Decadal seafloor geodesy reveals changes in the slip deficit rate along the Nankai Trough

Identifying the frictionally locked area of a plate boundary is an important geodetic issue in mitigating earthquake disasters. However, recent long-term geodetic observations have revealed that actual geodetic data contain transient behaviors due to slow slip events or changes in slip deficit rates. Therefore, the use of a snapshot of temporally averaged geodetic data is far insufficient to understand the accurate frictional state. This problem is particularly serious at sub-seafloor plate boundaries, where continuous or periodic geodetic observations for a long time are rarely realized. 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 slip deficit rate changes occurring throughout the entire shallow side adjacent to the locked area.