Periodicity of large-scale landslide revealed by the landslide-lake sediments and ancient documents

在地形陡峭的日本中部地区，预测超过100万立方米的大规模滑坡非常重要，因为它们会对基础设施造成严重破坏。三次最大的滑坡（南野谷，新白谷和德山山下滑坡）在过去的一百年中已知发生在米诺地块。其中两个发生在具有特殊地质结构的地点：抵抗风化和侵蚀的石灰石被推到弱混杂岩上。这些地质构造是在吸积时期形成的，对预测米诺地块未来的滑动很有用。类似的地质构造也出现在船夫山、井山、良善山和藤原湖地区。在井喜山西坡发生的大规模滑坡堵塞了流经山脚下的阿内川河，形成了滑坡湖。我们测量了湖泊沉积物的14C年龄，发现湖泊的年龄分别是距今34000年和5200年。秋山以南的关原山口一直是连接日本东西最重要的交通要道，许多古代文献都描述了秋山的滑坡事件。我们分析了这些文献，发现滑坡事件大约发生在公元890年、1500年、1700年和1909年（Anegawa地震）。这些事件没有形成滑坡湖，而距今34000年和5200年前形成滑坡湖的事件规模相当大

Prediction of large-scale landslides with more than one million cubic meters of landslide debris is important in central Japan with steep-slope topography, because they cause heavy damages on infrastrucutes. Three largest landslides (Nan'notani, Neoshiratani and Tokuyamashiratani landslides) are know to occur during last one hundred years in the Mino terrane. Two of them occurred at the sites with specific geologic structures : the resistant limestones against weathering and erosion are thrust over the weak melanges. These geologic structures formed at the time of accretion, and are useful to predict the future slides in the Mino terrane. Similar geologic structures are present in the Funafuseyama, Ibukiyama, Ryozensan and Fujiwaradake areas. Very large-scale landslides in the western slope of Ibukiyama dammed-up the Anegawa River flowing on the foot of the mountain, and formed landslide lakes. We measured the 14C ages of the lake sediments and revealed the ages of the lakes to be 34,000 and 5,200 years before present. The Sekigahara pass south of Ibukiyama has been the most important traffic route connecting eastern and western Japan, and many ancient documents described landslide events at Ibukiyama. We analyzed these documents and found that the landslide events occured at about 890, 1500, 1700 and 1909 (Anegawa earthquake) AD. These events made no landslide lakes, and the events 34,000 and 5,200 years before present making the landslide lakes were quite large in scale