Block movement and internal deformation of the Nankai forearc sliver associated with oblique subduction of the Philippine Sea plate in southwest Japan

We investigate block movement and internal deformation of the Nankai forearc sliver in southwest Japan. The forearc has been deformed by oblique subduction of the Philippine Sea plate: interseismic crustal shortening in the direction of plate convergence and long-term lateral movement along the Median Tectonic Line (MTL) that divides the forearc from the rest of the overriding plate. In this study, we decompose crustal deformation field into these two components using three-dimensional surface displacement rates from nationwide continuous GPS array and supplementary campaign networks across the MTL. We use the following procedures: (1) we correct the original velocity data to remove the forearc movement, assuming that the forearc moves at a constant rate of 3-10 mm/yr along the MTL but its fault plane is fully locked from surface to a depth of 15 km. (2) Using the corrected velocity data, we estimate interseismic slip deficit distribution on the plate interface reproduced by more than 500 triangular elements. Then the site velocities calculated from the above plate coupling distribution are compared with the original velocities. (3) Residuals between the original and calculated velocities illustrate the forearc lateral movement and the locking effect of the MTL fault plane. Now we can use the residual velocity field to estimate slip-locking distribution on the MTL fault plane. (4) We check the first-assumed constant rate of the forearc movement by comparing it with the estimated slip deficit rate on the MTL. In this analysis the optimal rate of the forearc movement is estimated as large as 5-6 mm/yr. In the eastern Shikoku the slip pattern is nearly pure strike-slip at a rate of 2-4 mm/yr. In contrast significant normal component is recognized together with strike-slip component of about 5 mm/yr in the western Shikoku.