Spatial Variation of Slip Deficit Rate at the Nankai Trough, Southwest Japan Inferred from Three-Dimensional GPS Crustal Velocity Fields-Repeated Geodetic Inversion Analyses for the Shifted Target Area

The Philippine Sea plate has subducted beneath southwest Japan at the Nankai Trough, causing megathrust earthquakes every 100-140 years. The subduction process shows lateral variations along the trough, such as a bending of the plate interface beneath the Kii Peninsula and rapid changes of strike and dip off the east coast of Kyushu. We invert three-dimensional GPS velocities using ABIC geodetic inversion technique to estimate interseismic slip de.cit distribution at the Nankai Trough. The studied area is roughly from Kii peninsula to eastern Kyushu (131.5-137E, 31-35N). In the ABIC geodetic inversion some initial constraints are needed to stabilize solutions, such that the slip at the outer margin of the fault is set to zero. For the estimation of interseismic slip de.cit on the plate interface, however, these constraints are unrealistic because slip de.cit distribution continues later.ally to the outside of the target area. In this study we propose a repeated inversion method that shifts target area laterally along the plate boundary. This method averages slip de.cit estimates at the same point obtained from more than three consecutive inversions. The estimates obtained near the lateral margin, which may be strongly affected by the non-slip constraints at the outer margin, are excluded from the averaging. As a result final solutions are stabilized regardless the setting of the region. The final slip de.cit distribution at the Nankai Trough shows high contrast of strong and weak plate locking zones. Down-dip variation of the slip de.cit is emphasized more clearly but the lateral variation becomes moderate compared with the single inversion result.