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

南海地震是沿着南海海槽发生的板间地震，是世界上历史记载最多的地震之一。在公元687年的白凤-南海地震之后，从历史文献中确定了9次南海地震（Usami，1996;Tuji，1999）。本研究的目的是利用地质证据揭示南海地震的史前发生。本研究选择四国岛进行。1946年南海大地震时，忠须池发生同震性沉降，被海啸海水覆盖，我们从该池中采集了6个5 m长的沉积物岩心，鉴定出14层海啸沉积物。放射性碳测年表明，这些地层沉积在1300 yBP和3800 yBP之间的时间段。这些结果表明，南开地震的复发间隔在过去4000年中的平均速率为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 Nine Nankai earthquakes have been identified from historical documents after the Hakuho-Nankai earthquake at A.D.687(Usami,1996;Tuji,1999).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.