Evaluation of recurrent intervals of Nankai Earthquakes recorded in lake and marsh sediments

南开地震是南开海槽沿线的板间地震，也是世界上记载最多的地震之一。自公元687年白河-南开地震(Usami，1996；Tuji，1999)以来，从历史文献中已识别出9次南开地震。这项研究的目的是利用地质证据揭示南开地震的史前发生。一个名叫Tadasuga-ike的小池塘位于南开南部。四国岛被选为本次研究的对象。1946年的最后一次南开地震，中津池同震下沉，被海啸的海水覆盖，我们从池塘中采集了6个5m长的沉积物岩芯，鉴定出14层海啸沉积物。放射性碳测年表明，这些地层是在1300yBP至3800yBP之间的时间段沉积的。这些结果表明，近4000年来，南开地震的平均重现率为230年。

The Nankai earthquakes are interplate earthquakes along the Nankai Trough and one of the most historically documented earthquakes in the world. Nine Nankai earthquakes have been identified from historical documents after the Hakuho-Nankai earthquake at A.D.687(Usami,1996;Tuji,1999). The purpose of this study is to reveal pre-historical occurrences of the Nankai earthquakes by using geological evidence.A small pond named Tadasuga-ike located in the southern. Shikoku Island is selected for this study. The last Nankai earthquake of 1946,Tadasuga-ike subsided coseismically and was covered by seawater of tsunami.We collected six 5m long sediment cores from the pond and identified fourteen layers of tsunami sediments. Radiocarbon datings indicated these layers deposited during the time period between 1300 yBP and 3800 yBP. These results suggest the recurrence intervals of the Nankai earthquakes are 230 years in average rate during the last 4000 years.