Uplift as a function of time from 1951 through 1990 has been deduced from annual mean sea level measured at 15 tide gages along the Nankai subduction zone, southwest Japan. The recurrence interval for rupture of the Nankai subduction zone is about 100 years, and the most recent rupture was in late 1946. Thus the 1951--1990 uplift record covers most of the first half of the earthquake cycle. The precision of the uplift record is better than could be obtained currently by annual Global Positioning System (GPS) measurements. The pre-1960 uplift record shows rapid deformation that appears to terminate in 1959.

After 1959 the uplift record is remarkably linear in time: Significant curvature in the uplift-versus-time plots is detected at only three of the 15 tide gages. The inferred uplift rates are not quantitatively consistent with the predictions of either the viscoelastic coupling or elastic half-space models of subduction, but the agreement is sufficient to suggest that the causative physical mechanisms have been identified. The immediate postseismic response is controlled by the propagation of slip downward along the downdip extension of the coseismic rupture, and the interseismic response is caused by the accumulation of a slip deficit on the main thrust zone. Asthenosphere relaxation is not required to explain the observations.