Abstract On the Pacific coast of Hokkaido, historical documents and seismic detections spanning the last 200 years show that large earthquakes (magnitude 7–8) have occurred every 50–100 years along the Kuril Trench. Geological reconstructions of past earthquakes in the region show extend records to the last 6000 years and show that unusually large (>M8) earthquakes occurred at intervals of several hundreds of years. However, the magnitude and extent of tsunamis induced by these unusually large earthquakes are not fully understood. In this study, we found nine anomalous sand layers embedded in peat sediments that dated between 5.5 and 1.2 ka on the southern Hidaka coast, Hokkaido. Paleoenvironments reconstructed from the peat sediments using diatom, chemical, and mineralogical analyses showed an evolution of the site from a saltmarsh, to a supratidal marsh, to a back-barrier wetland environment. Radiocarbon dating of peat sediments showed that these environmental changes correspond to a sea-level fall following the mid-Holocene highstand. The anomalous sand layers showed inland thinning and shape contacts with the underlying peat. Amongst the layers, the most recent layer (1.7–1.2 ka) was designated as a storm deposit due to its limited distribution of less than 100 m from the beach ridge. Eight additional sand layers from 5.5 to 2.2 ka were distributed more than 300 m away from the beach ridge, and displayed a normal grading of sand particles, a characteristic typical of tsunami deposits. Because there was no record of tsunamis striking the Hidaka coast from the observed large earthquakes (M7–8) that recur every 50–100 years, we interpret the sand layers as tsunamis deposited by unusually large earthquakes. Because the eight inferred tsunami sand layers were deposited during the highstand period, prior to 2.2 ka, we infer that the inundation area due to tsunamis in the coastal zone has changed with relative sea-level (RSL) in the past. We highlight the necessity of evaluating the influence of long-term RSL change.

中文翻译：

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从日本北海道日高海岸南部获得的全新世海啸、风暴和相对海平面记录

摘要 在北海道太平洋沿岸，过去 200 年的历史文献和地震探测表明，沿千岛海沟每 50-100 年发生一次大地震（7-8 级）。该地区过去地震的地质重建显示，记录延伸到最近 6000 年，并表明异常大（> M8）地震每隔数百年发生一次。然而，由这些异常大地震引发的海啸的震级和范围尚未完全了解。在这项研究中，我们在北海道日高海岸南部的泥炭沉积物中发现了 9 个异常沙层，泥炭沉积物的年代在 5.5 至 1.2 ka。使用硅藻、化学和矿物学分析从泥炭沉积物重建的古环境显示该地点从盐沼、到潮上沼泽，到背障湿地环境。泥炭沉积物的放射性碳测年表明，这些环境变化与全新世中期高位后海平面下降相对应。异常砂层显示出内陆变薄和与下伏泥炭的形状接触。在这些层中，最近的层（1.7-1.2 ka）被指定为风暴沉积，因为它距离海滩脊不到 100 m 的有限分布。8 个额外的 5.5 到 2.2 ka 的沙层分布在离海滩脊 300 多米的地方，并显示出正常的沙粒等级，这是海啸沉积物的典型特征。由于观测到的每 50-100 年发生一次的大地震（M7-8）没有海啸袭击日高海岸的记录，我们将沙层解释为异常大地震沉积的海啸。由于 8 个推断的海啸沙层是在高位期沉积的，在 2.2 ka 之前，我们推断沿海地区海啸淹没面积在过去随相对海平面 (RSL) 发生了变化。我们强调评估长期 RSL 变化影响的必要性。