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[SCG50-02] Effect of the Philippine Sea slab–overriding plate contact on the stress field in the Southwest Japan arc
Keywords:Stress loading, Southwest Japan arc, Philippine Sea slab, Kinki triangle, 1891 Nobi Earthquake, Finite element method
The Southwest (SW) Japan arc is formed by the subduction of the Philippine Sea plate, the movement of which is currently northwestward. The regional stress field is east-west compression in general. While the Philippine Sea slab has almost no deformation at the beginning of the subduction, which is inferred by the linear geometry of the Nankai Trough, it is strongly deformed at depth, which is rare compared with the other subducted slabs. In particular, a remarkable folded geometry can be seen under the Nobi plain and the Kii Peninsula, in the middle of the SW Japan arc. A recent receiver function analysis revealed that the Philippine Sea slab strongly indents into the bottom of the lower crust of the overriding plate under the Nobi plain. Above and west of the slab-crust contact zone, a lot of reverse faults develop forming a part of the deformation zone of the Kinki triangle. East of the slab-crust contact zone is the zone of strike slip faults, including the source faults of the 1891 Nobi earthquake, which has been the maximum inland earthquake (M8) ever observed in the Japanese islands. The recently revealed slab-crust contact raises the problem of how it has contributed to such a pattern of crustal activity.
In order the investigate the slab-crust contact, we construct a 3D finite element model including the region. The geometry of the plate interface in the model is taken from the previous study based on the distribution of interplate earthquakes. Previous studies show that the regional inland earthquakes can be explain by the long-term east-west compression. Therefore, we impose the boundary condition of lateral compression, while we ignore the direct effect of subduction of the Philippine Sea slab. On the plate interface, we impose the constraint of no relative vertical movement and allow the relative horizontal movement only in the direction along the Philippine Sea subduction. We also assume that the overriding lithosphere and the slab are elastic and that the region under them are viscoelastic, since we consider the long-term stress loading. We consider the variation of the thickness of the overriding lithospheric between 20 km and 70 km while we fix the thickness of the slab to be 70 km. Taking the standard viscosity value of 1019Pa s, the model achieves the steady stress loading state. In this state, we perform the parameter study for the lithospheric thickness, the angle of the lateral compression, and the maximum depth of the slab.
We first take the lithospheric thickness to be 35 km, the angle of lateral compression to be 0 degree (east-west), and the maximum slab depth to be 100 km, as the reference model, and computed the stress loading rates. The stress rate field at the depth of the seismogenic zone (5-20 km) shows the pattern of reverse-type stress above and west of the slab-crust contact zone and strike-slip type stress east of the slab-crust contact zone. This pattern is well consistent with the observed stress pattern. Then we examine the stressing pattern change with the change of the lithospheric thickness. We obtained similar stressing pattern to the reference model for the lithospheric thickness of or below 40 km, but uniform reverse-type stress pattern for the lithospheric thickness of or more than 50 km. The effect of the angle of lateral compression and the maximum slab depth is negligible.
These results suggest the significant effect of slab-crust contact on the east-west compression in the SW Japan. The lithospheric thickness under the SW Japan is estimated to be 40 km. This model cannot explain the stress pattern of the western SW Japan (the Chugoku district). In this area, the direct effect of the subduction of the Philippine Sea plate might be dominant.
In order the investigate the slab-crust contact, we construct a 3D finite element model including the region. The geometry of the plate interface in the model is taken from the previous study based on the distribution of interplate earthquakes. Previous studies show that the regional inland earthquakes can be explain by the long-term east-west compression. Therefore, we impose the boundary condition of lateral compression, while we ignore the direct effect of subduction of the Philippine Sea slab. On the plate interface, we impose the constraint of no relative vertical movement and allow the relative horizontal movement only in the direction along the Philippine Sea subduction. We also assume that the overriding lithosphere and the slab are elastic and that the region under them are viscoelastic, since we consider the long-term stress loading. We consider the variation of the thickness of the overriding lithospheric between 20 km and 70 km while we fix the thickness of the slab to be 70 km. Taking the standard viscosity value of 1019Pa s, the model achieves the steady stress loading state. In this state, we perform the parameter study for the lithospheric thickness, the angle of the lateral compression, and the maximum depth of the slab.
We first take the lithospheric thickness to be 35 km, the angle of lateral compression to be 0 degree (east-west), and the maximum slab depth to be 100 km, as the reference model, and computed the stress loading rates. The stress rate field at the depth of the seismogenic zone (5-20 km) shows the pattern of reverse-type stress above and west of the slab-crust contact zone and strike-slip type stress east of the slab-crust contact zone. This pattern is well consistent with the observed stress pattern. Then we examine the stressing pattern change with the change of the lithospheric thickness. We obtained similar stressing pattern to the reference model for the lithospheric thickness of or below 40 km, but uniform reverse-type stress pattern for the lithospheric thickness of or more than 50 km. The effect of the angle of lateral compression and the maximum slab depth is negligible.
These results suggest the significant effect of slab-crust contact on the east-west compression in the SW Japan. The lithospheric thickness under the SW Japan is estimated to be 40 km. This model cannot explain the stress pattern of the western SW Japan (the Chugoku district). In this area, the direct effect of the subduction of the Philippine Sea plate might be dominant.