Local variation of inundation, sedimentary characteristics, and mineral assemblages of the 2011 Tohoku-oki tsunami on the Misawa coast, Aomori, Japan
Local variation of inundation, sedimentary characteristics, and mineral assemblages of the 2011 Tohoku-oki tsunami on the Misawa coast, Aomori, Japan
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DOI：
10.1016/j.sedgeo.2012.06.003
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发表时间：
2012-12-30
影响因子：
2.8
通讯作者：
Putra, Purna Sulastya
中科院分区：
文献类型：
--
作者：
Nakamura, Yugo;Nishimura, Yuichi;Putra, Purna Sulastya
2011年的东北海啸对日本东部沿海地区造成了严重破坏，并在受灾地区留下了一层沉淀物。我们从沉积物来源、海岸地形和水流高度的角度讨论了2011年东北海啸沉积特征的差异。研究区域位于东北北部的三泽海岸，包括一条20公里长的海岸线，海岸线上有沙滩、沿海沙丘和缓坡的低地。这一景观组合为研究地形对海啸沉积物特征的影响提供了一个机会。在2011年4月10日至5月2日进行的实地调查中，我们描述了海啸沉积物的厚度、相和结构。我们还在内陆延伸至550米的13个海岸垂直断面上以大约20米的间隔采集了砂样，用于粒度和矿物组合分析。海啸的高度是通过测量树木、折断的树干或建筑物上的水痕中发现的碎片的高度来估计的。沿海低地的性质影响了洪水的高度和淹没距离。在研究区南部，有一条狭窄的、宽100米的低洼海岸带，向陆地的梯田坡面上的爬升高度达到了10米。在北部，最大洪水达到550米，在更宽的低洼沿海地形上，最高水位为32米。平均水流高度为4-5米。海啸侵蚀了海岸沙丘，沿海岸形成了小峭壁。海啸沉积物立即向海岸沙丘靠岸，厚度超过20厘米，但从这一点起，内陆地区的厚度明显变薄。在沙丘附近，矿床主要由中砂(1-2Phi)组成，具有平面和平行的层理，但没有明显的向上细化或粗化。沙丘的颗粒大小与沿海沙丘相似，我们推测沙丘是当时海啸沉积的当地物质来源。海啸沉积的矿物组合以斜方辉石和单斜辉石为主，与沙丘和海滩砂相似。在超过一半的内陆淹没距离的地点，较薄的海啸沉积物主要由细沙(2.375 phi)组成，并有一些向上细化。颗粒大小和沉积特征的差异可能是由于泥沙运移和沉积过程的不同造成的。我们推断，分选较好、较细的沉积物是从悬浮物中沉积出来的，而相对较粗的沙子则是从牵引力流中沉积下来的。2011年东北海啸沉积物的沉积特征似乎主要受到沿海地形和任何一点的侵蚀程度的影响，而不是受水流高度的影响。(C)2012爱思唯尔B.V.保留所有权利。
The 2011 Tohoku-oki tsunami caused severe damage to the coastal regions of eastern Japan and left a sediment veneer over affected areas. We discuss differences in depositional characteristics of the 2011 Tohoku-oki tsunami from the viewpoint of the sediment source, coastal topography and flow height. The study area on the Misawa coast, northern Tohoku, includes a 20 km long coastline with sandy beaches, coastal dunes and a gently sloping lowland. This landscape assemblage provides an opportunity to examine the effects of topography on the characteristics of the tsunami deposit. During field surveys conducted from April 10 to May 2, 2011, we described the thickness, facies, and structure of the tsunami deposit. We also collected sand samples at approximately 20 m intervals along 13 shore-perpendicular transects extending up to 550 m inland, for grain size and mineral assemblage analysis. The tsunami flow height was estimated by measuring the elevation of debris found in trees, broken tree limbs, or water marks on buildings.The nature of the coastal lowland affected the flow height and inundation distance. In the southern part of the study area, where there is a narrow, 100 m wide low-lying coastal strip, the run-up height reached 10 m on the landward terrace slopes. To the north, the maximum inundation reached 550 m with a run-up height of 32 m on the wider, low-lying coastal topography. The average flow height was 4-5 m.The tsunami eroded coastal dunes and formed small scarps along the coast. Immediately landward of the coastal dunes the tsunami deposit was more than 20 cm thick, but thinned markedly inland from this point. Close to the dunes the deposit was composed largely of medium sand (1-2 Phi) with planar and parallel bedding, but with no apparent upward fining or coarsening. The grain size was similar to that of the coastal dune and we infer that the dunes were the local source material for the tsunami deposit at this point. The mineral assemblage of the tsunami deposit was dominated by orthopyroxene and clinopyroxene and was also similar to the dune and beach sand. At sites more than half the inundation distance inland, the thinner tsunami deposit consisted mainly of fine sand (2.375 Phi) with some upward fining. The difference in grain size and sedimentary characteristics was probably caused by differences in sediment transportation and depositional processes. We infer that the well-sorted, finer sediments were deposited out of suspension, whereas the relatively coarse sands were laid down from traction flows. The depositional characteristics of the 2011 Tohoku-oki tsunami deposit appeared to have been affected mainly by the coastal topography and the extent of erosion at any one point, as opposed to flow height. (c) 2012 Elsevier B.V. All rights reserved.