Interplate coupling in northeast Japan deduced from inversion analysis of GPS data

The spatial distribution of the strength of interplate coupling between the subducting Pacific plate and the overlying continental plate in northeast Japan was investigated through an inversion analysis of GPS data, employing Akaike’s Bayesian Information Criterion. The GPS data used for the inversion analysis are rates of baseline length changes and vertical displacement rates of crustal movements during the period from April 6, 1996 to March 20, 1998, which were obtained at 161 continuous GPS observation stations operated by the Geographical Survey Institute of Japan. The result shows the average drag rates of the overriding plate on the model source regions off the Tohoku and Hokkaido districts are 6.7 cm/yr and 6.8 cm/yr, respectively, which are 0.85 and 0.87 as interplate coupling ratio. These values are much higher than those in previous studies. The average directions of drag rates are oriented N73°W±15° and N78°W±13° on the model source regions off the Tohoku and Hokkaido districts, respectively. These directions agree well with the direction of coseismic slip of the foot-wall side (N75°W) of the 1978 Miyagiken–Oki earthquake (M 7.4), but differ slightly from the direction estimated from the plate motion model (N65°W) [Seno et al., J. Geophys. Res. 101 (1996) 11305–11315]. We also found afterslip of 5.5 cm/yr associated with the 1994 Sanriku–Haruka–Oki earthquake (M 7.5) off Aomori prefecture. Interestingly enough, large afterslip occurred on the plate boundary even when more than a year had passed after the Sanriku–Haruka–Oki earthquake. The amount of drag rate becomes dramatically small at depths greater than about 50 km on the model source regions, indicating weak interplate coupling there.