Millennial-scale crustal movements inferred from Holocene sedimentary succession of the Omoto plain, northern Sanriku coast, Northeast Japan: Relevance for modeling megathrust earthquake cycles

Crustal movements on a millennial timescale (10³–10⁴ years) are important data for assessing the deformation of subduction zones during megathrust earthquake cycles. Such data did not exist until recently for the Sanriku coast, near the source region of the 2011 Tohoku-oki earthquake. Since the 2011 earthquake, studies of Holocene sedimentary successions have documented subsidence on the central to southern segment of the Sanriku coast on a millennial timescale, but comparable data are lacking for the northern Sanriku coast. This study derived millennial-scale crustal movements from a well-dated Holocene sedimentary succession in the Omoto plain on the northern Sanriku coast. Sedimentary and fossil evidence record the following sequence of environmental changes influenced by sea-level changes: upward growth and landward progradation of a barrier and flood tidal delta from 10,000–9000 cal BP, an estuarine environment from 9000 to 8000 cal BP after growth and progradation of the barrier and flood tidal delta ceased, and a regressive delta and subsequent fluvial environment after 8000 cal BP. Thick deposit of delta to floodplain sediment since mid-Holocene is not apparent in the Omoto plain, whereas this feature typical of subsidence area is documented along the central Sanriku coast. Although the absolute direction of the vertical deformation trend could not be well constrained in the study area because of compaction effects, the evidence indicates that subsidence during the mid and late Holocene was either absent or not noticeable on the Omoto plain. This difference in deformation histories corresponds to a contrast in coseismic subsidence during the 2011 earthquake between the northern Sanriku coast (small subsidence) and the central to southern Sanriku coast (large subsidence). The results of this study suggest that the northern and central to southern parts of the Sanriku coast should be considered separately when assessing regional crustal deformation related to subduction.