Iwai Shohei, Goto Kazuhisa
Department of Earth Science, Tohoku University, Sendai, 981-8578, Japan.
International Research Institute of Disaster Science, Tohoku University, Aoba 468-1 Aramaki, Aoba-ku, Sendai, 980-0845, Japan.
Sci Rep. 2021 Jun 28;11(1):13434. doi: 10.1038/s41598-021-92917-2.
Around the world, numerous coastal boulders with weight of few thousand tons are suspected to have been transported by very large tsunamis, although their origins remain enigmatic. For clarifying origins of these boulders, the relation between the tsunami flow depth and the movement of meter-size boulders should be clarified but there is no proper field dataset. Here we collected first comprehensive dataset of both moved and unmoved boulders as well as the maximum flow depths along the Sanriku coast of Japan, where was affected by the 2011 Tohoku-oki tsunami based on satellite image analyses and field survey. The dataset revealed that up to ca. 1500 tons of boulders and concrete blocks were moved by the 2011 tsunami with approx. 28 m flow depth. We further revealed that most unmoved boulders were not moved because of the local setting rather than their heavy weights. The threshold of moved/unmoved boulders is estimated against the flow depth. The threshold predicted that approx. > 20 m flow depths are required to move approx. > 1000 tons boulders. The results imply that even a few thousand tons of enigmatic boulders in the world could have been moved by these sizes of the tsunami flow depths, although applicability of our results to other examples should be evaluated in the future work. We further tested the validity of an earlier proposed inverse model. Although the model result is consistent with the field observation, assumption of the appropriate parameters is problematic and further improvement of the model is required to estimate hydrodynamic features of the tsunami and to discriminate tsunami boulders from storm ones. Regarding such future work, our dataset is expected to be important for the evaluation of the improved numerical models.
在全球范围内，许多重达数千吨的沿海巨石被怀疑是由特大海啸搬运而来，尽管它们的来源仍然成谜。为了弄清这些巨石的来源，需要明确海啸水流深度与米级巨石移动之间的关系，但目前尚无合适的实地数据集。在此，我们基于卫星图像分析和实地调查，首次收集了日本三陆海岸沿线移动和未移动巨石以及最大水流深度的综合数据集，该地区曾受2011年东北海啸影响。数据集显示，2011年海啸致使重达约1500吨的巨石和混凝土块发生移动，当时水流深度约为28米。我们还进一步发现，大多数未移动的巨石未被移动是由于当地环境，而非其重量。针对水流深度估算了巨石移动/未移动的阈值。该阈值预测，移动约1000吨以上的巨石需要约20米以上的水流深度。研究结果表明，尽管未来工作中应评估我们的结果对其他实例的适用性，但世界上即使是重达数千吨的神秘巨石也可能是由这种规模的海啸水流深度搬运而来。我们还测试了一个早期提出的反演模型的有效性。尽管模型结果与实地观测一致，但合适参数的假设存在问题，需要进一步改进该模型以估算海啸的水动力特征，并区分海啸巨石和风暴巨石。关于此类未来工作，我们的数据集有望对改进数值模型的评估具有重要意义。
