An acceleration of the background seismicity and a shortening of the slow slip events on the Boso peninsula (Japan) recurrence intervals suggest a slow decoupling of the Philippine Sea-North America (PHS-NAM) subduction interface from 1990 to 2011. Motivated by these observations, we used GPS (Global Positioning System) time series to study the 14-year evolution of interface coupling offshore Honshu with a specific focus on the Kanto region. We processed the GPS data in double difference and analyze them with a trajectory model that accounts for seismic and aseismic variations, and that includes an inter-seismic acceleration term. We inverted the surface acceleration obtained, on both the Pacific-North America (PAC-NAM) and the PHS-NAM interfaces. The inverted slip rate changes over time compares well with previous studies: we observe slip deceleration between 39$^o$-41$^o$ N and slip acceleration between 37$^o$-39$^o$ N, with a maximum amplitude of 3.45 mm/yr$^2$ corresponding to an equivalent geodetic coupling change of 0.64. Our analysis reveals a novel and robust slip acceleration South of 36.5$^o$ N that we interpret as a decoupling of the PAC-NAM interface. It is located noticeably far from the 2011 Tohoku earthquake rupture and is therefore unlikely connected to it. We link the slip rate changes to the background seismicity changes and retrieve the slip acceleration from either the seismicity rate or the surface displacement. Our results further demonstrate that inter-seismic slip rate can significantly evolve over years to decades, and suggest a simple relationship between the background seismicity and the slip on the subduction interface.