A Study Of The Beach Terraces On Muroto Peninsula, Island Of Shikoku, Southwestern Japan

Abstract

The three sets of raised beach terraces exposed on Muroto Peninsula, Shikoku, Japan are the result of a landward tilting of the lithosphere which results from a seismicly induced elastic rebounding. The terraces all tilt to the north and converge near the hinge line of the great 1946 Nankaido earthquake. A model for terrace formation is developed which is a combination of simple beam theory and thrust fault elastic rebound theory. Axial stress drops of 320 to 425-bars are subtracted from axial loads which are applied to an initially downwarped 24 km thick lithosphere which overlies an asthenosphere of 10^19 poise viscosity. This results in a modeled coseismic uplift of 120 cm; in agreement with observations on Muroto Point. Also consistent with the observations is the modeled coseismic landward shift of the hinge line position, and the modeled post-seismic seaward shift of the hinge line position after the earthquake. Finally, a tectonic uplift rate of between 17.5 and 24 cm/century is obtained by the model in agreement with Yoshikawa's et al. (1964) uplift rate for Muroto Point.