DEFORMATION OF THE JAPANESE ISLANDS AND SEISMIC COUPLING - AN INTERPRETATION BASED ON GSI PERMANENT GPS OBSERVATIONS

The entire area of the Japanese Islands has been covered by the perman ent GPS observation network of the Geographical Survey Institute since 1994, In this paper we use a solution for the vectors of motion durin g 1995 for a selection of 116 stations to discuss the origin of the ob served deformation field. We refer the displacement field to Eurasia u sing the VLBI-determined motion of Kashima and demonstrate that other choices such as the Okhotsk or North American plates for north Japan a re not compatible with the data. 1 yr GPS velocities are much higher t han geological constraints would allow because these short-term measur ements include transient elastic deformation. However, the good qualit ative agreement between the observed geodetic deformation tensors and those inferred from active faults and earthquakes suggests that the Qu aternary permanent deformation is essentially the result of the transf er of part of the subduction-induced elastic deformation into permanen t plastic deformation. We then compute the elastic deformation of the Japanese Islands caused by interseismic loading of the Pacific and Phi lippine subduction planes. The geometry of the coupled zone and its do wnward extension are determined from the distribution of earthquakes f or the Pacific slab. For the Philippine slab we use the geometry propo sed by Hyndman et al. (1995). These elastic models account for most of the observed velocity field if the subduction movement of the Philipp ine Sea Plate is 100 per cent locked and if that of the Pacific Plate is 75-85 per cent locked. We note that the boundaries of the areas whe re significant elastic deformation is predicted (more than 10 mm yr(-1 ) of motion with respect to Eurasia) coincide with the main zones of p ermanent deformation: the Eastern Japan Sea deformation zone for the P acific subduction elastic deformation field and the Setouchi/MTL defor mation zone for the Nankai field. Each zone probably accommodates 10-1 5 mm yr-l of motion in the long term (convergence in the Eastern Japan Sea; strike-slip in the Setouchi/MTL zone), To account for this defor mation, the effect of elastic loading from the trench must be combined with 5-10 mm yr(-1) of motion of the Amur Plate with respect to Euras ia. Because loading during the subduction earthquake cycle causes an i ncrease in stress in the Eastern Japan Sea and Setouchi/MTL deformatio n zones, the probability of earthquake occurrence in these zones may b e higher near the end of the cycle.