectonics and geological structure.
Beneath the land area, the forearc region shows radial anisotropy of VPH > VPV and the backarc region show radial anisotropy of VPH < VPV, respectively. This distribution pattern suggests the existence of large-scale structure that separates the forearc region from the backarc region.
Lower crust (25 km depth)
E–W-trending azimuthal anisotropy is dominant beneath the inland area. The direction is almost coincides with the direction of motion of the PAC slab relative to the Okhotsk plate, and is presumed to reflect the plastic deformation and flow field of the lower crust caused by the subduction of the PAC plate.
The N–S-trending azimuthal anisotropy is zonally distributed beneath the coastal areas of the Pacific Ocean and Japan Sea. This anisotropy can be attributed to plastic flow in the N–S direction or the developed internal structure (such as laminated structure) orientated in the N–S direction. However, since it is unlikely that orthogonal deformation fields exist next to each other, the N–S anisotropy can be attributed to the geometry of the internal structure.
In addition, a characteristic zonal region with NW–SE azimuthal anisotropy is found beneath the Honjo-Sendai tectonic line. This may reflect the deep structure of the tectonic line.
The coastal regions indicating N–S azimuthal anisotropy shows distinct radial anisotropy of VPH > VPV, which suggests that the laminated structure has gentle slope. On the other hand, strong radial anisotropy of VPH < VPV are observed beneath the inland area, suggesting that the lower mantle in the Tohoku region dose not have a horizontal layering structure.
Mantle wedge (deeper than 40 km)
N–S-trending azimuthal anisotropy is widely distributed beneath the land area. The direction is almost parallel to the strike of the iso-depth contours of the PAC slab, which suggests that the flow field in the mantle wedge would be controlled by the geometry of the PAC slab.
Radial anisotropy of VPH < VPV is found beneath the land area, suggesting the presence of small scale convection (e.g., Honda and Yoshida, 2005). Although it is not clear, the areas around the active volcanoes indicate strong VPH < VPV anisotropy.
In this study, the anisotropic features the upper crust, lower crust, and mantle wedge are presented and interpreted individually. As the next step, we aim to relate them and comprehensively interpret the anisotropic structure of the Tohoku region.