17:15 〜 18:30
[HQR04-P03] Holocene coastal evolution and vertical deformation in the Hachinohe plain, NE Japan, inferred from sedimentary analyses
キーワード：三陸海岸、完新世、地殻変動、堆積物コア、東北地方太平洋沖地震
We report geomorphological and sedimentological analyses in the Hachinohe plain (northernmost Sanriku coast, NE Japan) that yield new insights into vertical deformation around the source region of the 2011 Tohoku-oki earthquake. Landform classification and analyses of seven sediment cores document the transgression and regression of the sandy barrier and back-barrier estuary during the Holocene.
Fluvial environment, which prevailed at lower part of the plain at 10,000 to 9,500 cal BP, changed to tidal river environment at 9,500 to 9,000 cal BP. Further transgression induced deeper bay environment at seaward site at 8,500 cal BP, whereas fluvial environments prevailed at upper part of the plain at that time. Because the Hachinohe plain is surrounded by Pleistocene marine terraces and the Mabechi River narrowly crosses them, we suggest that a wave-dominated estuary enclosed by a barrier formed in the present Hachinohe plain during the Middle Holocene. Transgressive lag sand and gravel layers and overlaying shoreface and foreshore sandy sediments imply that a barrier, which had been offshore during the Early Holocene, transgressed and truncated the back-barrier sediments. Consequently, the barrier has been located at present coastline after middle Holocene. The existence of barrier and upward-coarsening silt to sand layer, which filled in water area behind the barrier, indicates progradation of bay-head delta during the Middle Holocene. After progradation of the bay-head delta, fluvial environments again prevailed behind the barrier.
Comparison of height of a surface of the onset of tidal river environment (-22.76 m) at 9,460 cal BP, when sea-level was on or higher than the depositional surface, with hydro-isostatic sea-level prediction that do not account for tectonic deformation suggests maximum subsidence rate on a millennial scale in the Hachinohe plain is 0.7 mm/yr, if there is a millennial-scale subsidence trend at all. Similarly, comparison of a surface of onset of marsh environment (4.51 m) at 6,340 cal BP with hydro-isostatic sea-level predictions suggests that maximum uplift rate in the plain is 0.7 mm/yr. Therefore, the vertical deformation rate on a millennial scale in the Hachinohe plain ranges from -0.7 to 0.7 mm/yr (uplift positive). This vertical deformation is consistent with relative uplift in Kuji and Omoto, northern Sanriku coast and contrasts with subsidence along central to southern Sanriku coast. Our results also support a variation along the Sanriku coast in vertical deformation during the last 100 kyr; the southern part is subsidence while northern part has uplifted at least relative to southern part. Agreement of this trend with deformations during/before the 2011 Tohoku-oki earthquake that the amount/rate of subsidence decreased from south to north implies that effect of subduction and slip on megathrust for coastal deformation persist at least during 100 kyr around the source region of the 2011 earthquake.
Fluvial environment, which prevailed at lower part of the plain at 10,000 to 9,500 cal BP, changed to tidal river environment at 9,500 to 9,000 cal BP. Further transgression induced deeper bay environment at seaward site at 8,500 cal BP, whereas fluvial environments prevailed at upper part of the plain at that time. Because the Hachinohe plain is surrounded by Pleistocene marine terraces and the Mabechi River narrowly crosses them, we suggest that a wave-dominated estuary enclosed by a barrier formed in the present Hachinohe plain during the Middle Holocene. Transgressive lag sand and gravel layers and overlaying shoreface and foreshore sandy sediments imply that a barrier, which had been offshore during the Early Holocene, transgressed and truncated the back-barrier sediments. Consequently, the barrier has been located at present coastline after middle Holocene. The existence of barrier and upward-coarsening silt to sand layer, which filled in water area behind the barrier, indicates progradation of bay-head delta during the Middle Holocene. After progradation of the bay-head delta, fluvial environments again prevailed behind the barrier.
Comparison of height of a surface of the onset of tidal river environment (-22.76 m) at 9,460 cal BP, when sea-level was on or higher than the depositional surface, with hydro-isostatic sea-level prediction that do not account for tectonic deformation suggests maximum subsidence rate on a millennial scale in the Hachinohe plain is 0.7 mm/yr, if there is a millennial-scale subsidence trend at all. Similarly, comparison of a surface of onset of marsh environment (4.51 m) at 6,340 cal BP with hydro-isostatic sea-level predictions suggests that maximum uplift rate in the plain is 0.7 mm/yr. Therefore, the vertical deformation rate on a millennial scale in the Hachinohe plain ranges from -0.7 to 0.7 mm/yr (uplift positive). This vertical deformation is consistent with relative uplift in Kuji and Omoto, northern Sanriku coast and contrasts with subsidence along central to southern Sanriku coast. Our results also support a variation along the Sanriku coast in vertical deformation during the last 100 kyr; the southern part is subsidence while northern part has uplifted at least relative to southern part. Agreement of this trend with deformations during/before the 2011 Tohoku-oki earthquake that the amount/rate of subsidence decreased from south to north implies that effect of subduction and slip on megathrust for coastal deformation persist at least during 100 kyr around the source region of the 2011 earthquake.