The 2011 off the Pacific coast of Tohoku Earthquake generated huge tsunami waves causing severe damage to coastal areas throughout the Tohoku region. Tsunami waves also affected the seafloor and biological conditions in coastal areas. This research project (Tohoku Ecosystem-Associated Marine Sciences, TEAMS) supported by Japan Ministry of Education, Culture, Sports, Science and Technology has been conducted to analyze changes in marine ecosystems in areas off the Tohoku coast after the earthquake. Bathymetric data accumulated using acoustic instruments not only provide a distribution of sea debris and existing artificial fish reefs; they also provide basic information related to the various scientific uses of the project results. Several research efforts were undertaken in and around Otsuchi Bay before the earthquake. This area is regarded as an important area for this project. We conducted detailed bathymetric and subbottom profiling surveys off Otsuchi Bay. The results obtained from the surveys indicate the existence of a narrow channel extending west‒east in our observation area. Sub-bottom profiling data show that the channel structure is very shallow: less than 2 m. This channel only erodes the uppermost layer (Unit 1). Therefore, these results imply that this channel structure was formed in newer geological time. Unit 1 covers the lower layer (Unit 2) with disconformity. Therefore, unit 1 probably formed after Unit 2 was eroded at the time of marine regression. Unit 1 becomes thinner toward offshore. It became difficult to recognize Unit 1 at 140‒150 m depth around 8 km west of Line A. Unit 2 exists throughout Line A. This difference might be attributable to their different distances from the shoreline, which supplies sediment, or to different sea floor inclination angles. Moreover, a buried flexural structure was detected clearly at the lower portion of Unit 1 and Unit 2 around 2.7 km from the north of Line B. However the seafloor is almost flat. Therefore, we concluded that the flexural structure was formed during the formation of Unit 1. These flexure structures imply the existence of a deeper buried fault structure. Therefore, studying not only the lineament of bathymetry but also the sub-seafloor sedimental structure is important.

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