An assessment of coastal tectonics along the Sanriku coast, northeast Japan, from a Holocene sedimentary succession in the Kuji plain

Holocene sedimentary successions in tectonically active regions are key to understanding coastal tectonics, because they provide information on millennial-scale crustal movements. Recent analyses of Holocene sedimentary successions along the Sanriku coast, near the source region of the 2011 Tohoku-oki earthquake, have reported millennial-scale subsidence in central to southern parts of the coast, but only one comparable dataset is available for the northern part of the coast. To obtain a comprehensive understanding of millennial-scale crustal movements along the Sanriku coast, we obtained well-dated Holocene sediment cores from the Kuji plain in the northern part of the coast. These cores record a transgressive barrier-lagoon system from ca. 9500 to 8000 cal BP, a regressive prodelta to delta front from ca. 8000 to 6500 cal BP, and a subsequent wave-influenced shallow marine and fluvial environment. The early to mid-Holocene relative sea level in the study area estimated from the sedimentary environment at that time is higher than that in the Tsugaruishi plain, central Sanriku coast, where millennial-scale subsidence was previously inferred. This discrepancy indicates that the Kuji plain has been uplifted relative to the Tsugaruishi plain on a millennial scale. The contrast in the millennial-scale crustal movements between relative uplift on the northern Sanriku coast and recently inferred subsidence on the central to southern Sanriku coast confirms that the Sanriku coast contains multiple areas of crustal deformation. Furthermore, this spatial distribution of millennial-scale crustal movements is in good agreement with the recently reported MIS 5 marine terrace distribution, suggesting a continuous southward tilting trend along the Sanriku coast since ca. 100 ka.