論文

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 a sandy barrier and back-barrier estuary in a wave-dominated estuary system during the Holocene. We compare the heights of (i) the earliest Holocene marine flooding surface overlying fluvial sediments and (ii) the onset of a Middle Holocene marsh environment with non-tectonic hydro-isostatic sea-level predictions to constrain the millennial-scale vertical deformation rate in the Hachinohe plain to within −0.7 to +0.7 mm/yr (uplift positive). This result is in contrast to observed subsidence along the central to southern Sanriku coast but is consistent with relative uplift along the northern Sanriku coast. Our results support deformation characterized by southward tilting along the Sanriku coast during the last 100 kyr as suggested by previous studies south of Hachinohe. Furthermore, this tectonic pattern is consistent with deformation trends before and during the 2011 Tohoku-oki earthquake. This collective consistency implies that the effects of subduction and megathrust slip on coastal deformation have persisted for at least 100 kyr around the source region of the 2011 earthquake.

To investigate the relationship between paleoenvironmental changes and marine reservoir effects, the radiocarbon ages of marine shells and terrestrial plants from the same horizons of sediment cores in the Holocene Epoch were compared. Three sediment cores were obtained from the central to northern parts of the Sanriku coast, corresponding to the boundary between the warm Kuroshio and cold Oyashio currents. These drilling sites are also located in a tectonically active area associated with the subduction of the Pacific Plate in Japan Trench. Based on lithological analysis, molluscan assemblage investigation, and radiocarbon dating, we interpreted five sedimentary facies. These paleoenvironmental changes were mainly related to sea level rise during the last deglaciation period. Reservoir ages from 33 pairs of terrestrial plants and marine shells (420 ± 130 14C years) were found to span the period from 2,300 to 9,000 cal BP.