Temporal change of interplate coupling in northeastern Japan during 1995–2002 estimated from continuous GPS observations

SUMMARY Temporal change of deformation in northeastern Japan is clarified by continuous Global Positioning System (GPS) observations from 1995 April to 2002 March. The observed GPS velocity is approximately parallel to the direction of plate convergence on east and west plate boundaries of northeastern Japan and shows post‐seismic transient deformation around source regions of the M w 7.8 1993 Hokkaido‐Nansei‐Oki and the M w 7.7 1994 Sanriku‐Haruka‐Oki earthquakes. We interpret the source of the observed deformation as contemporary interplate coupling on the east subducting boundary to the Pacific Plate and the west collision boundary to the Amurian Plate. Using elastic dislocation theory, we inverted horizontal and vertical velocities of 212 GPS stations to estimate interplate coupling on both boundaries. The estimated coupling during 1995–2002 is spatially heterogeneous, however, it is temporally almost constant except for the region around the 1993 and 1994 earthquakes. After‐slip of the 1994 earthquake occurred over the coseismic rupture area and its downdip extension on the plate boundary for 0.3–1.3 yr after the earthquake. After‐slip continued only in the downdip extension for later periods and decayed with time. Weak coupling was recovered in the eastern part of the coseismic rupture area 3.3 yr after the earthquake. Interplate coupling on the Pacific Plate was strong in two regions, Miyagi‐Oki and Tokachi‐Oki. The west plate boundary is tightly coupled except for the source areas of three large earthquakes that occurred in 1964, 1983 and 1993. The apparent decoupling of the source areas of these earthquakes implies long‐term post‐seismic deformation as a result of viscoelastic relaxation in the subseismogenic lithosphere.