Full interseismic locking of the Nankai and Japan-west Kurile subduction zones: An analysis of uniform elastic strain accumulation in Japan constrained by permanent GPS

We analyze permanent Global Positioning System (GPS) data obtained over Jap an between 1995 and 1997 to estimate the instantaneous interseismic couplin g ratio of the seismogenic zones due to the subduction of the Pacific and P hilippine Sea plates below the Japanese islands. We first derive the GPS st rain rate fields that characterize the crustal deformation of southern and northern Japan and invert them to determine the effective subduction veloci ty along the central Nankai trough on one side and the Japan-west Kurile tr ench on the other. These "reference free" velocities are close to those pre dicted by plate motion models with respect to Eurasia. We conclude that the Eurasian reference frame gives a good approximation to the subduction moti on and that to first order, both subduction zones were fully locked during the period of measurements. We then test whether the coupling ratio shows l ocal variations within the seismogenic zones. To do this, we divide the sub duction interface into 35 km x 30 km elements that we model by point source groups, and we invert the GPS velocity field referenced to Eurasia to deri ve the coupling ratio (between 0 and 1) on each fault element. The results are coherent over the 3 years and confirm that both the central Nankai and the Japan-west Kurile seismogenic zones are homogeneously fully locked. Mos t of the coupling ratios are close to 1 and a few are close to 0; intermedi ate values are rare. The zones of decoupling correspond either to strong po stseismic afterslip associated with the 1994 Sanriku-Oki interplate earthqu ake (Japan trench) or to a small overestimation of the actual lower limit o f the locked zone. We conclude that within the resolution of the GPS data a nd the model, (1) partial coupling did not exist during these 3 years along the Nankai and Japan-west Kurile trenches; (2) the small seismic coupling ratio previously derived from earthquakes analysis for the Japan and Kurile trenches may indicate that a significant part of the elastic energy is dis sipated silently through slow earthquakes and postseismic afterslip; and (3 ) the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading. Finally, we discuss the nonelastic residual deformati on within the frame of the long-term deformation of the Japanese islands.