2023 Volume 81 Issue 1-2 Pages 27-41
In this study, the submarine sediments change due to the 2011 off the Pacific coast of Tohoku Earthquake and Tsunami and in subsequent temporal variations of sediments were identified in Ofunato Bay, Iwate pref. Sediment distribution and sedimentary environment changes associated with natural and human activities (such as reconstruction works etc.) were considered in combination. The understanding of changes in sedimentary environments caused by natural and human activities is important for infrastructure development for disaster prevention and mitigation in coexistence with the natural environment in coastal areas. The results of this study indicate that the distribution of sediments in Ofunato Bay did not change significantly before and after the earthquake and tsunami. However, some observation sites changed the sediment characteristics shows variety after the tsunami. In the southern part of Sangoshima Island in the central part of the bay, the ratio of gravel sediments increased. Off the bay-mouth breakwater, sandy sediment changed to mud immediately after the tsunami, and muddy sediment changed to sandy sediment again 5 years after the tsunami (October 2016). After October 2016, the characteristics of sediments offshore of the bay-mouth breakwater is similar to that before the earthquake and tsunami. These sediment changes could have been caused by the collapse by the tsunami and subsequent reconstruction of bay-mouth breakwater. Wave ripples were observed on the seafloor after the reconstruction of the breakwater, but not before, suggesting that the effect of the waves on the seafloor has changed by breakwater reconstruction. Construction of structures by humans was considered to have affected sedimentation in coastal sea area and shallow marines.
