Holocene Crustal Movement in the Kesennuma Okawa Plain, Southern Sanriku Coast, Northeast Japan, Estimated from Coastal Geology

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  • 三陸海岸南部・気仙沼大川平野の完新世における地殻変動
  • サンリク カイガン ナンブ ・ ケセンヌマ オオカワ ヘイヤ ノ カンシン セイ ニ オケル チカク ヘンドウ

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Abstract

 The Sanriku coast was believed to have followed an uplifting trend in the geological time-scale on the basis of it being a flat surface interpreted as a Pleistocene marine terrace. On the contrary, geodetic and tide-gauge data show a rapid trend of this area subsiding during the last several decades. This area also experienced extensive subsidence during the 2011 Tohoku-oki earthquake (Mw 9.0). The discrepancy between long-term uplift and short-term subsidence has been pointed out by several researchers, but remains unresolved. Knowledge of sedimentary facies and deposition ages of the Holocene sequence is required to consider this problem. The depositional process of the Holocene sequence and crustal movement is discussed on the basis of coastal geology. A sediment core, KO1, was acquired from the lower reach of the Kesennuma Okawa Plain, southern Sanriku coast. Core sediments show typical deltaic succession influenced by Holocene sea-level change. Relative sea-level (RSL) at 9.6 to 10.1 cal BP is estimated to be −37 to −34 m based on the depositional surface of tidal flat deposits including molluscan shells living in the intertidal zone. Estimated RSL is lower than the theoretical RSL without tectonic effects. A probable cause of this discrepancy is Holocene tectonic subsidence in the area studied. Unlike previous arguments for a long-term uplift, particularly in northern Sanriku, the results of this study reinforce the theory that the south Sanriku coast subsided in the geologic time-scale.

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