Imaging Interseismic Locking at the Nankai Subduction Zone, Southwest Japan

Recent geodetic and seismological observations have revealed that brittle-plastic transition zones at subduction zone interfaces are loci of slow slip episodes and nonvolcanic harmonic tremors. It is therefore important to estimate the depth range of brittle-plastic transition zones and how interplate locking changes with space and time within the brittle-plastic transition zone, not only for understanding the interseismic stress accumulation process in subduction zones but also to explore the causes of slow earthquakes and nonvolcanic harmonic tremors. With this point of view, the depth variations of interplate locking status in the Nankai trough, southwest Japan, were investigated by inverting vertical Global Positioning System (GPS) velocities. Although vertical component of GPS velocities have rarely been used as important information because of its higher noise level, we employed vertical, rather than, horizontal, velocities because the vertical deformation field will enable us to separate rigid plate motions from deformation due to interplate locking, horizontal displacements contain both rigid plate motion and deformation from interplate locking, whereas vertical displacements contain only the interplate locking effect. Interplate locking was estimated for three profiles: the Source region of the 1946 Nankaido earthquake, that of the 1944 Tonankai earthquake, and the Tokai seismic gap. The results show a gradual decrease of interplate coupling between about 20-25 and 35-45 km depth with a plateau region between 25-35 km for all three profiles. These plateaus may be interpreted as stable-unstable boundary in rate- and state-dependent friction or represent the change in mechanism (say, frictionally to plastically) or material. The depth of the brittle-plastic transition zone in the Tokai profile seems to be shallower than other two probably because of the younger and hotter Philippine Sea plate in the Tokai area.