INTERSEISMIC DEFORMATION AT THE NANKAI TROUGH, JAPAN, SUBDUCTION ZONE

Uplift along the coast of southwestern Japan following the 1944 Tonankai (M(s) = 8.0) and 1946 Nankaido (M(s) = 8.2) earthquakes has been inferred from the 1950-1985 tide gage records. Although uplift rates vary with geographic position, the temporal dependence at each site can be described as the superposition of an exponentially decaying (time constant approximately 5 years) transient, significant only in the first decade postseismic, and a steady interseismic rate. The steady interseismic uplift rate is defined only over the 1956-1985 interval, but the linearity in time of that uplift record is such that uplift over the remainder of the interseismic cycle (approximately 80 years) might reasonably be expected to occur at the same rate. Moreover, the spatial pattern and magnitude of the steady uplift rate are reasonably close to those predicted by a simple dislocation model of subduction. However, the currently observed steady uplift rate is not consistent with the uplift rate inferred from geodetic leveling for the final half of the previous earthquake cycle. Thus, either the hypothesis of a steady interseismic uplift rate after the first decade postseismic is wrong or the strain accumulation pattern for one earthquake need not be the same as for the preceding earthquake on the same segment.