Vertical tectonic crustal movements along the Japanese coastlines inferred from late Quaternary and recent relative sea-level changes

摘要

Observed relative sea-level (RSL) changes during the past ~130 kyr make it possible to evaluate the vertical crustal movement due to several tectonic processes such as the subduction of oceanic plates. Here we infer average rates of tectonic crustal movement along the Japanese coastlines on three typical timescales of ~50 yr, ~6 kyr and ~125 kyr based on tide gauge and Holocene RSL observations and the altitudes of marine terraces formed at the Last Interglacial (LIG) phase at ~125 ka. The rates on a timescale of ~50 yr are derived from tide gauge data, thermosteric sea-level changes due to thermal expansion of the oceans and predictions due to the glacio-hydro isostatic adjustment (GIA) for the last deglaciation and also recent melting of the mountain glaciers and Greenland and Antarctic ice sheets. Those for ~6 kyr and ~125 kyr are based on the RSL observations and the predictions by GIA modelling, considering uncertainties for temporal changes in eustatic sea-level for the mid- to late-Holocene and LIG phase. The inferred rates for ~50 yr are significantly different from those for ~125 kyr in most sites, particularly for sites along the coastline from eastern Hokkaido to northeastern Japan, Shikoku and south Kyushu facing the Pacific Ocean. In these regions, the rates for ~125 kyr and ~50 yr are positive (uplift) and negative (subsidence), respectively. Also, the observed RSL changes at ~6 ka are consistent with the inferred RSL changes using the rates for ~125 kyr and GIA-predictions in many sites, but inconsistent with those for ~50 yr in most sites except for a few sites. These results suggest that the rates on a timescale of ~50 yr are not representative of the tectonic crustal movements for timescales longer than ~6 kyr in most sites along the Japanese coastlines. However, the inferred rates on these timescales may be useful in discussing the recurrence of megathrust earthquake with its interval of ~1 kyr such as the 2011 earthquake off the Pacific coast of Tohoku.