Identifying run-up and backwash units from sediments is important to distinguish tsunami deposits from layers formed by other events such as river floods. To reconstruct run-up and backwash of past tsunami, we conducted grain imbrication analysis using statistical hypothesis testing on X-ray computed tomography (CT) images of a gravelly tsunami deposit. Grain imbrication analysis was conducted on the gravel in a layer namely S1. Three cores taken at 2 sites were used for the analysis. The results were shown as rose diagrams, then we conducted statistical hypothesis testing to examine the distribution patterns of the angle data of the grain imbrication. Grain imbrication analysis and statistical hypothesis testing revealed a run-up and a backwash units in the layer S1. Units of run-up in the layer S1 are thicker and more frequently observed than units of backwash.
A re-investigation was conducted for a continuous outcrop of the Plio-Pleistocene Kobiwako Group located on the riverbed at the Yasu River in Shiga Prefecture, Japan. The outcrop is famous for producing fossil footprints of elephants, deers and others of about 2.6 million years ago. As a result of applying sedimentary facies analysis to the strata, the following new facts were found out. The strata are constructed by the progradation of shallow lacustrine delta system. The delta topset shows horizontal alternation of floodplain silt and sand of distributary channels and/or mouth bars with wave ripples, cross stratification, mud-drape, flood sand, plant debris, tree stumps and footprints. The delta foreset is gently inclined down-lap alternation of sand and mud with subaqueous gravity-flow sand and slump beds of delta slope or delta lobes. The bottomset is massive clay of prodelta deposits. The delta deposits record the lake-level changes caused by weather change and tectonic movement. Paleocurrent directions obtained from the delta deposits, such as forward direction of fluvial bed-form indicated by cross-lamination and prograding direction of delta by foreset stratification, are approximately east to west that is significantly different from previously reported direction. These paleocurrents indicate that the paleogeography at the time needs to be reexamined.
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.