[H-QR23_1AM1] ヒト-環境系の時系列ダイナミクス

[HQR23-04] 第四紀および最近の海水準変動より推定される日本列島沿岸域の鉛直地殻変動

*奥野 淳一¹、中田 正夫²、石井 正好³、三浦 英樹¹ (1.国立極地研究所、2.九州大学大学院理学研究院、3.気象研究所)

キーワード：地殻変動, 海水準変動, 第四紀, 検潮儀, 海水膨張

Observed relative sea-level (RSL) changes during the past 130 kyr are mainly caused by change of ocean volume, tectonic crustal movement and glacio-hydro isostatic adjustment (GIA) of the Earth in response to the redistribution of ice and water loads. Here we examine the tectonic crustal movements along the Japanese coastlines on three typical timescales (50 yr, 6 kyr and 125 kyr) based on several sea-level observations and their predictions due to GIA process and recent melting of mountain glaciers and both polar ice sheets. We use the observations of RSL based on tide gauge and Holocene RSL observations and the altitudes of marine terraces formed at the last interglacial (LIG) phase at about 125 kyr. 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 GIA for the last deglaciation and also recent melting of the mountain glaciers and both polar ice sheets. Those for 6 kyr and 125 kyr are based on the RSL observations and the predictions by GIA modeling, 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 kyr BP 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. The inferred rates on these timescales may be useful in discussing the recurrence of megathrust earthquake with its interval of about 1 kyr like the 2011 off the Pacific coast of Tohoku Earthquake.