Interseismic deformation of the Nankai subduction zone, southwest Japan, inferred from three-dimensional crustal velocity fields

We have studied crustal deformation in the Nankai subduction zone, southwest Japan, based on three-dimensional GPS velocity fields. Oblique subduction of the Philippine Sea plate has caused two different modes of deformation of the overriding plate: interseismic crustal shortening in the direction of plate convergence, and permanent lateral movement of the forearc. The block boundary dividing the forearc is the Median Tectonic Line (MTL); however, we assumed that its shallower portion is fully or partially locked to a certain depth. The plate boundary and the MTL are represented by many rectangular faults. We carried out inversion analyses with a priori information to estimate simultaneously slip deficit rates at those rectangular faults, together with the rate of lateral movement of the forearc. The results show that the seismogenic subduction faults at a depth of 5–25 km are strongly locked. As for the transition zone at 25–35 km, the inversion analysis results in stronger locking than shown in previous studies, especially when the vertical velocity data are weighted. The rates of lateral forearc movement and slip deficit at the MTL are nearly comparable but in a reverse sense to each other. This shows that the shallower portion of the MTL is strongly locked but that stationary aseismic slip is occurring in the deeper part.