[SCG61-P01] Vertical deformation style along the Sanriku coast inferred from coastal geology examined after the Tohoku earthquake
キーワード：alluvial plain、coastal geology、millennial-scale crustal movements、Sanriku coast
Along the Sanriku coast, northeast Japan, discrepancies in crustal movement have been reported between uplift on a timescale of 105 years estimated from presence of flat surface previously recognized as Pleistocene marine terrace (Koike and Machida, 2001) and subsidence on a timescale of 101 – 102 years from geodetic record (Kato, 1983; Ozawa et al., 2011). Some hypothetical models, which explain these discrepancies, are proposed in relation to megathrust earthquake cycle (e.g., Nishimura, 2014). These models assume that crustal movement style is same within whole part of the Sanriku coast, and show that amount of accumulation of uplift exceeds that of subsidence during one earthquake cycle. However, because of lack of reliably identified MIS5 (ca. 125ka) marine terrace with age data along the Sanriku coast except for northernmost of the coast (Miyazaki and Ishimura, 2018), and of putative marine terrace along the southern part of the Sanriku coast (Koike and Machida, 2001), spatial distribution of crustal movement cannot be estimated on the basis of only feature of Pleistocene marine terrace.
After the 2011 earthquake, we have examined Holocene sedimentary successions of alluvial plains distributed along the Sanriku coast in order to obtain data on millennial-scale crustal movements, which help to solve the problem about the temporal inconsistency of crustal movement. Recent studies have reported millennial-scale subsidence in central to southern parts of the coast (e.g., Niwa et al., 2014; Niwa et al., 2017), but only one comparable dataset is available for the northern part of the coast (Niwa et al., 2019). 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. Relative sea-levels in the Kuji plain are higher than –20.1 m, higher than –4.3 m, higher than –4.8 m, and higher than –5.9 m on the basis of sediments deposited under the sea-level dated to 9000 to 8730 cal BP, 7580 to 7470 cal BP, 6940 to 6740 cal BP, and 6400 to 6260 cal BP. These sea-levels are higher than that in the Tsugaruishi plain, central Sanriku coast, where millennial-scale subsidence was previously estimated (Niwa et al., 2017). This discrepancy indicates that the Kuji plain has uplifting trend relative to the Tsugaruishi plain on a millennial-scale. By the addition of relative uplifting trend of the Omoto plain (Niwa et al., 2019), millennial-scale crustal movements along the Sanriku coast are best characterized as subsidence along the central to southern parts and relative uplift along the northern Sanriku coast. Spatial distribution of millennial-scale crustal movements accords with marine terrace distribution that uplifting trend during the past 100ka is confirmed only in northernmost of the Sanriku coast (Miyazaki and Ishimura, 2019). This agreement suggests a continuous southward tilting trend along the Sanriku coast since ca. 100 ka.
Kato (1983) Tectonophysics, 97, 183-200. Koike and Machida (2001) The University of Tokyo Press, p105. (in Japanese with English abstract) Miyazaki and Ishimura (2018) J. Geogr. (Chigaku-zasshi), 127, 735-757 (in Japanese with English abstract). Nishimura (2014) J. Disaster Res., 9, 294-302. Niwa et al. (2014) Quat. Res. (Dai-yonki kenkyu), 53, 311-312 (in Japanese with English abstract). Niwa et al. (2017) Quat. Int., 456, 1-16. Niwa et al. (2019) Quat. Int., 519, 10-24. Ozawa et al. (2011) Nature, 475, 373-377.
After the 2011 earthquake, we have examined Holocene sedimentary successions of alluvial plains distributed along the Sanriku coast in order to obtain data on millennial-scale crustal movements, which help to solve the problem about the temporal inconsistency of crustal movement. Recent studies have reported millennial-scale subsidence in central to southern parts of the coast (e.g., Niwa et al., 2014; Niwa et al., 2017), but only one comparable dataset is available for the northern part of the coast (Niwa et al., 2019). 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. Relative sea-levels in the Kuji plain are higher than –20.1 m, higher than –4.3 m, higher than –4.8 m, and higher than –5.9 m on the basis of sediments deposited under the sea-level dated to 9000 to 8730 cal BP, 7580 to 7470 cal BP, 6940 to 6740 cal BP, and 6400 to 6260 cal BP. These sea-levels are higher than that in the Tsugaruishi plain, central Sanriku coast, where millennial-scale subsidence was previously estimated (Niwa et al., 2017). This discrepancy indicates that the Kuji plain has uplifting trend relative to the Tsugaruishi plain on a millennial-scale. By the addition of relative uplifting trend of the Omoto plain (Niwa et al., 2019), millennial-scale crustal movements along the Sanriku coast are best characterized as subsidence along the central to southern parts and relative uplift along the northern Sanriku coast. Spatial distribution of millennial-scale crustal movements accords with marine terrace distribution that uplifting trend during the past 100ka is confirmed only in northernmost of the Sanriku coast (Miyazaki and Ishimura, 2019). This agreement suggests a continuous southward tilting trend along the Sanriku coast since ca. 100 ka.
Kato (1983) Tectonophysics, 97, 183-200. Koike and Machida (2001) The University of Tokyo Press, p105. (in Japanese with English abstract) Miyazaki and Ishimura (2018) J. Geogr. (Chigaku-zasshi), 127, 735-757 (in Japanese with English abstract). Nishimura (2014) J. Disaster Res., 9, 294-302. Niwa et al. (2014) Quat. Res. (Dai-yonki kenkyu), 53, 311-312 (in Japanese with English abstract). Niwa et al. (2017) Quat. Int., 456, 1-16. Niwa et al. (2019) Quat. Int., 519, 10-24. Ozawa et al. (2011) Nature, 475, 373-377.