Interplate coupling in the Tohoku district, Japan, deduced from geodetic data inversion

The spatial distribution and the temporal change of the strength of interplate coupling between the subducting Pacific plate and overlying continental plate in the Tohoku district, northeast Japan, have been investigated through the inversion analysis of geodetic data. The geodetic data used for this analysis are the rates of vertical (1966-1995) and horizontal (1979-1991) displacements, which presumably represent average crustal movements during the interseismic period. We incorporate smoothing constraints on back slip distribution into the observed data. The degree of the constraints is controlled by hyperparameters whose optimal values are determined by minimizing Akaike's Bayesian Information Criterion (ABIC). Under the assumption of ABIC, we can obtain a unique solution. The result of the inversion analysis shows that there are two regions of high coupling, the first extending from 20 to 45 km in the north (40.5 degrees N) and the second extending from 25 to 55 km in the south (38.5 degrees N). The average back slip rate in the northern region reaches 55 +/- 11 mm yr(-1), and in the southern region 41 +/- 14 mm yr(-1). Also, the inversion analysis shows that the direction of the plate convergence at the Japan trench is N71 degrees +/- 3 degrees W in the northern region and N48 degrees +/- 5 degrees W in the southern region. The interplate coupling becomes weaker toward the shallower and deeper portions and exhibits significant temporal variation.