Crustal deformation and interplate coupling in the Shikoku district, Japan, as seen from continuous GPS observation

Abstract

Data from the nationwide GPS continuous tracking network, which has been operated by the Geographical Survey Institute (GSI) of Japan since April 1996, were used to study crustal deformation in the Shikoku district, Japan. The crustal deformation data are inverted to estimate interplate coupling between the subducting Philippine Sea (PHS) plate and the overriding continental plate. For this purpose, we first extracted site coordinates from daily SINEX files for the period from 1 April 1996 to 31 March 1998, and estimated average site velocities by linear regression. Estimated velocities under ITRF96 are converted into a kinematic reference frame [K. Heki, J. Geophys. Res. 101 (1996) 3187–3198] to discuss the crustal deformation relative to the stable interior of the Eurasian (EUR) plate. To segregate the signal and noise in the thus obtained velocity vectors, we employed the least-squares prediction technique. Estimated signals (displacement vectors) were used to reconstruct the strains (dilatations, maximum shear strains, principal axes of strains) and to estimate the interplate coupling in the area. Results of crustal strains clearly portray tectonic straining of the Shikoku island: (1) dilatational strains show that the Shikoku island is under a compressive strain regime induced by the subducting PHS plate; (2) maximum shear is high in the island, which may be due to the oblique subduction; (3) the principal axes of the strains indicate that the Shikoku island is under the influence of the converging oceanic plate beneath the island. In contrast, the results of the inversion analysis show: (1) there is a high coupled region extending from 10 to 30 km deep; (2) the average back slip rate in this region reaches 43±5 mm/yr; and (3) the direction of the plate convergence at the Nankai trough is N42±6°W.