We study the preseismic, coseismic, and postseismic vertical surface displacements in the Shikoku, Kinki, and Chugoku districts, which are associated with the 1946 Nankaido earthquake (M=8.2), one of the largest interplate earthquakes which occurred in southwest Japan. These displacements are precisely estimated from the first-order leveling data (1890-1980) by using an epoch reduction method.
The preseismic distribution of vertical surface displacement is characterized by the seaward tilts in the Kii peninsula and the eastern part of Shikoku, which can be interpreted in terms of a steady state subduction of the Philippine Sea plate. The coseismic distribution is the same as that caused by the low-angle thrust faulting near the plate boundary between the Philippine Sea and Asian plates. The most notable feature of the postseismic distribution is the uplift localized in the coseismically subsided region. Such an uplifting tendency lasts at least 30 years after the earthquake, and then would turn gradually into the preseismic one as described above.