Decadal seafloor geodesy constrains frictionally locked areas along the Nankai Trough

Identifying the frictionally locked area of a plate boundary is an important geodetic issue in mitigating earthquake disasters, but slow slips make it impossible to understand the frictional state via temporally averaged geodetic data. This problem is particularly serious at sub-seafloor plate boundaries, where long-term geodetic observations are difficult. Here, we evaluated long-term slip deficit rate variations in the Nankai Trough, the only area in the world for which high-density data on horizontal and vertical components have been accumulated via seafloor observations with a sufficient frequency on the decadal scale. Consequently, we constrained the constantly locked areas and found that it was limited mainly to a depth of 10–20 km, with slow slips occurring throughout the entire shallow side adjacent to the locked area.