津波堆積物を河川の氾濫など他のイベントで形成された層と区別するためには堆積物から遡上流・戻り流れを識別することが重要である.過去の津波の古流向を復元するため,Takada et al.(2016)が報告した礫質津波堆積物のX線CT画像を対象にして統計的仮説検定を用いた粒子インブリケーション解析を行なった.解析はTakada et al.(2016)のTSd1に相当する層(S1)に対して行い,2地点で採取した3本のコアを使用した.解析の結果をローズダイアグラムで示し,さらに得られた長軸方向角度データが従う分布型や,長軸方向角度データが統計学的に有意な集中を持つかを調べるために統計学的仮説検定を行った.その結果,S1層には遡上流と戻り流れのユニットが存在することが示された.S1層の中において遡上流のユニットは戻り流れのユニットよりも厚く,より頻繁に観察された.

We studied onshore sediment cores obtained from a coastal marsh on the south coast of Beppu Bay, northern Kyushu Island, Japan, to ascertain whether prehistoric intraplate earthquakes in the bay had generated tsunamis, and to establish a chronology and recurrence intervals for these earthquakes. The latter had been inferred from investigations on shallow offshore faults, but the timing of the fault movements has large uncertainties. We identified five sand layers in the up to 8.8 m long sedimentary sequence, which is mainly composed of non-marine organic-rich mud and marine organic-poor mud. Most sand layers, 1-20 cm thick, exhibit sharp upper and lower contacts with the surrounding muds, implying that they were deposited by sudden events. The sand layers are geochemically distinct, with peaks in Si, K, Ca, Ti, Mn, Fe, and Sr, and diatom assemblages dominated by brackish-marine and marine species. This suggests that the sand came from the sea bottom and/or shore rather than from the freshwater marsh. Based on radiocarbon age modeling, the depositional ages of these sand layers are: 3270-3450 cal yr BP, 4250-4510 cal yr BP, 4970-5280 cal yr BP, 5750-6750 cal yr BP, and 6430 -7070 cal yr BP. The oldest deposit is inferred to record a tsunami associated with the 7.3 ka calderaforming eruption of the Kikai volcano, southern Japan. Thus, we exclude it from the calculation of the recurrence interval of tsunamigenic intraplate earthquakes in the bay. Recurrence is approximately 460 -1850 years, including the 1596 CE Keicho Bungo earthquake and a fault rupture 1700-2200 years ago, which have been reported in studies on ruptures of shallow offshore strike-slip and normal faults. Brackish-marine and marine species dominate the fossil diatom assemblages in the upper muddy sand layer (younger than 2750-2870 cal yr BP) above the non-marine organic-rich mud, implying that the muddy sand was deposited in an inner bay or tidal flat environment. The facies change from non-marine to marine sediments after 2750-2870 cal yr BP cannot be explained by a change in the regional sea level because the latter has fallen in the last 6000 years. Therefore, the most likely interpretation for this environmental change is a local coseismic subsidence of the marsh, probably attributed to a rupture of the active fault located south of the marsh. This study is the first to report on recurrent tsunamigenic intraplate earthquakes based on research of onshore tsunami deposits, and suggests that investigating tsunami deposits can contribute to an assessment of tsunami risks for intraplate earthquakes elsewhere. (c) 2021 Elsevier Ltd. All rights reserved.

Erosional and sedimentary features associated with flooding have been documented in both modern and past cases. However, only a few studies have demonstrated the relationship between these features and the corresponding hydraulic conditions that produced them, making it difficult to evaluate the magnitude of paleo-flooding. This study describes the characteristics associated with inundation depth and flow direction, as well as the erosional and sedimentary features resulting from the disastrous flooding of the Kinu River, central Japan, in September 2015. Water levels rose rapidly due to heavy rainfall that eventually overtopped, and subsequently breached, a levee in Joso City, causing destructive flooding on the surrounding floodplain. Distinctive erosional features are found next to the breached levee, while depositional features, such as a sandy crevasse-splay deposit are found further away from the breach. The deposit can be divided into three units based on sedimentary facies. The vertical and lateral changes of these sedimentary facies may be the result of temporal and spatial changes associated with flow during the single flooding event. These observations and quantitative data provide information that can be used to reveal the paleohydrology of flood deposits in the stratigraphic record, leading to improved mitigation of future flooding disasters.