Abstract
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.