Crustal movements on Shikoku, southwestern Japan, inferred from inversion analysis of levelling data using ABIC

With an inversion technique based on Akaike's Bayesian Information Criterion (ABIC) we analyzed levelling data for 1893-1983 on Shikoku, southwestern Japan, where large interplate earthquakes have periodically occurred at intervals of about 120 years, releasing tectonic stress produced by steady relative motion of the Philippine Sea and the Eurasian plates. Through the inversion analysis we reconstructed the pattern of crustal movements on Shikoku over the last 120 years, including the occurrence of the 1946 Nankaido earthquake (M 8.1). The result clearly shows that the crustal movements on Shikoku include significant secular vertical motion (uplift in the south and subsidence in the north) in addition to cyclic motion related to the periodic occurrence of interplate earthquakes at the Nankai trough. Contrary to the widely accepted theory, we could not find any correlation between the secular vertical motion and the coseismic vertical displacement. The secular uplift motion on southern Shikoku estimated from the levelling data completely agrees with that inferred from the present heights of marine terraces formed by eustatic sea level changes and crustal uplift for the last 10⁵ yr. This suggests that the fundamental causes of the short-term (10² yr) and long-term (10⁵ yr) movements on southern Shikoku are the same, the steady subduction of the Philippine Sea plate at the Nankai trough. On northern Shikoku, on the other hand, the pattern of secular crustal motion estimated from the levelling data is quite different from that of the Quaternary uplift inferred from the present heights of eroded flat surfaces, indicating the existence of some unknown tectonic process controlling the very long-term (10⁶ yr) crustal movements.