Spatiotemporal distribution of interplate coupling in southwest Japan from inversion of geodetic data

We determine the spatiotemporal distribution of slip (or slip deficit) on the subduction interface of the Nankai trough over an entire earthquake cycle using geodetic data (including leveling, triangulation, and trilateration, sea level, and GPS surveys) obtained during the past 100 years in southwest Japan. We develop a new inversion method that accounts for long‐term crustal deformation, coseismic (earthquake) displacements, and stress relaxation of the viscoelastic asthenosphere. From this analysis we obtain a model that shows postseismic afterslip on the deeper part of the plate interface following the 1946 Nankaido earthquake. Significant afterslip is found beneath central Shikoku that totals about 0.8 m. The slip deficit rate during the interseismic period is 5–6 cm/year in the N50°W–N60°W direction, which is consistent with the relative plate motion between the Philippine and Amurian plates. A fully locked region is found in the shallower portion (<30 km), and the slip deficit rate has a maximum at a depth of about 20 km. The plate interface deeper than 30 km is slowly slipping. The amount of slip deficit reaches about 3.3 m off Shikoku and about 2 m off the Kii peninsula 50 years after the 1946 earthquake. This suggests that fault healing and full interplate coupling has occurred by this time along the Nankai trough.